Planetary Balloon Missions Revisited

Transcription

1 Planetary Balloon Missions Revisited

2 John Vistica November 2016 Introduction: NASA introduced the concept of sending a superpressure balloon to Mars over a decade ago. I ve always liked the idea of exploring Mars with up close images of ever-changing landscapes and had hoped that we would see this happen. NASA and JPL have also proposed a number of other balloon missions to Venus and elsewhere; however, it seems that nothing has gone beyond the proposal and fundamental testing stages and the latest papers on the subject are years old.

3 What s New: A standard superpressure balloon has no means of changing its altitude, but there has been a fundamental advancement in superpressure balloons in the last few years allowing for a low tech means of variable altitude control. I am referring to GoogleX s Project Loon to bring internet access to far off places here on Earth. Along with their ingenious means for altitude control they have also made significant advancements in producing consistent flawless polyethylene superpressure balloons able to maintain altitude up to six months without loss of helium. How can we re-imagine previous mission proposals for superpressure balloons on Mars and Venus by applying this new method of variable altitude control? How can variable altitude control expand the mission capabilities for planetary exploration? Personal Note: I am a space enthusiast and a recent Aerobot fan. I don t know anyone at Google, NASA, JPL, SpaceX, or any other organization mentioned in this presentation.

4 Recent Mars and Venus Balloon Mission Proposals Here are three missions that will be covered in this presentation: Mars Aerobot / Balloon: This was a 2001 mission proposal for a 10,500 m 3 superpressure balloon with a float altitude of 6.5 km and a payload of 15 kg and a diameter of 27 m. It would be able to traverse the majority of Mars landscapes at that altitude. For reference, since altitude is measured by the average elevation at Mars equator and Curiosity s Gale Crater landing site is at -4.4 km, it would fly 10.9 km above Gale Crater. VALOR: Part of the Venus Design Reference Mission (VDRM). Two superpressure balloons designed for a fixed altitude of 55.5 km where Earth-like temperature and pressure conditions exist. This is in the mid-level cloud layers of Venus where clouds range from 40 km to 60+ km. Mission duration: 30 days (battery dependent). Scientific Payload 22.5 kg. Diameter 7.1 m. Additional references: VEXAG: Venus Exploration Analysis Group VEXAG: Roadmap for Venus Exploration (2014) VME: Venus Mobile Explorer: A surface lander probe that, after first analyzing an initial landing site, expands a stainless steel bellows with helium causing the probe to float to second site for additional analysis. Mission duration: 1 hour descent and 5 hours after landing. Lander 650 kg + Bellows System 1132 kg. Mars Aerobot - Image credit: NASA/JPL Venus VALOR - Image credit: NASA/JPL Venus VME - Image credit: NASA/JPL

5 GoogleX s Project Loon Background: I highly recommend watching the set of 2013 GoogleX Project Loon YouTube videos for a very good overview of their balloon development and altitude control system. Image Credit: Google Loon Balloon (15m x 12m) Image Credit: Google Loon Altitude Adjustments Variable Altitude Control: Basically GoogleX partnered with Raven Aerostar to develop and successfully demonstrate altitude and flight path control utilizing an inner bladder envelope within the main fixed-volume superpressure envelope. Filling the bladder with outside air increases overall density of the balloon causing it to lower its altitude. By changing altitude it can catch different directional winds and thus steer itself using known weather patterns. This method completely avoids the classic and limiting ballast and venting methods for altitude control. Their balloons can maintain altitude for as long as six months before being remotely brought down for recovery. Image Credit: Google Patent for Bladder Method Image Credit: Google Patent for Diaphragm Method Patents: Google has patents for both the bladder buoyancy control method and an alternate method using a diaphragm connected at the circumference of the balloon. For a Loon balloon the bladder method is used, but for more extreme altitude control to dive to the surface of Mars or Venus the diaphragm method would use much less material so that is what is assumed in this study.

6 Buoyancy Fundamentals For A Loon-Based Balloon Buoyancy Equation: For simple balloons, lift mass for neutral buoyancy is based on the density of the air minus the density of the internal gas (helium) times the volume of the balloon (V0). For a more complex diaphragm balloon (Figure 1) this becomes: Lift mass = (Density_Air0 * V0) (Density_He * V1) (Density_Air2 * V2) Design Lift Mass: (See Appendix B for full equations) 1. Select a target max altitude for the balloon and determine the air density conditions using available atmospheric profile equations or data. 2. Select a V0 volume. 3. Calculate design lift (kg) for V0. This is when V2 = 0 and V1 = V0. This is the total mass of the balloon + payload for neutral buoyancy at the maximum altitude. 4. Select a design ΔP1 at max altitude. This is the differential pressure between P1 and P0 and dictates the initial helium load. It should be sufficiently high to ensure that the balloon does not depressurize under any ambient conditions expected in the operating altitudes. Ideally it should be well below the design limit of the balloon ΔPmax to facilitate altitude changing operations without adding excessive stress. Figure 1: Superpressure balloon with diaphragm P1 = P2 ΔP1 = P1 P0 V0 = V1 + V2 T0 = T1 = T2 * * This ignores heat transfer rates. In reality there would be sensors monitoring internal and external conditions to drive real-time responses with known heat transfer info. Altitude Control System: 1. Pump in external air into V2 volume thus compressing V1. As V2 increases the overall weight of the balloon increases and the balloon will drop until the outside density balances the equation again. 2. Lowest altitude is only restricted by external conditions such as temperature and pressure that the balloon and payload material can handle (e.g. Venus). 3. Using target altitude s ambient values calculate V1, V2, and P1. Adjust V2 to the desired volume. 4. The differential pressure (ΔP1 = P1-P0) cannot exceed the maximum operating internal pressure differential limit (ΔPmax). Increase V2 until ΔP1 = ΔPmax. Let the balloon drop in altitude so that P0 increases. Increase V2 again in this controlled fashion until V2 equals the target value. Once conditions (T1, P1,V1,V2) settle out the balloon should be at target altitude.

7 Mars Balloon Mission

8 Mars Red Loon Mars Aerobot / Balloon: This was a 2001 mission proposal for a 10,500 m 3 superpressure balloon with a fixed float altitude of 6.5 km and a payload of 15 kg and a diameter of 27 m. It would be able to traverse the majority of Mars landscapes at that altitude. For reference, since altitude is measured by the average elevation at Mars equator and Curiosity s Gale Crater landing site is at -4.4 km, the balloon elevation would be 10.9 km above Gale Crater. Introducing the Mars Red Loon: Mars Aerobot - Image credit: NASA/JPL and Global Aerospace ( Red Loon is just what I am calling a Mars version of a diaphragm-controlled balloon in this presentation.) Take GoogleX s off-the-shelf Loon balloon, and adapt it for Mars (diaphragm rather than bladder). Select the design maximum altitude. Upsize as necessary for larger payloads. For Earth at 27 km altitude, the standard operating altitude for a Loon, the conditions are already very close to those of Mars. GoogleX and Raven Aerostar have engineered a superpressure balloon for commercial purposes that is ideal for the next stage of exploring Mars.

9 Mars Red Loon Mission Capabilities Able to navigate over most the planet (Mars Elevation Map). Pretty much everything in the northern hemisphere is in reach. Limited only by the design altitude (maximum attainable altitude when V2 = 0). A set of design altitudes from 4.0 km to 6.5 km are examined in the presentation. Steerable navigation utilizing wind directions at varying altitudes. Won t be quite as accurate without Earth s weather satellite info, but that is part of the adventure. Able to land anywhere it can fly. The balloon can land and park itself as long as we want as many times as we want. By continuing to increase V2 after landing, the gondola becomes an anchor. Vent off V2 and the balloon is airborne again. Characterize atmospheric conditions at varying altitudes (gas analysis, temperature, pressure, wind direction, wind speed, etc.). Greatly expand our understanding of the Martian atmosphere. Provide ever-changing panoramic images and data on a daily basis. Provide up close validation of what we are seeing in the satellite imaging. Able to travel over the landscape at whatever altitude we choose up to the design altitude with appropriate aerobot safeguards to avoid approaching hazards. Want to cruise at 1 km and then drop into that approaching canyon at 100 m for a closer look?...no problem! Presently we are limited to one landing site per mission with years in between. So far they are limited to low altitude safe places to optimize the chance of a successful landing. I suspect that there are a lot of interesting places to explore on Mars that don t make the list. A Red Loon would be able to drop in on dozens if not hundreds of diverse places. The challenge will be deciding to skip over areas never to return on the way to the next big reveal.

10 Zero Pressure Diaphragm Fixed Volume Outer Envelope V1 Weighted Center V2 He Air2 Figure 2: Red Loon Cross-Section V0 Air0 Increase V2 volume using local air to increase the weight of the balloon, causing the balloon to lose buoyancy. The Red Loon would use the same polyethylene envelope material as GoogleX s Loon. If an Earth Loon can handle the solar heating and other stresses in Earth s atmosphere for six months, it should handle anything Mars can throw at it. Mars Red Loon The Mars Red Loon would use the diaphragm rather than the bladder used in GoogleX s Loon. V2 will have to inflate up to 70% of V0 for the full altitude range and a diaphragm would use only 65% of the material as a bladder. GoogleX has provided a basic dimension for their balloon as a spheroid 15 m wide and 12 m tall. That calculates out to 1414 m 3 based on a perfect spheroid just as a reference. Maximum V2 volume is determined by how far down from the design altitude we wish to descend. For this study I use -8.2 km for Mars lowest elevation at the Hellas impact basin just to be sure we can land anywhere. For operations very near the surface it will be critical to maintain neutral buoyancy to handle the large swings in temperature and pressure during the daily weather cycles. Mars weather cycles are discussed in Appendix A. During the heat of the day the pressure drops and the temperature spikes. This leads to a drop in air density. To keep altitude V2 must be vented otherwise the balloon will drop and possibly impact the ground for an unplanned landing. During the evening the temperature rapidly drops and pressure increases. This leads to a rapid increase in air density. To keep altitude V2 must be inflated otherwise the balloon will rise.

11 Mars Red Loon - Delivery Methods There are a few options for delivering Red Loon to Mars. The balloons going to Venus are rather robust compared to those going to Mars and more likely to survive an aerobrake entry, descent, and deployment. For Mars there is much more to contend with as the balloon is a bit more delicate with limited atmosphere to make it all happen. Option 1: Image Credit: NASA/GAC Option 2: Image Credit: NASA/AFS Option 1: Atmospheric Entry System This is how the Venus balloons would be deployed. After aerobraking and parachute deployment to reduce speed, the balloon is deployed and inflated. The inflation tanks fall away and the balloon flies away. So far drop tests have been rather unsuccessful in this method. Either the balloon fails to fill properly or the cords get tangled. There are a lot of things that have to go exactly right to make this work and so far this option is showing a low probability of success. Option 2: Land a deployment package on the surface and deploy the balloon from there. NASA awarded funding in 2009 to Aurora Flight Sciences to study a Shielded Mars Balloon Launcher (SMBL) in association with a future lander. There is little detail about what NASA had in mind regarding the balloon s size and capabilities but it at least indicates that someone at NASA was still working out options for balloon exploration of Mars. Option 3: Red Loon meets Red Dragon. SpaceX will hopefully demonstrate a propulsive landing on Mars in All of the crazy hard stuff of getting a balloon to Mars can be accomplished by the Red Dragon. For a future Red Dragon mission a modified version of the top hatch would provide an excellent means for deploying a balloon. This seems the best option for successfully deploying a Red Loon with a reasonable payload capable of globetrotting Mars. Option 3: Image Credit: SpaceX

12 Mars Red Loon SpaceX Red Dragon Deployment Here are some of the benefits for a Red Dragon delivery: Red Loon travels to Mars in a pressurized temperature controlled compartment mitigating potential damage due to low temperature and exposure to vacuum for the long trip to Mars. Red Loon could be staged directly under the Red Dragon s modified top hatch within a protective deployment container. Countless deployment run-throughs could be performed on Earth refining the process by which the balloon needs to be stored and inflated to assure no damage and proper deployment. Red Loon should already have a readily available supply of helium. Red Dragon uses an ultra-high pressure helium tank to pressurize the SuperDraco fuel tanks for the landing. Just preheat the helium when filling the balloon. A 10,000 m 3 (29 m diameter) balloon will take about 12 kg of helium. Tether the balloon to Red Dragon after inflation to perform operational checks on all systems. NASA would have total control over the deployment and release. Red Dragon should have plenty of cameras available to monitor the deployment progress. Figure 3: Artist Concept Red Loon Deployment From Red Dragon The cost of putting Red Dragon 2018 on Mars is speculatively about $320 million. Even if Red Loon was the only payload on a later mission, which is highly unlikely, that would be quite a bargain compared to developing a customized stand-alone mission for decent deployment with questionable chances of success.

13 Payload Mass (kg) Planetary Balloon Missions Revisited Mars Red Loon Payload Mass and Design Altitude Payload Mass by Balloon Volume Design altitude is the maximum altitude achievable for a given payload. As the design altitude goes up the payload capacity goes down. - Higher design altitude allows for exploring higher terrain. - Lower design altitude allows for a larger payload. 4.0 km 4.5 km 5.0 km 5.5 km 6.0 km 6.5 km Diameter (m): (29) (33) (36) (39) 0 5,000 10,000 15,000 20,000 25,000 Balloon Volume (m 3 ) Figure 4: Payload Mass by balloon volume for different design altitudes The chart above is based on an outer V0 envelope density of kg/m 2 and a V2 bladder density of 0.01 kg/m 2, a payload mass can be calculated for select design altitudes. Actual material density for Google s Loon is unknown, but Raven Aerostar s zero pressure balloons of the same thickness is about 0.01 kg/m 2. The payload here is all non-balloon items including the altitude control system. Whereas much of the balloon mass is driven by the surface area of the envelopes, only real analysis by GoogleX and NASA will determine the requirements for the materials, design, tendons, end plates, altitude control system, solar power, etc. which will drive scientific payload mass.

14 Altitude ( km ) Planetary Balloon Missions Revisited Project Red Loon Altitude Control Red Loon V2 Volume Percent By Altitude - Design altitude is 6.0 km for this example. - Each data point represents 500 m descent. - V2 volume increase reduces buoyancy thus altitude. - Example: To reach the surface of Gale Crater which is at -4.4 km, V2 must be inflated to 53% of V Altitude ( km ) Temp ( C ) Press ( Pa ) ΔP1 ( Pa ) V2 as Percentage of V0 Gale Crater -4.4 km Figure 5: Red Loon V2 Volume Percent of V0 By Altitude Using the standard Mars atmosphere profile equations for pressure and temperature, this chart shows an example of changes to V2 to obtain a desired altitude. If we want the balloon to drop from 6 km to -2 km, then V2 must be inflated to about 44% of V0. The table in the chart gives a sampling of conditions using the profile equations and its impact on ΔP1, the differential pressure between P1 and P0.

15 Topic Altitude Control System Planetary Balloon Missions Revisited Mars Red Loon SpaceX Red Dragon Deployment Comments GoogleX s Loons use an altitude control system they call a Croce which uses a centrifugal air compressor and vent valves to change their V2 bladder volume. How much air needs to be moved depends on the air density and the total V0 volume. Sizing of equipment will depend on selected balloon volume, operational surface altitudes, and expected daily and seasonal environmental variations. Mars weather cycles are discussed in Appendix A. Cameras Along with the scientific cameras should be at least one 360 degree panoramic camera. The balloon will be passing over vast unexplored regions never to return again. There is no time to point and shoot. It should be able to automatically document its own journey. Relaying that data is discussed on the next slide. Ideally we should be getting the HD 360 degree video tour across canyons, valleys, mountains, craters, and everything else Mars has to offer. For reference here is one 360 camera I found with a quick search: 360Fly 4K camera (172g) 1800x1800 pixel with live streaming capability. List $499 Shock resistant - built for extreme sports. Able to take 10MB photos while video capturing. Solar Power Google s Loon uses a flat panel of solar cells above the payload for recharging the batteries. That may be sufficient for Red Loon, but it will also depend on season and dust accumulation. The solar powered Mars rovers such as Spirit and Opportunity benefited greatly by wind cleaning events. It may be that a balloon would have similar opportunities, but here are a couple of possible alternatives: Use the centrifugal air compressor with a diverter valve that connects to a manifold that blows off accumulated dust. Instead of a flat solar panel, add a flexible solar skirt hanging from the perimeter of the balloon so that we don t have to worry about accumulating dust nor low solar inclinations during the winter. Here is an article on extremely lightweight flexible solar cells being developed.

16 Mars Red Loon SpaceX Red Dragon Deployment Topic Package Options Comments For a Red Dragon landing site that is kilometers below the design altitude, it will be possible to attach and lift temporary payloads and deliver them hundreds if not thousands of miles away from the landing site. One possibility is to deploy seismic monitoring stations at various parts of Mars to allow triangulation of seismic events and give a better view of the internal structure of Mars that wouldn t be available with a single sensor. Propulsion Option Landing a balloon is always tricky. Landing on autopilot on another planet that may have slight winds and various hazardous objects is going to be more challenging. My ideal Red Loon would add at least a couple of bi-directional and rotatable propulsion blades above the payload to facilitate more controlled landings and hazard avoidance. It should be able to at least compensate for low wind conditions. Alternative justification: What if we land in an interesting spot, but from the images we get we see something that is VERY interesting just a few hundred meters over that way. With some mode of propulsion a Red Loon would be able to lift off and maneuver over to where we want it to go rather than being completely dependent on drifting with the wind. Also, while travelling during daylight hours when more power is available, the same rotors could help direct the path of the balloon to a more desired flight path so that we aren t completely dependent on finding an altitude with an appropriate wind direction. Any propulsion option is greatly dependent on the balloon s drag coefficient, wind speeds, air density, and available power to provide sufficient thrust to make it workable and worth the extra weight. Data Transmission The level of data and image gathering will easily exceed any bandwidth available to send to Earth. There would not be any way to determine what should be sent in real time and what isn t sent could be lost forever. Instead set up a relay from the balloon back to the Red Dragon via orbiters to store the terabytes of data onto a special mass storage device on the lander. Low resolution uploads to Earth can be sent and higher resolution subsets can be queried upon request. The data can be accessed long after the balloon mission is complete or until we get sufficiently high enough bandwidth communications orbiters so that we can upload everything back to Earth.

17 Mars Red Loon - Summary If we are going to get serious about exploring Mars, then we are going to have to come up with a better way to do it than these limited single missions that are restricted to their immediate surroundings. Rovers help a little, but they take literally years to travel a few miles. There are 144,798,500 km 2 of Martian surface to explore. A Red Loon superpressure balloon can dramatically expand our capability to do just that. With Red Loon we can: Circumnavigate the globe at varying latitudes. Future Missions: Provide steerable navigation utilizing wind directions at varying altitudes. Land anywhere it can fly. Characterize atmospheric conditions at varying altitudes. Provide ever-changing panoramic images and data on a daily basis. Travel over the landscape at whatever altitude we choose up to design altitude. Provide up close landscape characterization unavailable from orbital imaging. Eventually we will need true steerable aerobot airships able to perform detailed scans over large regions with surveillance and ground penetrating radar to fully assess local resources for the proper siting of human colonies. The Red Loon aerobot is a very important step in getting there.

18 Venus Balloon Missions

19 Venus VALOR Balloon Modification (VALOR+) Proposal The VALOR superpressure balloons are relatively simple passive missions because of the limited battery life, 22.5 kg scientific payload restrictions, and the fixed 55.5 km altitude. One was to be dropped at the equator and the other at higher latitudes. A variation on the VALOR balloon is part of a VISION Discovery mission proposal that does use a solar array for power and has radar for mapping and a camera for a final dive to the surface at the end of the mission. VALOR+ Modifications Add a diaphragm-based variable altitude control system with an altitude target range of 40 km to 60 km which should cover the full cloud cover range. Normal operational range is 55 km to 60 km with temporary dives to 40 km due to higher ambient temperatures. Add solar power recharging which should be viable near the 60 km altitude. Upsize the balloon for larger payload capacity. Add two-way communication to allow remotely navigating the balloon utilizing directional wind variations at differing altitudes. Mission duration could extend for months as it is only limited by helium retention. Eliminate one of the balloons as one balloon can effectively cover a wide range of latitudes with an atmosphere rotation rate of about four days and the ability to actively navigate the balloon. There is also some mission overlap with the deep dive mission that will be covered next. Venus VALOR - Image credit: NASA/JPL Wiring run Zero Pressure Diaphragm Helium Fill Lines Sensor Platform V1 He Weighted Center V2 Altitude Control V0 Air0 Fixed Volume Outer Envelope Air2 Figure 6: VALOR+ cross sectional details

20 Mass (kg) Planetary Balloon Missions Revisited Venus Balloon Modifications With Altitude Control System Venus Payload Capacity by Balloon Volume Assumptions: Design Altitude: 60 km Balloon V0 Material: kg/m 2 Diaphragm Material: 0.2 kg/m 2 Spherical Balloon VDRM Lander (payload kg) VME Lander (payload kg) VALOR Balloon (payload - 44 kg) Diameter (m): (10) (12) (14) (16) (17) (18) Balloon Volume (m 3 ) Figure 7: Total, balloon, and payload mass by balloon volume For any Venus balloon with a diaphragm-based altitude control system, the chart above shows the total design lift, balloon mass, and the available mass for both payload and altitude control system with a 60 km design altitude. The payload mass for the VALOR balloons and VDRM and VME landers are given for reference.

21 Altitude ( km ) Planetary Balloon Missions Revisited Venus Balloon Modifications With Altitude Control System 60 Venus Balloon V2 Volume Percent By Altitude 50 1 % % Altitude ( km ) Temp ( C ) Press ( Atm ) ΔP1 gauge (Pa / psig) / / / / Each data point represents 1 km descent. - V2 volume increase raises the diaphragm (red). - Need V2 to reach 99% of V0 to get to the surface. - As balloon descends internal gauge pressure increases modestly. 99 % V2 as Percentage of V0 Figure 8: V2 as percentage of V0 to achieve a specific altitude. This chart shows the change in V2 necessary to lower buoyancy to achieve a target altitude. The table shows details of four altitude conditions where 2 psig was set as the initial overpressurization at the 60 km design altitude.

22 Venus VALOR Balloon Modification (VALOR+) Discussion (1 of 3) Topic Comments V2 Pumps V2 Backup Relief Valve Solar Power Heat Mitigation (balloon) GoogleX s Loons use an altitude control system they call a Croce which includes a centrifugal air compressor and vent valves to change their V2 bladder volume. How much air needs to be moved depends on the air density at altitude (see Figure 8). For a 1000 m 3 balloon on Venus for example: At 60 km it takes 122 m 3 added to V2 to lower 1 km. At 40 km it takes 16 m 3 added to V2 to lower 1 km. At 20 km it takes 2.6 m 3 added to V2 to lower 1 km. At 1 km it takes m 3 added to V2 to lower 1 km. For full vertical decent mobility a balloon on Venus may need both a centrifugal air compressor for high altitudes and a smaller positive displacement pump for more refined control at lower altitudes. Similarly, to ascend there should be two sizes of relief valves to reduce V2. Especially for those excursions below 55 km on Venus it would be prudent to have a backup relief valve with an in-line regulator such that P2 never dips below ΔPmin. Emergency venting V2 will return the balloon to its design altitude at 60 km without collapsing the V0 envelope along the way. My assumption is that at 60 km (above or near the cloud canopy) there is sufficient solar energy to recharge the batteries to support a 6 month mission even if still in the Venus haze. With the 4 day rotation of the atmosphere, the batteries should likely be recharged with at least one excursion to 60 km per solar day depending on the battery capacity and power usage. It would be interesting to characterize the light availability at different altitudes for solar cell purposes for future missions. The basic VALOR design uses aluminum coatings to mitigate heat buildup inside the balloon at 55.5 km. Solar heating in the balloon may be an issue at 60 km even with the -10 C ambient temperature and large aluminum surface area. If removing heat is still a problem, then consider this: Add a radiator to the bottom of the Altitude Control Box that can circulate helium, using an in-line leak-proof fan assembly, from V1 through the radiator to reduce temperature. Use the two internal helium fill lines that pressurized V1 upon descent for the circulation. The fan should only be necessary when solar power production is available.

23 Venus VALOR Balloon Modification (VALOR+) Discussion (2 of 3) Topic Comments Heat Mitigation (payload) Altitude Control System Sensor Wish List V0 Envelope Diaphragm V2 Volume For the brief excursions down to 40 km (~150 C) use lander technology for temperature control using lithium nitrate with phase change material and insulation. This ensures that the temperature does not exceed 30 C for the duration of the excursion. The amount of heat control needed will depend on the duration below 55 km and the thermal characteristics of the payload container. All electronics that control the Altitude Control System should be in the Payload container where thermal conditions are controlled. Altitude Control System components (V2 Pumps, valve controllers, etc.) need to be rated for the expected operational ambient conditions (-10 C to 150 C). Add a sensor platform at the top of the balloon. Also add camera to take image showing horizon, top of balloon with strobe to catch a photo of the rain hitting balloon. Add microphone inside V1 envelope should be good indicator of rain intensity as the balloon is acting like a big drum. Everything should be lightning strike hardened of course. Instrument wires go through the same sleeve as the upper helium fill line inside V0. Images from above cloud tops could be spectacular depending on the intensity of the ever-present haze which may extend from 30 km to 87 km. Initial assumption: As a starting point plan to use the existing VALOR design material and spherical shape: Teflon, Aluminum, Mylar, Vectran with Polyurethane (0.272 kg/m 2 ). Note that each material and adhesive will have to be rated for brief excursions to 150 C. Polyurethane has an upper temperature limit of 130 C which is a problem. Kevlar, for example, will lose 10% strength after 500 hours at 160 C. Teflon (PTFE Polytetrafluoroethylene) is rated up to 260 C. The diaphragm that separates V1 from V2 is effectively a zero-pressure membrane. The diaphragm is a full hemispherical-sized volume of V0 connected along the V0 circumference. At design altitude: V0 = V1 and V2 = 0. Calculations assume 0.2 kg/m 2 since it doesn t require the reinforcement as the V0 envelope. The entire V2 volume (lower V0 half + lower diaphragm) will also need the same Teflon coating applied to the outer surface for corrosion resistance. V2 would likely see corrosive condensates form as internal temperature changes. Cycling a properly positioned relief value on the bottom plate should expel unwanted liquids. Should add a liquid sensor to the bottom plate.

24 Venus VALOR Balloon Modification (VALOR+) Discussion (3 of 3) Topic Helium Loading Sensor Wiring Navigation Com Link Aerobot Technology Comments Need to load helium from the helium tanks below the payload to the upper V1 volume during the deployment sequence. One option is from the lower balloon plate connect two fill lines and run up the opposite sides of V0 on the inside (Figure 8). Create a sleeve of Vectran against the outer envelope to run the lines through to prevent abrasion damage to the diaphragm. Assuming that the helium heat exchanger is being implemented, the tank lines can connect into the heat exchanger thus having access to the two fill lines to V1. There would have to be a couple of creative zero-leakage penetrations for the fill lines between V2 and V1 at the perimeter. If instruments are to be placed at the top of the balloon, then run wiring through the bottom plate, up one of the sleeves used by the helium fill lines, and through the top plate to keep them all internal. GoogleX has been quite successful with their Project Loon balloons in utilizing changes in wind direction at different altitudes to actually steer a balloon across the globe. With a nominal 55 km to 60 km altitude range we may find that Venus provides its own opportunity to do the same. With hopefully a 6 month mission lifespan, we could eliminate the second planned VALOR balloon and traverse to all parts of the globe riding the winds with just one balloon. Communications between the balloon and the orbiter should be upgraded to two-way to allow controlling schedules for deep dive excursions to 40 km, navigation options, upgrading operational algorithms, and adjusting investigations based on exploratory findings. Venus represents an amazing opportunity to demonstrate and showcase aerobot technology in one of the most dynamic atmospheric environments in the solar system. There would be orders of magnitude difference in scientific return between sending a passive fixed-altitude balloon for limited duration vs sending a navigable, rechargeable, and variable altitude balloon that can explore the full 40 km to 60 km cloud canopy range across the globe. The lessons learned here would be applicable to Mars, Titan, and any other planet or moon with an atmosphere.

25 Venus VME Balloon Modification (VME) Proposal Solar Option Venus Mobile Explorer (VME): A proposed surface lander probe mission that, after first analyzing the initial landing site, expands a stainless steel bellows with helium causing the probe to float to second site for additional analysis. Mission duration: 1 hour descent and 5 hours after landing. Lander 650 kg + Bellows System 1132 kg. VME+ Deep Dive Modifications Venus VME - Image credit: NASA/JPL Use the more mature designed Venus Design Reference Mission (VDRM) lander as a payload starting point. Add a hard core version of the VALOR+ balloon capable of handling surface conditions. Design operational range is 0 km to 60 km. All material would be rated for a minimum of 500 C capability. Add solar recharging capability for staging altitude of 60 km. Balloon envelope would be an aluminum-based metallic film matrix with metallic thread reinforcement. Designed for deep dives to the surface with surface times of 1 to 2 hours. Mission duration could extend for weeks or months as it is only limited by the helium retention and the expected material cycling during deep dives. The modest differential pressure should mitigate the helium loss rate. Able to perform surface deep dives every fourth day targeting numerous regions of the surface. Surface samples analyzed at staging altitude rather than at surface to simplify process and allow for extended analysis.

26 Topic VME+ General Discussion Venus VME Balloon Modification (VME+) Discussion (1 of 3) Comments Skip the one-time lander concepts altogether. Instead take a hard core version of the VALOR+ balloon modifications and add a surface-capable payload. I ll call this deep dive option Venus Mobile Explorer Plus (VME+). As mentioned before the only limitation to the variable altitude control method is the material that the balloon and payload are made of to handle the ambient conditions. With a design altitude of 60 km the VALOR+ balloon requires V2 to max out at 89% of V0 to get down to 40 km (see Figure 8). To get to the surface V2 has to achieve 99% of V0. For this study I set a design ΔP1 at 60 km of 2 psig. At 0 km ΔP1 would max out at a modest 5.6 psig. The main issue will be descent, surface, and ascent times to keep payload electronics within safe operating limits. For reference, the current VDRM lander is designed for a 1 hour descent and 5 hours on the surface until the temperature increase fails the equipment. The balloon s descent rate would be expected to be much slower than a parachute and ascent time now comes into play which will limit time on the surface. Let s assume that the balloon only has an hour or two on the surface. - All critical readings on surface conditions should be met. - Great panoramic photo opts and atmospheric data collected on descent, landing, and ascent. - Sample grab for later analysis (discussed further down) - Able to anchor by increasing V2 a bit more thus keeping the probe in one spot. Now also consider the new mission capabilities for VME+ - After lifting off, continue to drift over the landscape taking images for a fuller characterization of the region. - Land at a second site within the same descent (maybe the first landing failed to capture a sample). - Don t land at all during some of the deep dives but instead descend to an altitude low enough to travel over the surface and scan the region with radar and photos. Surface wind speeds are assumed to be less than 3 mph, but the further up you go the faster they pick up and the more travel distance so the more ground covered. - With the full range of altitudes available, do a deep dive to any desired level to perform detailed environmental analysis that isn t available by a lander just passing through at the end of a parachute. - ABLE TO PERFORM A DEEP DIVE TO THE SURFACE EVERY 4 DAYS.

27 Topic Mission Reality Check Balloon Envelope and Diaphragm Payload Venus VME Balloon Modification (VME+) Discussion (2 of 3) Comments - If a VME+ balloon lands at one site and fails to ascend, then you have a one-time lander that you can still get data from and vital feedback on the technology assuming it at least made it to the surface in one piece. - If a VME+ balloon manages two deep dives, then it just paid for itself over the cost of sending a second standard lander if and when it were to get authorized (years to get funding, building, launch, travel time, and arrival). - If a VME+ balloon manages to perform multiple deep dives, aside from being a major game changer in exploring Venus, it would represent decades of data gathering by the old method within a single one to six month mission. All materials have to be rated for at least 500 C. For this exercise I assume the same material weight as VALOR but it will need to be replaced with a flexible metallic matrix material. Pure aluminum has a melting point of 660 C. As a reference standard household heavy duty aluminum foil is kg/m 2. Obviously there are many factors to consider in a proper alloy material matrix selection which I ll leave to more knowledgeable people but as an example here is a commercial flexible aluminum coated fiberglass material rated to 537 C. For the calculations I assume kg/m2 for V0 and 0.2 kg/m2 for the diaphragm since it sees less stress. - Extreme corrosion and temperature resistant alloy/oxide coating to replace the Teflon functionality. - Flexibility of the diaphragm to cycle through min and max V2 volume changes. Fortunately it is still effectively a zero-pressure membrane. - At max V2 and highest temperature the diaphragm and the outer envelope will be in contact and unfortunately standard Teflon is rated only up to 260 C. Metallic surfaces will have to be coated with something that is resistant to bonding under all expected conditions. - The higher the designed maximum operating differential pressure (ΔPmax), the better the achievable descent rate. Use a metallic thread in the material matrix to increase internal pressure capabilities. Lander payload for original VME is calculated at 650 kg excluding the bellows system. The current standard VDMR lander weighs in at 686 kg. The new payload will have to account for the new Altitude Control System mass. Figure 7 s chart is marked with both landers to indicate potential upper limit payloads for a surface mission.

28 Topic Design Altitude V2 Pump Sample Retrieval Solar Cells Propulsion Option Venus VME Balloon Modification (VME+) Discussion (3 of 3) Comments The design/staging altitude is the maximum altitude where V1 = V0 and V2 = 0. I am assuming this to be 60 km, but it should be set by the lowest altitude where there is sufficient light that the solar cells can recharge the batteries. The lower the staging altitude, the smaller the balloon can be. After crunching the numbers for a design altitude with an overpressurization of 2 psig, dropping down to the surface results in modest 5.6 psig inside the balloon. For reference the VALOR s 7.1 m balloon s burst test was at 23 psig. The pump and valves will have to work under an extreme range of temperatures and pressures. The electronic controls for the pump will be safely within the payload pressure vessel. Should probably look to rocket engine technology for materials and fabrication of a robust design of the pump, motor, and valves under such a large set of operating conditions. Lander sample analysis for VME and VDMR seem to be quite a challenge for taking a sample at 92 atmospheres and getting it into an analysis chamber at about one atmosphere and then sitting around for a couple of hours getting results. Fortunately we don t have to really do this for a VME+. - Engineer a scoop-type device under the pressure vessel that activates upon landing. - Contain the sample at ambient pressure outside the pressure vessel. - Continue with all of the other great stuff we want to do while on the surface and then start back up. - During ascent let pressure escape the sample container while ensuring corrosive aerosols do not get to the sample. Use a vent check valve or something. - Analyze sample at our leisure at the staging altitude. This should be a challenge but maybe not. If solar cells can not be rated for surface conditions, then skip the idea of having solar cells outside the pressure vessel. Instead have a panel of solid corrosion resistant light concentrators overlaid onto high temperature optical fiber. The fiber optic bundle would penetrate the pressure vessel and direct the light to the solar cells safely in the pressure vessel thus charging the batteries when at staging altitude. For references look up Concentrating Photovoltaics (CPV) for the concept and here is a gold coated optic fiber rated to 700 C. Assuming the V2 centrifugal air pump s motor windings are made to withstand the extreme temperatures and pressures, then we could apply that same technology to adding a couple of rotor motors to add propulsion control. Near the surface the altitude control system may not have enough finesse to manage landings where conditions are not ideal such as for object avoidance or wind speed issues.

29 Venus VME Balloon Modification (VME) Proposal RTG Option This is a single slide of a more complex option for VME+ based off of an RTG-powered balloon study. Low-altitude Exploration of the Venus Atmosphere: A 2010 study by Geoffrey Landis for operating a RTG-powered spherical rigid tank balloon. - Titanium tank (3.6 m diameter, 28 m 3 volume, 510 kg, 2mm thickness) - Fixed altitude: 5 km (66 atm, 424 C) well below the haze layer. - Lifting gas: 70 kg N 2 and 530 kg H 2 O (see link for explanation) - Payload: 320 kg - Active cooling of payload powered by 4 RTGs adapted for Venus - Differential pressure (ΔP1): 1 atm - H2O used over helium due to transport issues and avoid leakage - Visual imaging during daylight hours - Continual radar scanning day or night - Venus Surface Power and Cooling Systems by Geoffrey Landis Venus Low Altitude Balloon at 5 km altitude VME+ Modifications Add altitude control bladder to tank. Suspend bladder away from lower tank to avoid damage by liquid H 2 O during transport and atmosphere entry. V2 requires 23% of V0 for 5 km altitude control V2 requires 42 % of V0 for 10 km altitude control Note: Venus surface has 13 km of elevation deviation with the highest peak at 10.4 km above mean elevation. Selectable altitude while travelling over the landscape. Drop to the surface as often as we like. Steerable navigation using winds at different altitudes. VME+ (RTG Option) Discussions Less thermal cycling on components than the previous VME+ option. MUCH more time studying the surface. The arguments for the N 2 /H 2 O mixture is well made in the study for low-altitude ballooning. The 3.6 m diameter limit for the tank was based on the smaller fairing size for the Delta-IV. To get larger payloads than the 320 kg since we now have to include the altitude control system, consider taking advantage of the larger Delta-IV fairing limit of 4.8 m or the Falcon 9 s fairing limit of 4.6 m. Also, the tank doesn t have to be spherical it could be a cylindrical tank with spherical ends to increase the volume and, thus, the payload capability.

30 Venus Modified Missions - Conclusions (1 of 2) Multiple Mission Options - Whether the balloon is fixed or flexible a Venus mission can be significantly enhanced by applying Google s variable altitude control method. Having the capability to drop to the surface and return to design altitude over large sections of the planet will significantly improve the scientific exploration of Venus. - The altitude control system should be designed by the same type of engineers that design rocket engines. It would have to function flawlessly under an extreme range of environmental conditions using similar materials. Failure Mode Considerations for Deep Dive Mission - Since the V0 envelope will be dealing with going from -10 C to 470 C, there will be expansion of V0 and all of the thermal stresses that entails. That will also have to be considered in the buoyancy equations among other things. An aluminum skinned envelope would expect a 4% increase in volume of V0 at the surface with minimal impact to parameters. - As V2 goes from 0% to 99% any catching of the diaphragm fabric against the V0 envelope or abrasive brushing may cause premature failure of either V0 or the diaphragm. To help mitigate this possibility, add extra mass (thicker material) to the center of the diaphragm such that it rises very much like shown in Figure 1. This way the fabric should evenly lay out against V0 as V2 is increased. - Fortunately a failure of the diaphragm during a deep dive to the surface would not end the mission. V1 would mix with V2, but overall density would be the same. As long as we are injecting outside air into the balloon, the descent would continue normally. Only when trying to vent V2 on ascent would we lose helium and fail to rise. The mission just turns into a standard one-way surface lander mission but with some lessons learned. Cloud Mission Payload - Yes, I did completely skip that all of the instruments for the VALOR balloon project were expecting to operate at 24 C rather than 150 C at 40 km (at least the exposed parts) and would have to be redesigned along with the containment. But on the plus side they won t have to work on a trickle power usage. With a rechargeable battery and, if the balloon is designed for the deep dive, it certainly can handle larger and more capable scientific instruments than allotted with the original balloon.

31 Missing Wow Factor Venus Modified Missions - Conclusions (2 of 2) OK, I get it. The flagship Venus Design Reference Mission (VDRM) mission for an orbiter, two mid-altitude balloons, and a lander has been worked on for over a decade by a lot of dedicated people at NASA, JPL, VEXAG, and elsewhere. There is a lot of skin in the game, it would accomplish great science, and has reached a high level of development. It is basically ready to go if it ever gets the funding to happen. But with more focus going to Mars, NASA s stripped down DAVINCI atmospheric probe is the only non-orbiter mission currently getting the green light for Venus that I know of. From a layman s and dare I say congressional perspective here is the problem. The lander and balloon missions are somewhat one dimensional. You send a lander to the surface, get a few photos from one spot to send home, and then it dies. My apologies to the scientists who have been eagerly awaiting the data from the many other instruments, but from the rest of the world s perspective they get to see the few photos of the surface of Venus and that is it. People are used to the Mars rovers lasting years which makes this a harder sell. For the VALOR balloons it is almost worse. Since they are supposed to cruise at 55.5 km in the middle of a cloud bank, they are not even equipped with a camera from what I can tell. It is just raw data collected over a month. Again, great for the scientists in a single-altitude-sampling-ignoringeverything-else-above-and-below kind of way, but not much for anyone else. Now add the enhanced balloons with variable altitude control. - For the deep dives the surface of Venus can be explored in earnest. Ever-changing landscapes will reveal themselves on a weekly basis via images and radar...maybe even come across a volcano or two. - Achieving decades of research from the surface all the way up to the staging altitude while characterizing numerous spots across Venus could all be available in a single mission. - For either balloon at the staging altitude there should be great photo opportunities. What does a Venus sunrise or sunset look like over the cloud tops? How about a photo of an acid rain squall hitting against the balloon? Just what do sulfuric acid clouds look like and how do they compare with Earth s water vapor clouds? What does it look like when the atmosphere crashes through one of those gravity waves? It promises to be a wild ride. - Learn how to navigate the balloons across the globe using varying wind directions at different altitudes. - Master the art of real aerobot exploration of another world. Maybe a little over the top, but just trying to make a point.

32 Final Word

33 Final Word (1 of 2) This started out with wondering about launching a balloon from SpaceX s Red Dragon on Mars which led to GoogleX s Project Loon which led to the history of proposals for exploratory balloons on Venus and Mars. After working up the basic Red Loon modeling I decided to apply it to the other missions and see where they went. In this presentation I have tried to give enough information so that anyone not familiar with balloon exploration could follow along. My interest is in seeing what these missions could look like if we throw in GoogleX s variable altitude control method. Maybe NASA and JPL are already talking with GoogleX and are looking into these types of mission modifications, but I have not found any mention on the internet for applying this method to planetary exploration. None of the NASA-related websites acknowledge the capability.

34 Final Word (2 of 2) It is my hope that some very smart people at NASA, JPL, and GoogleX can come together, sign whatever proprietary agreements that are necessary, and explore these mission options in earnest. It should at least be part of the narrative. The returns in terms of capabilities and data promise to be orders of magnitude better than the original missions. This approach isn t just theoretical. GoogleX has flown hundreds of these balloons which have actively navigated around the world with over 17 million km of flight data to date per their website. Yes, this presentation is a bit wordy, but I was trying to anticipate some of the many topics and issues that would come up in any type of discussion. I welcome feedback so please feel free to contact me at the link below. John Vistica jvistica1@gmail.com Seattle, WA

35 Appendix A - Mars Weather

36 Appendix A - Mars Elevation Map Figure 9: Mars MGS MOLA elevation map. Image Credit: NASA In selecting a design altitude and payload limit, it will also be important to select the regions the mission will be exploring. The image above shows a Mars elevation map that gives an indication of where a Red Loon could travel. A design altitude limit of 4 km should allow access to at least 80% of the planet just don t navigate a path where the prevailing winds will push it into mountains or plains higher than it can go.

37 Appendix A - Mars Weather - Daily Cycles Click images to link to the larger original image Curiosity REMS Weather Data The most current information regarding the daily and seasonal changes on Mars comes from the Curiosity rover which has collected over two Martian years of temperature and pressure readings using its REMS instrument at Gale Crater at an elevation of -4.4 km. Figure 10: Curiosity REMS Pressure Data Image Credit: NASA/JPL-Caltech REMS data sets by sol can be obtained here. Select only the REMS data. The files will have timestamps and air temperature (column 16) and pressure (column 38). Exclude Sol 1 and Sol 598 for unreliable data. For my purposes I condensed down the 17.5 million samplings to a more manageable representative 40 thousand records averaged on ten minute intervals. That is good enough to capture extremes and quantify rates of change during the day. From the air temperature and pressure, the air density can be calculated for the Red Loon example. This is only one spot on Mars, but it gives an indication to density cycles and what a Red Loon will have to contend with. There is an amazing daily swing in air density near the surface of Mars that will have to be dealt with in order to station keep near the surface. The Red Loon should be able to maintain a relatively fixed distance above ground as it traverses across the landscape. Pressure drops and temperature rises in the afternoon will require venting V2. Pressure rises and temperature drops in the evening will require increasing V2 to maintain altitude. The rate of change and size of V0 dictates the required capacity to vent or pump. Figure 11: Curiosity REMS Temperature Data Image Credit: NASA/JPL-Caltech

38 Appendix A - Mars Weather - Seasonal Cycles Click image to link to original web image Mars seasons also affect the pressure and temperature ranges. Figure 12 shows the seasonal variations in pressure observed by the Curiosity REMS instruments. Curiosity landed on Mars Sol 319 at the peak of the summer season. A Red Loon mission seasonal arrival will also have an impact on the capabilities of the mission during its expected operational life. Figure 12: Curiosity REMS Pressure Data Image Credit: NASA/JPL-Caltech Arriving in summer will limit the maximum altitude that can be obtained compared to the other months.

39 V2 as Percent of V0 Planetary Balloon Missions Revisited 60 Appendix A - Mars Weather - Daily V2 Cycle Examples Red Loon V2 Daily Cycle Volume Percent By Season Based on Curiosity REMS data from Gale Crater - An example sol for each season. - Shows V2 as a percentage of V0 to maintain neutral buoyancy at surface altitude. Spring (Sol 510) Summer (Sol 16) Autumn (Sol 169) Winter (Sol 892) Hour Curiosity Sol Figure 13: Red Loon V2 Daily Cycle Examples Curiosity Gale Crater: For operations near the surface a balloon will have to deal with the constant changing buoyancy issues with ever changing temperature and pressure. This chart gives examples of the types of V2 volume changes necessary to maintain neutral buoyancy during a sol across different seasons. The rate of V2 change and balloon size will drive the sizing of the pumps and vents. For times of rapid changing conditions the safest place is either anchored or maintain a higher altitude away from the surface.

40 Appendix B Balloon Equations

41 Appendix B Balloon Equations Balloon Calculations Introduction The equations in this appendix are meant to define conditions for neutral buoyancy for the balloon. They are sufficient to give an overview of balloon performance in an alien environment to coax out limitations and possibilities. They are not meant to deal with all of the dynamic stuff such as solar heating, drag coefficients, and altitude transitions that cause unbalanced buoyancy effects. Select the balloon s physical criteria to calculate general values like shape, volume, surface area, initial pressurization, etc. With the surface areas of the outer volume and the diaphragm, calculate the total mass of the balloon material. Create a table of desired altitudes with associated ambient temperature and pressure from atmospheric profile equations or data. Choose a design altitude and the corresponding temperature and pressure. This determines the design Lift mass which is the total mass of the balloon and payload. Design Altitude Criteria Examples Location Altitude Temp (C) Press (Pa) ΔP1 (Pa) Source Mars 6 km Atm. Profile Equations Venus 60 km Wikipedia - Venus For each altitude calculate balloon conditions (P1, P2, V1, V2, Δ P1, V2 as a percent of V0, etc.) for the given ambient temperature and pressure to establish neutral buoyancy.

42 Appendix B Balloon Equations A. Superpressure Balloon Characteristics Assume Spherical Balloon for Venus: Diameter = (Volume * 6 / PI) (1 / 3) Surface Area of V0 = PI * Diameter 2 Surface Area of Diaphragm = (Surface Area of V0) / 2 Assume 15 x 12 Spheroid Balloon for Mars: Volume of V0 = PI / 6 * (Diameter) 2 * Height Diameter of V0 = (V0 / PI * 15 / 2) (1 / 3) Height of V0 = Diameter * (12 / 15) Surface Area of V0 = PI * (Diameter) 2 / 2 * (1.739) Surface Area of Diaphragm = (Surface Area of V0) / 2 Assume P1 = P2: To be technically correct P2 = P1 + (mass of diaphragm) * g c / (cross sectional area of V0). This is normally negligible compared to the absolute pressure of P1 and P2. The diaphragm acts like a weighted piston compressing V2. For Venus with a 0.2 kg/m2 diaphragm this equates to 44 Pa with an ambient pressure of 23,882 Pa at 60 km so not much effect.

43 B. Balloon Stress Discussion Stress on the balloon membrane dictates when the balloon would burst (Figure 14). Stress = P * r / (2 * t) Appendix B Balloon Equations P differential or gauge pressure of the balloon r radius of the balloon t thickness of the balloon membrane Figure 14: Stress on a balloon membrane Stress in spherical balloons: The VALOR balloon s burst pressure was about 23 psig with a diameter of 7.1 m. Since this is a simple spherical balloon, if the radius is doubled with the same material, then the burst pressure should be about half of the original. Knowing the burst pressure for one set of radius and thickness allows us to predict the burst pressure for other conditions. Stress in spheroid balloons: The GoogleX s Loon balloon has the hallmark pumpkin shape for a superpressure balloon. Each bowed section or gore of the balloon reduces the local radius of the material thus reducing the stress. Here is a good NASA website explaining superpressure balloon design to reduce and manage stress. The Loon was mentioned to have a typical burst pressure of 1000 Pa in one of their videos. Upsizing requires full stress recalculations to determine the proper number and curvature of the gores along with the other design requirements to mitigate stress. I just set my Mars design altitude overpressurization ΔP1 value to 100 Pa to keep it simple since the goal here is to see what happens to ΔP1 over the expected range of ambient conditions.