Planetary Balloon Missions Revisited Planetary Balloon Missions Revisited 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. Planetary Balloon Missions Revisited 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. Planetary Balloon Missions Revisited 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 m3 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. Mars Aerobot - Image credit: NASA/JPL 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 Venus VALOR - Image credit: NASA/JPL 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. Venus VME - Image credit: NASA/JPL Planetary Balloon Missions Revisited 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. Variable Altitude Control: Basically GoogleX partnered with Raven Aerostar to develop and successfully demonstrate Image Credit: Google Loon Balloon (15m x 12m) 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 Image Credit: Google Loon Altitude Adjustments months before being remotely brought down for recovery. 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 Image Credit: Google Image Credit: Google Patent is assumed in this study. Patent for Bladder Method for Diaphragm Method Planetary Balloon Missions Revisited 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. Altitude Control System: 1. Pump in external air into V2 volume thus compressing V1. As V2 increases the Figure 1: Superpressure balloon with diaphragm overall weight of the balloon increases and the balloon will drop until the outside • P1 = P2 density balances the equation again. • ΔP1 = P1 – P0 2. Lowest altitude is only restricted by external conditions such as temperature and • V0 = V1 + V2 pressure that the balloon and payload material can handle (e.g. Venus). • T0 = T1 = T2 * 3. Using target altitude’s ambient values calculate V1, V2, and P1. Adjust V2 to the * This ignores heat transfer rates. In reality desired volume. there would be sensors monitoring internal 4. The differential pressure (ΔP1 = P1-P0) cannot exceed the maximum operating internal and external conditions to drive real-time pressure differential limit (ΔPmax). Increase V2 until ΔP1 = ΔPmax. Let the balloon drop responses with known heat transfer info. 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. Planetary Balloon Missions Revisited Mars Balloon Mission Planetary Balloon Missions Revisited Mars Red Loon Mars Aerobot / Balloon: This was a 2001 mission proposal for a 10,500 m3 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. Planetary Balloon Missions Revisited 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.