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For this post I invited back fellow thermodynamics enthusiast Mike Pauken, principal engineer at NASA’s Jet Propulsion Laboratory and author of Thermodynamics for Dummies, to complete this 3-part series related to his work on developing balloons for Venus. His first post covered the developmental history of balloons while his second dove into the fundamental reasons why balloons float to begin with. For his third and final post, Mike provides a general discussion of how his group at NASA designs balloons to fly on other planets and especially Venus. Please extend a warm return welcome to Mike! – Bob Hanlon
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Getting to fly a balloon on another planet is not an easy venture. The first step involves telling a compelling story on why it’s important to fly a balloon at places such as Venus, Mars or even Titan. It’s not the designer or builder of balloons that gets to tell this story. Rather, the argument for scientific exploration, using a balloon, may come from a collective agreement within a community of planetary scientists interested in studying how planets are similar or differ from each other. Or it may come from political will, to demonstrate as a matter of national pride, that a nation can pursue and accomplish something very difficult to display superiority among nations.
A compelling story of exploration, whether scientifically or politically motivated, weighs the risks and benefits, looks at all options and makes recommendations on the best path forward to accomplish the objectives of a new mission. This story must reach the right ears and eyes. It has to reach those that are capable of funding and supporting it and carrying it out. Once that is done, then the design, build, testing of a planetary balloon mission can begin.
A good story though, needs artwork to illustrate new concepts and help people see the vision. One of my friends loves to create artwork of space exploration. His concept of a balloon flight at Venus is shown as the front image of this post. I hope it caught your attention.
Earth is not the only planet in our solar system where balloons can fly. In fact, two balloons flew at Venus in 1986. The Soviet Vega missions carried a balloon as part of their payload. You can click on the Vega missions link to read more. I think the fact that the Soviets flew balloons on Venus in the cold war era was a matter of political will, to demonstrate superiority and they did a great job in discovering much of what we know about Venus today.
The primary motivations today for exploring other planets is to improve our understanding of how the universe was formed and whether life exists on other places than Earth. There are amazing telescopes today that allow us to detect planets at far away stars. These are called exoplanets. But how can we tell what they are like? Well, for starters, lets get a better understanding of the exoplanets right in our own backyard: the solar system! It’s likely many star systems are similar to our solar system. But there are also some that are very much different. What causes these differences? How did stars and their solar systems form? Can we detect signatures of life at other stars or planets? There are many questions to ask and in the course of scientific discovery, it seems that for every question we find an answer for, we get at least two or more new questions popping up.
There are many ways to explore the solar system: telescopes on the ground or in orbit around earth were our first eyes outward. These were followed by spacecraft probes that orbit around other planets/moons/asteroids, and space vehicles that land on the surface and deploy science instruments in situ to make measurements. Some of these are stationary landers, others are mobile vehicles. Some vehicle are capable of exploration in the “air” of other planets. There are many ways we have explored the universe around us. But for now, our focus is on one particular mode of exploration: the Planetary Balloon.
When developing a balloon for other planets the first question that needs to be addressed is “Why a balloon?” What are the advantages and risks of flying a balloon compared to other means of collecting data? The competition for balloons includes everything we have in our Earthly airspace such as airplanes, helicopters, gliders, and blimps. Just as we have many choices of aerial vehicles on Earth, each fills a specific need, each has their own advantages and disadvantages.
Without going into specific details on each type of aerial vehicle, I will describe the major strengths and weaknesses of balloons for exploring planetary atmospheres. Balloons do not need to expend power to stay aloft like airplanes and helicopters. Since power can be limited on planetary missions, it is an advantage to not need much power for staying up in the air. Another strength: balloons can act as tracers to the atmospheric winds providing in situ data on wind conditions around a planet. But this is also a weakness for balloons, for they are at the mercy of the wind and cannot fly directly to any specific location. Blimps can stay aloft without much power and can move about relative to the wind and may be able to reach specific target locations. However, blimps are significantly more complex to deploy in the atmosphere of another planet compared to balloons.
Even a balloon has competition with other balloons. The simplest balloon has a fixed volume and will have a fixed gas density inside. It would be helpful to overfill the balloon and have some reserve gas inside to extend the lifetime of the balloon. All balloons will develop leaks over time and slowly descend. If the balloon gas has a fixed density, it will generally float at the same altitude all the time except if vertical winds push it up or down. The Soviet Vega Balloons experienced vertical winds quite frequently in their 2-day missions indicating the Venus has a turbulent atmosphere. More complex balloons are able to change the density of the gas inside by either compressing the lift gas into a storage vessel or by compressing atmospheric gas into a bladder within the balloon. These allow the balloon to change altitude and since wind may change direction at different altitudes, it could be possible to perform a bit of balloon “steering” by taking advantage of differing winds.
Let’s say we’ve managed to put together a compelling story about exploring the atmosphere of another planet and we’ve convinced ourselves that a balloon mission is the best means for accomplishing our objectives. There are still a lot of decisions that need to be made to convince a panel of reviewers to recommend our project for funding a space exploration mission. The biggest thing in our story is, what kind of science are we going to do? What instruments to we need to have to accomplish our science objectives? We start our balloon mission concept by developing a list of instruments. We figure out how much power each one consumes, how much they weigh, how much space they occupy. In addition to the science instruments, the balloon payload will need many support systems: mechanical structure, thermal protection, electrical power, onboard computer processing, data storage, and telecommunications.
We sum up the estimates for cost, mass, volume and power to determine if it fits within the scope of the project. If not, then we must make decisions on what and where to make cuts. Once we have these figured out, we can determine what size the balloon needs to be to support the payload at the altitude range we desire to conduct our mission. We then figure out how all of the systems are going to be packaged inside a vehicle that will deliver the balloon and payload to the planet…working our way backwards, we figure out how big of a rocket is needed to get us on our way. It takes a lot of iteration back and forth to figure all these things out.
Even within the balloon design itself there are many decisions. What materials should be used? How will the balloon be assembled and tested? What lift gas should we use? Helium is generally safer to work with, hydrogen provides a bit more lift. Are there other factors that need to be considered? All of these questions and more need to be answered in designing a balloon system. Let’s move on from here and go over a bit of detail in developing planetary balloons.
Fundamentals of Balloon Design
What are the differences between flying a balloon on another planet compared to Earth? We will start with the thermodynamic equations describing balloons and jump in from there. In my previous post I showed the lift force of a balloon can be calculated from gas density, gravity, and volume using this equation:
FL = V·g·(ρa – ρb)
Where FL is the lift force, V is the balloon gas volume, g is the gravitational acceleration, ρa is the density of the atmospheric gas, and ρb is the density of the balloon gas.
The other force acting on a balloon is the mass gravitational force, FB, which is the weight of the balloon plus its payload. We calculate the balloon mass gravitational force, FB, using the following equation:
FB = g·(mb + mp)
where g, is the gravitational acceleration, mb is the mass of the balloon and mp is the mass of the payload the balloon carries.
Physics Holds True Across the Solar System
These equations apply to any planet or moon with an atmosphere. The values you’ll use for the gravitational acceleration and atmospheric density will depend on the properties of the planet or moon. In Table 1, I’ve summarized a comparison of the gravitational acceleration, g, atmospheric gas composition and molar mass between 4 places in our solar system where one might want to fly a balloon:
|Destination||Gravitational Constant, m/s2||Atmosphere Composition|
|Venus||8.87||96% CO2, 4% N2||43.4|
|Earth||9.81||78% N2, 21% O2||28.97|
|Mars||3.71||95% CO2, 3% N2||42.6|
|Titan||1.35||97% N2, 3% CH4||27.6|
These things affect the size of the payload that can be carried by a specific size balloon on each planet/moon. One of the fun things about fantasizing about designing balloons for other planets and moons is getting to use different values of the physical parameters for balloon flight than what we customarily use here on the home planet. Let’s take a walk through our solar system and figure out how balloons or payloads will differ depending on the place we wish to fly. The first thing we’ll want to understand on this journey is the pressure/temperature and density profiles of different atmospheres as a function of altitude. It is convenient that density is defined by absolute pressure, temperature and molecular mass, so we can use density to compare the atmospheric profiles. We can then worry about temperature and pressure profiles later.
If we are to fly balloons at other places in the solar system, it may be useful to fly in conditions that are similar to our experiences on Earth. I put together a graph of the atmospheric density profiles of Mars, Earth, Titan and Venus in Figure 1 to illustrate how the density of the atmospheres of the different planets/moons compare with each other. Titan is a moon of Saturn with a wonderful atmosphere for flight. I placed black marks on the density profile for Earth that are equivalent to the density of the Mars atmosphere. Flying a balloon on Mars from 0 to 5 km altitude is like flying a balloon on Earth between 30 and 32 km altitude as I marked on the y-axis in the figure. This is the region of stratospheric Earth balloon flights where balloons the size of stadiums carry science payloads that weigh several thousand pounds.
In contrast, flying a balloon on Earth from 0 to 10 km altitude would be equivalent to flying a balloon on Titan at an altitude range from 30 to 48 km as shown by the blue marks on the Titan profile. For Venus, the balloon altitude would be from 52 to 62 km illustrated with red marks on the Venus profile. If we were to design, build and test balloons for other planets/moons, we need to consider where we would do this testing in our atmosphere. Testing a balloon destined for Mars would be performed in our stratosphere, while testing for Titan or Venus would be accomplished in the Earth’s troposphere. You can imagine testing in the stratosphere is more difficult than testing in the troposphere.
Let’s Design Balloons for Other Planets
We will compare balloon flight on different planets/moons by determining how much payload a particular balloon could carry on Mars, Titan and Venus. I suggest we use a 10-meter (33 feet) diameter balloon to fly our payload to make our comparisons. We can set the atmospheric density for our balloon flights on Earth, Titan and Venus at 1 kg/m3. This gas density occurs at approximately 2 km on Earth, 34 km on Titan and 54 km on Venus. The atmosphere of Mars does not reach this density, so we’ll do a separate comparison for Mars using a lower atmospheric density of 0.014 kg/m3. This density occurs around 0.8 km on Mars and 33 km on Earth. We could use this density on Titan and Venus, but the altitudes would be over 80 km on Venus and 100 km on Titan. This is so high in their atmospheres it is not of current scientific interest and most observations at these altitudes can be accomplished by an orbiting satellite anyway. Furthermore, making high altitude balloons for other planets is much more difficult than designing for flights at lower altitudes.
We will calculate the lift force, FL for Mars, Earth, Titan and Venus to compare our payload capacities for each location. The 10-m diameter balloon fixes the volume at 524 cubic meters. The gravitational acceleration is shown in Table 1 above and the density of the atmosphere is fixed at 1 kg/m3 except at Mars where we are using 0.014 kg/m3. All we need now the is density of the lift gas to complete our comparisons. If we choose helium as our lift gas we can compute the density of the helium for each destination very simply by using the molecular mass values. We don’t need to know what the temperature and pressure are of the atmospheres to do this, instead, the density of the balloon lift gas, ρb, can be found using:
ρb = Mb/Ma·ρa
Where Mb is the molecular mass of the lift gas, which for helium is 4, and Ma is the molecular mass of the atmosphere gases which are listed in Table 1 above.
The other data we need is the mass estimate of the balloon. For robust balloons designed for Venus we would use a material for the envelope that has an area density of around 150 grams/m2. If we assume for simplicity we use the same area density for Earth and Titan, the balloon mass would be around 57 kg allowing for features besides the balloon material such as fittings, tapes for seams, load lines, et cetera, that add up to the mass of a balloon. But this mass is much too heavy for Mars. In fact, the mass of a balloon for Mars needs to be 1/10 of that for other destinations in order to work. This is the strategy used for Earth’s high altitude balloons: very thin films for balloon envelopes. We will set the Mars balloon mass at 5.7 kg for this example.
The results of the comparisons are shown in Table 2. Despite the differences in gravity between Earth, Venus and Titan, the amount of payload carried by a 10-m balloon is about the same for each: 405 kg +/- 15 kg, less than 10% variation. If you look at the two equations we used to calculate the payload mass above, in the force balance between the lift force, FL, and the mass gravitational force, FB, the gravity drops out and we find that the variation in payload mass is due only to the differences in the helium density at the planets/moon.
|Destination||Altitude, km||He Density, kg/m3||Lift Force, N||Net Lift Mass, kg|
You will notice that even though we picked an atmospheric density of 1 kg/m3 for Earth, Venus and Titan, the Helium density is not the same for each. The molecular mass of each atmosphere is different because they vary in composition. Earth is mostly nitrogen and oxygen. Venus is mostly carbon dioxide with some nitrogen. In fact, Venus has 4 times more nitrogen (by mass) in its atmosphere than Earth. Titan’s atmosphere is mostly nitrogen with a small amount of methane. These differing atmospheric compositions have a density of 1 kg/m3 at temperatures and pressures different than on Earth as shown in Table 3. We assume here, for simplicity, that the helium inside the balloon is at the same temperature and pressure as the atmosphere. In reality, the gas inside the balloon will have a different temperature and possibly different pressure than the atmosphere due to heating from the sun (in daylight hours) and exchanging energy with the atmosphere, ground and sky surrounding the balloon.
It’s very interesting that the pressure and temperature conditions at Venus are not widely different than those here on Earth. This makes Venus a compelling destination for a balloon flight. While Titan is also a very good place to fly a balloon, since the thermodynamics are suitable, the materials challenges are significant. The temperature of the atmosphere is cryogenic. Mars atmosphere is very much like the Earth’s stratosphere and there have been plenty of balloon flights in our stratosphere. The chief difficulty is making very light weight balloons that can be deployed at Mars reliably. This is not an easy thing to do.
Verifying VEGA Balloon Data from Science Papers
I would like to close by presenting some actual data on the 1985 Vega balloon missions to Venus and show that the calculations we use for designing balloons on Earth hold for other planets as well. There are two significant publications with pertinent information about the balloons flown at Venus. I give you the references at the end. The first publication is “Overview of VEGA Venus Balloon in Situ Meteorological Measurements” by Sagdeev et al, (1986) and the second is “VEGA Balloon System and Instrumentation” by Kremnev et al, (1986). Sagdeev publishes several plots of the temperature and pressure of the Venus atmosphere and I’ve snipped a small section in Figure 2 to collect specific flight parameter data.
In Figure 2, the pressure starts out high (almost 900 mbar) and the altitude is low, nearly 50 km. This is the very beginning of the balloon mission. The balloon starts the mission out at a lower altitude because it has heavy tanks of compressed helium attached for inflating the balloon. When the helium is fully injected into the balloon, the tanks are dropped and the balloon rises up to its float altitude of roughly 53.6 km shown here. Data is transmitted intermittently, so there are gaps. I put a red circle about 7 hours into the mission to grab a place where we can use the data to calculate the balloon lift and compare it to the balloon mass. I estimate, from these plots, the pressure is 550 mbar (55 kPa) and the temperature is 311 K (38°C). From the ideal gas law, the atmospheric density is 0.92 kg/m3 under these conditions. The balloon is on the night side of the planet so we can assume the balloon is not heated by sunlight.
The 1986 paper by Kremnev gives us information about the balloon size and mass: diameter 3.54 meters, mass of balloon system: 12 kg, mass of payload: 6.9 kg. There are some disparities in the actual balloon size and mass. Some sources report the balloon diameter is 3.4 m and the balloon system mass of 12.5 kg. Either way, we’re just looking to get into the ballpark here with the data we have on hand. The pressure of the helium inside the balloon was not equal to the atmospheric pressure. It was slightly pressurized to provide a constant density balloon and provide some reserve helium to account for leakage over time. The balloon pressure was about 3 kPa over the atmospheric pressure. Using a pressure of 58 kPa and temperature of 311K, the helium density inside the balloon is about 0.09 kg/m3. This density is confirmed also by the report that 2.1 kg of helium was used to inflate the balloon which nominally has a volume of 23.2 m3.
Using our balloon lift equation, we can calculate how much lift force the VEGA balloon had at Venus:
FL = V·g·(ρa – ρb)
FL = 23.2m3 · 8.87m/s2 · (0.92 – 0.09)kg/m3 = 171N
And we can compare this to our mass gravitational force of the VEGA balloon and payload at Venus:
FB = g·(mb + mp)
FB = 8.87m/s2 · (12.5 + 6.9) kg = 172N
There you have it! We’ve been able to pick out data from scientific papers and run the numbers through the well known balloon equations and verify they even work on other worlds, not just Earth!
Sagdeev, R. Z., et al. “Overview of VEGA Venus Balloon in Situ Meteorological Measurements.” Science, vol. 231, no. 4744, 1986, pp. 1411–1414. JSTOR, http://www.jstor.org/stable/1696344. Accessed 29 Dec. 2020.
Kremnev, R. S., et al. “VEGA Balloon System and Instrumentation.” Science, vol. 231, no. 4744, 1986, pp. 1408–1411. JSTOR, http://www.jstor.org/stable/1696343. Accessed 29 Dec. 2020.