As an educator it’s quite exciting that an important unsolved problem may be one university or high school students, or even amateur scientists can address.
A possibly limiting technology for a manned Mars mission is the high energy requirements to produce the return propellant on Mars. Robert Zubrin estimated for a Starship-sized vehicle as it requiring possibly 10 football fields worth of solar panels, or possibly even a nuclear reactor placed on Mars:
Marcus House in his video series also discusses the problem:
SpaceX Starship can return from Mars without surface refilling.
Slide from the video showing proposed size of Mars solar panels for ISRU propellant production:
Beginning at about the 12 minute point, House runs the numbers and considers it so daunting, at least for initial missions, that he suggests it might be better instead to transport the propellant from Earth with Starship tanker flights all the way to Mars. This of course discounts the entire advantage of using Mars generated propellant to achieve a smaller mission size.
The problem is the method envisioned would be by either splitting CO2 into carbon and oxygen and/or H2O into hydrogen and oxygen by electrolysis to be used for the propellant. However, these are energy intensive operations, which explains why in the reverse direction you get so much energy out when you combine them for combustion.
What the researchers for this new idea are proposing instead is getting the oxygen for Mars atmosphere by filtering out the free O2 already there:
Feb 19, 2022 Breathing Mars Air: Stationary and Portable O2 Generation. Ivan Ermanoski Arizona State University The basis for TSSD is a two-step thermally-driven cycle operating below ~260 C. thermal swing sorption/desorption (TSSD)--to generate oxygen from the Mars atmosphere with 10x less energy than the state of the art. Our approach is motivated by thermodynamics: the minimum theoretical work to separate oxygen from the Mars atmosphere is ~30-50 times lower than to obtain it by splitting carbon dioxide. Efficiency: TSSD is expected to be ~10x more efficient than MOXIE. For MOXIE, the target power requirement for oxygen propellant production is 30 kW. The TSSD estimate is only 4 kW; i.e., 90% less than MOXIE. Applying TSSD in rovers, the estimated power for oxygen pro-duction is only ~50 W/person. https://www.nasa.gov/directorates/spacetech/niac/2022/Breathing_Mars_Air/
Here’s the breakdown of the Martian atmosphere:
Atmosphere of Mars. General information[2] Average surface pressure 610 Pa (0.088 psi; 4.6 mmHg; 0.0060 atm) Mass 2.5x1016 kg[1] Composition[3][4] Carbon dioxide 95% Nitrogen 2.8% Argon 2% Oxygen 0.174% Carbon monoxide 0.0747% Water vapor 0.03% (variable) https://en.wikipedia.org/wiki/Atmosphere_of_Mars
The authors so far are proposing just getting O2 from the Martian air. But note there is also free carbon monoxide(CO) in its atmosphere. Then it may also be possible to filter out CO from Mars air. This is quite important because CO can be made to combust with O2:
1.)2CO+O2→2CO2;ΔH=−569kJ/mol
However, when CO can be obtained from low energy filtration from the Martian atmosphere then free hydrogen for propulsion can be obtained by the reaction:
2.)CO + H2O → CO2+ H2, ΔH = -41 kJ/mol
This uses water that both orbiters and landers have shown to be wide spread on Mars even in low latitude locations in ice form.
You can then get methane, if that is the preferred fuel over hydrogen, by reacting the free hydrogen with CO2 by the famous Sabatier reaction:
3.)∆H = −165.0 kJ/mol
The nice thing about this proposal is that any decent high school or university lab or even well-appointed amateur lab can test it out themselves. Key question: what is the method requiring least energy to concentrate and filter out the O2 and CO when in the concentrations known in the Martian atmosphere?
About how to filter out the O2 and CO, three possibilities come to mind:
Of game-changing importance is that rather than football fields of solar cells or even a nuclear power plant being needed to generate the propellant, it may be the process of producing the propellant actually generates power.
But if this is so easy, why has this not been done before? Mars atmosphere is almost all CO2, so the obvious thing to investigate is getting O2 by splitting CO2 into carbon and O2 by electrolysis.
On the other hand, O2 is only a little more than 1/1000th of Mars atmosphere, which itself is only 1/100ths Earth’s air density. So the oxygen there would be in the range of only 1/100,000th the density we normally see on Earth. So this would require an extreme level of densification to be usable.
However, in this regard, it may be sufficient to just cool the oxygen to liquid form which is needed for the propellant application anyway. This might not require too high energy input since at night the temperatures drop to well below freezing on Mars.
The most optimal generation and separation/filtration methods have to take into account the conditions on Mars. For instance in eq. (2.) the energy released of -41 kJ/mol is when the water is in gaseous form. It's much less if it is liquid or frozen water form. But in this regard it is notable temperatures on Mars surface do reach above the melting point of water on large swaths of Mars during the the mid afternoon hours on Mars.
It had been frequently said that liquid water can not exist on Mars because Mars landers had shown the air temperatures were always below freezing. But what was missed was the actual surface temperatures were often well above the air temperatures. This is a rather obvious point that's easily understood when you compare for example on Earth the temperature of the air on a hot Summer day with what the temperature on the ground might be, especially if the ground is a dark surface absorbing sunlight rather than reflecting it. See the graphs from the Spirit and Opportunity MER rovers:
But because of the low atmospheric pressure the time at which the water will remain liquid is relatively short, even if the ground temperature will be above the melting point for hours. This is good for our propellant generation purposes though since we will get the water in gaseous form without additionally adding energy.
Hydrogen fuel from Mars to Earth?
The propellant generation from taking atmospheric oxygen and carbon monoxide might actually generate energy. But this means we could actually produce hydrogen and methane at low cost, unlike the electrolysis methods that require high energy input. This shows the importance of this in general not just for Mars. It could be a means of producing clean energy in the form of hydrogen being transported from Mars to Earth. Again, it is remarkable that this is something high school and university teams can contribute to establishing its feasibility.
But for it to be feasible, the energy requirements for producing the O2 and CO will have to be improved. The NASA release on the new TSSD method touts its reduced energy requirements over the usual electrolysis method, MOXIE, giving its power requirements as 4 kW. But the news release doesn't say how much oxygen is produced by the process for that power. This article allows us to estimate it:
MARCH 3, 2022 BY ANDY TOMASWICK
Mars Explorers are Going to Need air, and Lots of it. Here’s a Technology That Might Help Them Breath Easy.
...
All that power consumption makes for an expensive system. A MOXIE machine that can create 2 kg O2 per hour (enough to support two explorers) would require around 25 kW of power, or slightly less than the average American house uses per day. While that may not seem like a lot, utilizing solar energy is a much more difficult prospect on Mars. Any early solar farm built to run the MOXIE system would dwarf the habitat it could supply with oxygen for just two astronauts.
Enter the TSSD, which nicely eliminates all three major problems with MOXIE. The system itself relies on a thermochemical pumping system, which relies on heat differentials to move the atmosphere to the appropriate place, eliminating the need for a mechanical pump. It also doesn’t suffer from carbon fouling as it doesn’t break apart CO2. Lastly, it doesn’t require too much energy, with Dr. Ivan Ermanoski, the PI on the NIAC funded project and a research professor at Arizona State University, expecting that it will be 90% more efficient than MOXIE.
TSSD needing 1/10th the power of MOXIE would suggest 2 kg per hour of O2 for 2,500 watts, or 1 kg per hour for 1,250 watts. The process operating at an hour would require 1,250w*3,600s = 4,500,000 joules, 4.5 MJ, of energy per kg of O2 generated.
Then we also need to separate out the CO. The CO content in the Mars atmosphere is about half that of the O2 so as a first order estimate we can estimate it as taking 9,000,000 joules per kg generated, though to get the actual value would take closer examination of the whole process used to generate the O2 and how it would have to be adapted to generate the CO.
To get an idea of how much total energy this is in practical terms for both O2 and CO production, equation 1.) gives the energy generated by combusting CO in O2 as 569,000 joules per mol of O2. Since O2 is 32 grams per mol, this is 569,000J/32gm = 17,800 joules/gm or 17.8 MJ per kg of O2. But the CO needed for the reaction is 2 moles for each mole of O2, and since they are close to the same molecular weight you need about 2 kg of CO for each kg of oxygen.
So you need 4.5 MJ of energy to generate a kg of O2 and then 2*9 MJ = 18 MJ to generate the needed 2 kg of CO. This is a total of 22.5 MJ required to put in to get out the 17.8 MJ you would get by combusting the CO with the O2.
For only producing the H2 by equation 2.), you really need only the CO but the molecular weight of the CO is 15 times that of the H2 so you would need 15 times as much CO and its high energy cost by the TSSD process for each kg of H2 produced.
Then the major impediment for making this feasible is finding low energy methods of separating out O2 and CO at approx. 1 part per thousand concentrations from a CO2 atmosphere only 1/100th as dense as Earths.
TSSD is about 1/10th as power intensive as the usual electrolysis method. To put this in perspective, instead of 10 footballs fields of solar cells to run the propellant generating plant for the Starship on Mars, it would only take 1 football field worth of solar cells. The energy requirements probably need to be reduced again by at least another factor of 10.
Some recent advances in gas separation by absorption of surfaces might improve on the energy requirements of TSSD:
Weird Crystal Can Absorb All The Oxygen In A Room — And Then Release It Later
This could potentially make fuel cells, space travel, and scuba diving a lot more efficient.
BY SARAH FECHT | PUBLISHED OCT 2, 2014 1:15 AM EDT
...
Researchers from the University of Southern Denmark say they’ve invented a crystal that pulls oxygen out of the air and even water. Apparently, just a spoonful of the stuff can suck up all the oxygen in a room.
The crystal is a salt made from cobalt*, and it appears to be capable of holding oxygen at a concentration that is 160 times higher than the air we breathe. The paper notes that “an excess” of the substance would bind up to 99 percent of the oxygen in a room.
But what’s more remarkable is that the crystal can later release the oxygen when exposed to heat or low-oxygen conditions. In a press release, study author Christine McKenzie likens it to the hemoglobin in our blood, which uses iron to bind and release oxygen in the human body.
Unlike the windows of your house, nanoscale holes in graphene (named as “nanowindows”) can selectively choose which type of air molecules can pass through.
Scientists from Shinshu University and PSL University, France, theoretically proved concerted motion of the nanowindow-rim to selectively allow molecules to pass, in an energy-efficiently and fast way (Nature Communications, “Air separation with graphene mediated by nanowindow-rim concerted motion”). This brings up new possibilities to create an advanced molecular separation membrane technology.
The mechanism of separation by nanowindows is that the atomic vibration of the nanowindow-rim changes the effective nanowidow size. When the rim of one side is deviated and the other is deviated to the opposite direction, the effective nanowindow size becomes larger than when the rim does not move. This effect is very predominant for molecules of oxygen, nitrogen, and argon, inducing an efficient separation of oxygen from air.
Alternatively, this is low enough that a space elevator can do it using currently available materials. Still another possibility is by using a railgun, i.e., electromagnetic accelerator.
Mars Sample Return is again being discussed by NASA, as it was 10 years ago. And as was the case then the chief stumbling block is the $10 billion price tag. However, if done as a fully commercial space mission, i.e., no governmental funding required, it could be done for a fraction of the amount NASA is estimating, probably for a few hundred million dollars, including the launch cost on the Falcon Heavy.
SpaceX has shown that development costs for rockets can be done at 1/10th the cost of usual government financed rockets by following the commercial space approach. The same was proven for spacecraft in the form of capsules when SpaceX developed the Dragon at 1/10th the usual cost.
This plus using already existing in-space stages rather than developing entire new ones can greatly reduce the development cost of such a mission.
Here, I will propose a solution using a fully aerocapture approach to landing, meaning braking fully aerodynamically, at Mars to minimize the propulsive burns and therefore propellant that is needed on arrival at Mars. Below we'll discuss some possibilities for this hypersonic slowing. First, the delta-v requirements for such a mission.
Delta-V to and From Mars.
Here is a map of delta-v's for some locations in Earth-Moon-Mars space:
Now notice for the delta-v's after this leading into Mars they all have red arrows indicating this part of the trip can be done by aerocapture/aerobraking. So this portion of the flight leaving Earth orbit headed towards Mars, and landing on the surface is only 3.8 km/s, assuming all the slowing on reaching Mars is done aerodynamically.
After that, for the return trip:
Mars(surface) to low Mars orbit: 4.1 km/s
Low Mars orbit to Phobos transfer: .9 km/s
Phobos transfer to Deimos transfer: .3 km/s
Deimos transfer to Mars C3: .2 km/s
Mars C3 to Mars transfer: .9 km/s
Now the delta-v's after this leading from the graph into Earth all have red arrows indicating this part of the trip can be done by aerobraking. So the return part of the trip can amount to only 6.4 km/s, for a total of 10.2 km/s for the round trip, if the final part of the trip of returning to the Earth's surface is done fully by aerodynamic braking, i.e., not using propulsive burns.
As for the heat shield for these Mars return velocities notice that the SpaceX Dragon's PICA-X heat shield was designed to withstand such velocities. It reportedly weighs only half of Apollo era heat shields which would put it at about 8% of the landed mass.
However, for the sample being returned to Earth from Mars there is concern that there may be unknown microorganisms. So current plans include the sample being returned only to Earth orbit or to lunar orbit. Thereafter, the sample would be studied in some orbiting facility only or be placed in a special canister with several redundant layers of security for return to Earth designed not to be breached even if it crashes on return to Earth's surface.
In such case, we have two additional steps in the delta-v chart:
Mars transfer to Earth C3: .6 km/s
Earth C3 to GTO: .7 km/s
For a total of 6.4 km/s +.6 km/s + .7 km/s = 7.7 km/s.
This would be for when the sample is returned to geosynchronous transfer orbit(GTO). This is an intermediate orbit for getting to actual geosynchronous orbit. It is a highly elliptical orbit with closest point in low Earth orbit and farthest point at geosynchronous altitude of 35,700 km.
The other possibility would be to send instead to lunar orbit. Then the additional delta-v steps would be:
Mars transfer to Earth C3: .6 km/s
Earth C3 to lunar orbit: .7 km/s
The total delta-v for the return this time to lunar orbit would also be 7.7 km/s.
Now for the rocket stages for getting to Mars and returning a sample back. First, we'll use the Falcon Heavy for lofting the in-space stages first into space. Falcon Heavy has a payload capacity of 63.8 tons to LEO, but only 16.8 tons to Mars transfer orbit(MTO). This is a trajectory that sends a spacecraft to encounter Mars in its orbit about the Sun, but makes no attempt to actually enter orbit around Mars. This is the scenario we are considering where, once reaching Mars, the entire braking and landing on the surface is done aerodynamically.
So we have 16.8 tons to work with for in-space stages with capacity to lift off from Mars, fire a burn to direct the return craft back to Earth, and finally make the burn to put the craft in GTO orbit or lunar orbit.
We'll select existing stages using storable propellant for the in-space stages for this mission that may take up to 3 years round trip duration.
After that, we'll use two copies of the Integrated Apogee Boost Stage(IABS), at about 1.3 tons storable propellant load and about .275 ton dry mass.
This stage had an vacuum Isp of 312 s. However, for an in-space only stage vacuum Isp is primarily a function of expansion ratio so we'll assume we can also give it a vacuum isp of 340 s with sufficiently large nozzle of ca. 300 to 1 area expansion ratio. Astronautix.com lists its price as $15 million.
Then with these three stages we can get about .75 tons, 750 kg, payload to reach the 7.7 km/s delta-v needed for the round trip to Mars and back:
The total mass of all the stages and the payload is 15 tons, within the 16.8 ton limit of the Falcon Heavy to put into Mars Transfer Orbit(MTO).
Full Aerocapture/Aerobraking for Landing at Mars.
The question of using aerocapture at Mars is a major question at NASA now for large payloads in the 15 tons to 25 tons range for landing of human habitats for manned missions to Mars. The earlier methods for landing using to a large extent propulsive landing would require a prohibitive amount of propellant (for the usual propulsion methods. However see below.)
On the other land using just parachutes or spherical section reentry capsules because of the thin atmosphere would also be insufficient for such large payloads. See discussion here:
The Mars Landing Approach: Getting Large Payloads to the Surface of the Red Planet.
JULY 17, 2007 BY NANCY ATKINSON
Some proponents of human missions to Mars say we have the technology today to send people to the Red Planet. But do we? Rob Manning of the Jet Propulsion Laboratory discusses the intricacies of entry, descent and landing and what needs to be done to make humans on Mars a reality.
There’s no comfort in the statistics for missions to Mars. To date over 60% of the missions have failed. The scientists and engineers of these undertakings use phrases like “Six Minutes of Terror,” and “The Great Galactic Ghoul” to illustrate their experiences, evidence of the anxiety that’s evoked by sending a robotic spacecraft to Mars — even among those who have devoted their careers to the task. But mention sending a human mission to land on the Red Planet, with payloads several factors larger than an unmanned spacecraft and the trepidation among that same group grows even larger. Why?
One possibility for how to do it is hypersonic waveriders:
Hypersonic waveriders for planetary atmospheres.
December 1989 Journal of Spacecraft and Rockets -1(4)
DOI: 10.2514/3.26259
Anderson, John D., Jr MARK J. LEWIS, Ajay Kothari, Stephen Corda
International Hypersonic Waverider Symposium, 1st, University of Maryland, College Park, MD, Oct. 17-19, 1990, Proceedings
Article
January 1990
The concept of a hypersonic waverider for application in foreign planetary atmospheres is explored, particularly in regard to aero-assist for space vehicle trajectory modification. The overall concept of hypersonic waveriders is discussed in tutorial fashion. A review of past work is given, and the role of a new family of waveriders - the viscous optimized waveriders generated at the University of Maryland - is highlighted. The mechanics of trajectory modification by aerodynamic vehicles with high lift-to-drag ratios in planetary atmospheres is explored. Actual hypersonic waverider designs for Mars and Venus atmospheres are presented. These are the first waveriders ever presented for foreign planetary atmospheres. Moreover, they exhibit very high lift-to-drag ratios, as high as 15 in the Venus atmosphere. These results graphically demonstrate that a hypersonic waverider is a viable candidate for aero-assist maneuvers in foreign planetary atmospheres.
An advantage of this over usual caret-shaped hypersonic waveriders is that split in two parts and being curved they can they can more than double the underside surface area.
A key advantage of such high hypersonic L/D ratios, is that using lift we can curve the craft around the planet giving it further time to slow down in contrast to traveling in a straight-line and exiting the planets atmosphere with insufficient braking to fall below the planets escape velocity.
Possible Light Weight Propulsive Methods for Landing.
Because of the high delta-v requirements for such a mission it was thought the propellant requirements for a propulsive landing would be prohibitive. However, at least two different methods might make it possible, both by getting all or part of the propellant from the Martian atmosphere.
1.)On Earth, oxygen is the common oxidizer for burning. However some metals in such as magnesium and aluminum burn quite well in a carbon dioxide atmosphere, especially as fine powdered particles:
2.)Both oxygen and carbon monoxide from the Martian atmosphere.
That Mars atmosphere is overwhelmingly carbon dioxide is well known. However, it is notable that it contains small amounts of oxygen and carbon monoxide.
This is quite important because carbon monoxide can be made to combust in oxygen by the reaction:
2CO+O2→2CO2;ΔH=−569kJ/mol
This is not as high energy reaction as hydrogen or methane with oxygen but may be enough to provide sufficient thrust to slow down the craft to enable a soft landing via parachutes.
We have then though a similar problem as with scramjet propulsion on Earth. The craft will be moving so fast there might not be enough time for combustion to take place. The problem is made worse because there is additional time that must be taken to separate out by filtration the carbon monoxide and oxygen from the carbon dioxide.
Still, whether or not this problem can be solved, it is extremely important that this reaction be employed for ISRU once down on Mars. A criticism of the approach of SpaceX of landing the large Starship on Mars is the high energy requirements of producing the methane propellant requiring separating oxygen and hydrogen water(ice) in the soil by electrolysis.
For a vehicle the size of the Starship Robert Zubrin has suggested it might take 10 football fields of solar panels or even take a nuclear power plant. However, when CO can be obtained from low energy filtration from the Martian atmosphere then free hydrogen for propulsion can be obtained by the reaction:
CO + H2O → CO2 + H2, ΔH = -41 kJ mol-1
You can then get methane if that is the preferred fuel over hydrogen by reacting the free hydrogen with CO2 by the famous Sabatier reaction:
∆H = −165.0 kJ/mol
So obtaining free O2 and CO from the Marian atmosphere by low energy filtration makes obtaining propellant for the return flight for manned missions much more feasible.
Financing a Commercial Approach to a Mars Sample Return Mission.
If this is to be a fully commercial mission how is it to be funded?
Recall back in 1997 the great interest over the internet from people world-wide on the Mars Pathfinder mission. The Mars Pathfinder mission actually "broke the internet", with its sites getting up to 60+ million total hits per day, to the extent some mirror sites crashed or had to have access limited:
Traffic on Mars by Chuck Toporek Asst. Managing Editor Web Review However, the most interesting and little known fact about the amount of traffic to the mirror sites comes from France, where the government actually pleaded with computer users to stop accessing the two Mars Pathfinder mirrors. You see, the phone systems in France carry all of the Internet traffic in the country, so when people started visiting the mirror sites at VisuaNet and Le Centre National D'Etudes Spatiales (CNES), they tied up the phone lines and basically disabled the country. http://mars.jpl.nasa.gov/MPF/press/webreview/index4.html
The web traffic to the NASA web site for the Mars Exploration Rovers was even more extraordinary, measuring in the billions of hits:
NASA’s Web Site for 2005 By Digital Trends Staff — January 7, 2005 The U.S. National Aeronautics and Space Administration Web portal continues to drive high traffic numbers — more than 17 billion hits in 2004, report both NASA and Speedera Networks, a leading global provider of on-demand distributed application hosting and content delivery services. Speedera delivers content from the space agency’s portal to visitors seeking access to the site from around the world. Popular events on the NASA Web site, including the ongoing Mars Exploration Rover mission entering its remarkable second year, as well as upcoming major projects such as the launch and comet encounter of NASA’s Deep Impact satellite mission in 2005, are expected to drive continued high levels of traffic, according to NASA officials. http://www.digitaltrends.com/computing/nasas-web-site-for-2005/
It was estimated there were 142 million visits to the site during this period. So the question is how much advertising could be sold for a site this well visited?
It could be financed in the fashion of YouTube videos where the content creator is paid according to the number of views of the video:
How much do YouTubers make? 2023 facts and figures. Edited by: Erin Dunn • May 23, 2023 Curious about how much money YouTubers make per view? YouTubers make an average of $0.018 per ad view, according to Influencer Market Hub. Rates can range from $0.10 to $0.30 per ad view. However, the amount of money YouTube pays depends on a variety of factors, such as:
The most successful YouTube millionaires however make even more money by partnering with advertisers on their channels. Then the financial backers of the mission could sell the rights for products to be associated with financing the mission.
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Feng Ding
