the Earth Return Vehicle of Zubrin's Mars Direct plan

[JonClarke]

Mars -> Earth configuration <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[gunsandrockets]

Bizarre. A hybrid chemical/electro-thermal rocket engine, and using solar dynamic electric power generation!<br /><br />The trick with electro-thermal is getting the propellant to a high temperature for a high ISP while avoiding melting or eroding your electric heating element. It's why electro-thermal systems so far have mediocre ISP.<br /><br />I think the best electro-thermal system I have heard of was a U.S. Air Force satellite experiment using an arc-jet and ammonia propellant. As I recall it had something like an ISP of 800 seconds, 1/3 lbs thrust, and used 26,000 watts of power. Not really any better than any other electric thruster in terms of T/W.<br /><br />The closest thing to that Mars ERV hybrid rocket I can recall are hydrazine monopropellant thrusters that use resistojet heating to boost the low ISP monopropellant up to an ISP more equivalent of bi-propellant. Interesting for satellite thrusters, but not for the main engine of a manned spacecraft.<br /><br />I've actually contemplated hybrid chemical/electro-thermal rockets in the past. But I thought of a different approach than the Mars ERV designers did. I thought, the best method for a hybrid system is to apply electrothermal heating to propellant in pre-combustion rather than post-combustion. My method still adds the same total energy to the propellant but only subjects the electrical heating element to lower peak temperatures.<br /><br />I thought my hybrid electric system could supplement a standard LOX/LH2 upper stage. The stage could employ bi-modal thrust operation: one mode in pure chemical rocket fashion for high thrust, say during sub-orbital flight when accelerating to orbital speed; plus a low-thrust electric mode when in orbit, pre-heating hydrogen propellant before the hydrogen is injected into a thrust chamber for burning with oxygen. <br /><br />Because of the electric power demands of electro-thermal a hybrid engine would have to be a much smaller auxilary engine compared to a pure

 

[gunsandrockets]

Thanx for the images!

 

[JonClarke]

It seems overly complex to me as well.<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[j05h]

<i>> It seems overly complex to me as well. </i><br /><br />Separating power generation from solar collection via a mirror system allows the light to be used separately for some form of direct-heating STR engine. The power tower can then produce thrust directly or provide more operational power. Electric generation could be via concentrator and PV, Sterling generators or similar. A redeployable power and in-space engine would be a great system. This would mean some wasted mass on Mars (the engine chamber), but still allow power generation. I propose this setup like here, for NEO mining: <br /><br />http://www.projectsanbao.com/miner.html<br /><br />Direct use of solar power has a long future. There once was a proposal to test a solar-dynamic power system on ISS.<br /><br />Josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[gunsandrockets]

Google Ranking<br /><br />It's interesting to note that this discussion thread is the number one hit on google when searching for 'mars erv', or when searching for 'zubrin erv'. We are the number four hit when searching for 'earth return vehicle'.<br />

 

[gunsandrockets]

[perhaps the nasaspaceflight.com 'propforce' is our own space.com propforce?]<br /><br />yep, he is. Come on propforce and post something, I want to hear your ideas!<br /><br />I like propane as a propellant too; I'm fond of it's ability to use common bulkhead tanking with LOX. But your graphs on propellant performance were really eye opening to me. And now Ethylene looks to be the best candidate propellant because of it's high ISP and the fact it can be derived via ISRU. I haven't heard of any ISRU process for propane yet, so that seems to eliminate it as a prospect.<br /><br />I will even go out an a limb here and say LOX/methanol should be the first stage ERV propellants due to the very low ISRU power requirements, while LOX/ethylene should be the second stage ERV propellants because of high ISP. We keep the ERV smaller by not using all methanol propulsion, while still reaping the easy ISRU benefit of methanol because the 1st stage propellant should mass about 4X as much as the 2nd stage propellant. <br />

 

[JonClarke]

Logging in very briefly from Swan Hill...<br /><br />The efficiencies of using two different locally produced properllant mixes must be traded off against the weight penalty of needing two differenty plants to make the fuel. Also remember that methanol and ethylene are more complex organic molecules than methane, so require more plant to produce. It would be interesting to see the trade offs.<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[gunsandrockets]

<It would be interesting to see the trade offs. ><br /><br />I will have to double check, but I believe the same RWGS plant can produce different products by adjusting input and operating conditions. It entirely possible the same RWGS could manufacture both propellants!<br /><br />But assuming for a moment dual propellants would require two parallel ISPP systems, the main tradeoff would be added complication rather than mass since the chemical plant takes up remarkably little mass. According to Zubrin's numbers the production plant only accounts for 500 kg out of the ERV landed mass of 28,600 kg.<br /><br />

 

[gunsandrockets]

The massive landing gear and heatshield of your ERV concept adds a huge burden to the basic ERV mission. That is a lot of extra mass you end up carrying back to Earth for the sake of limited reusability.<br /><br />It seems you are focusing on a different goal compared to me. I'm looking for ways to trim the mass or make more efficient the mass of the original Zubrin ERV because the original seems too optimistic to even work. You are adding even more optimistic mass assumptions to the ERV than Zubrin did, all for the sake of increased utility.<br /><br />It might be possible to add the fudamental changes to the ERV you suggest, but off the top of my head I estimate the penalty for those changes would be a tripling of the total size/mass of the Zubrin ERV to accomplish the same core mission of returning four men to Earth. <br /><br />But in the interest of doing more than just raining on your parade, I have some suggestions to improve your reusable ERV.<br /><br />First off if you are going to bother lugging a large ERV heatshield all the way back to Earth orbit, there is no need for the ERV to carry the mass of a reentry capsule too. It's just wasted mass to carry that capsule. It makes more sense to EOR with a capsule launched from Earth.<br /><br />Secondly, return speed at Earth is very high, something like 14 km/s! That is much higher than the entry speed at Mars, so your heatshield will need special attention.<br /><br />Considering the volume of your ERV, I don't see the point in lugging around any inflatable habitat module either, it's just more wasted mass. Plus, building the habitat space into your ERV makes more sense for the multipurpose function you want your ERV to have.<br /><br />

 

[j05h]

<i>> The massive landing gear and heatshield of your ERV concept adds a huge burden to the basic ERV mission. That is a lot of extra mass you end up carrying back to Earth for the sake of limited reusability. </i><br /><br />Yes but in exchange for complete reusability, the concept doesn't really work otherwise. Realistically, the "ERV" I described is more in John Clarke's 50-ton range. If you can fly a larger craft to Mars and back several times, that infrastructure investment would be well worth it. EOR with a capsule is planned as part of the last-leg of flight, but assumed to start the discussion with a lifeboat capability. Ideally the heatshield is capable of full reentry if needed, sans lifeboat.<br /><br />If the craft is built bulky enough (it will be light during aerobraking) it could definitely be large enough for 4-6 people to live in. If built SSTO for Mars, it is going to have tankage for 100t of propellant, that can surround a sizable crew cabin. Since it is designed to be able to soft-land some amount of cargo, it stands that it could theoretically bring some module or other back to Earth. Nothing major if not, the vehicle itself would be valuable enough.<br /><br />Along with reusability of the craft, the basic assumption is for some kind of LEO (or L1, i'm not picky!) supporting infrastructure. The whole idea relies on ISRU as almost all Mars concepts do - part of my interest in a split-package of lander-orbiter is that the "sundancer" part of the cargo could include Phobos supplies. Again, I'm not picky, whatever works beyond flags-n-footprints. <br /><br />The basic Mars Direct architecture reminds me of ESAS and Apollo. Instead of building a network of craft and supply points, they sent singular "missions" that are easy to turn away from. Add to this the onerous expense of for NASA in those cases of operating an HLV that no one else buys into. We already have a robust medium ELV market, especially when including international providers. Any sucessfu <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[gunsandrockets]

<...Realistically, the "ERV" I described is more in John Clarke's 50-ton range ... Ideally the heatshield is capable of full reentry if needed, sans lifeboat ... If built SSTO for Mars, it is going to have tankage for 100t of propellant, that can surround a sizable crew cabin><br /><br />I think it's important to enumerate what a big bite you are taking. I've crunched a few numbers to narrow down just how much more massive your ERV concept would end up than the baseline Zubrin ERV.<br /><br />The Zubrin ERV during ascent grosses 100 tonnes and uses two rocket stages of 86.5 tonnes wet mass to propel the 13.3 tonne cabin from the Martian surface on a return trajectory to Earth. Your reusable ERV is going to propel a lot more than just a cabin back to Earth. But how much more?<br /><br />From information at this link...<br /><br />http://www.astronautix.com/craft/stcbrake.htm<br /><br />I found that a heatshield is 18% of an aerobraked Mars lander's mass. So based on that I estimated the mass of the Zubrin ERV heatshield to be over 8 tonnes.<br /><br />So with that heatshield estimate in hand I figure your reusable ERV would have to send a payload of about 26 tonnes back to Earth, almost double the 'payload' of the Zubrin ERV. That means your ERV at liftoff from Mars would need to be at least twice as massive as the Zubrin ERV.<br /><br />It's not easy to go direct from the surface of Mars back to Earth. Every gram counts. Which is why adding reusability to the ERV so readily baloons the size of the ERV.<br /><br /><...If you can fly a larger craft to Mars and back several times, that infrastructure investment would be well worth it...Along with reusability of the craft, the basic assumption is for some kind of LEO (or L1, i'm not picky!) supporting infrastructure....Again, I'm not picky, whatever works beyond flags-n-footprints...The basic Mars Direct architecture reminds me of ESAS and Apollo.

 

[j05h]

<i>> I think it's important to enumerate what a big bite you are taking. I've crunched a few numbers to narrow down just how much more massive your ERV concept would end up than the baseline Zubrin ERV. </i><br /><br />Certainly. It's a much different concept - it's a small spaceship instead of a disposable capsule. It's a Spider for Mars - I'll start a new thread for it Friday.<br /><br />That Cryogenic Aerobrake profile is pretty cool. That looks at first glance like a technically do-able Mars mission. <br /><br />My first critique of the Mars Direct ERV should stand, but let me rephrase it. The ERV should be a Mars-SSTO, with tankage integrated with a core crew cabin. This would provide the crew with excellent radiation protection inbound. Mass fractions for Mars SSTO are attainable, this makes sense if it can conserve structure in the ERV. I recommend exploring an all-composite structure with base-first heat shield. Possibly use an ARD-type ballute to increase cross-section during reentry. The masses for aeroshell, cabin structure and propulsion stages should in this context be one number, roughly 9 tons of primary structure (plus engines). That's a decent starting number to hold 100t of propellant and some people. My thinking on this is that the bulkier structure can use a lesser heat shield on Earth return. <br /><br />If possible, incorporate as much RCS duty into the main engines as possible. Ballard makes hydrocarbon fuel cells today, possibly include a methane-fuelled unit for power, using residual fuel instead of photovoltaics for power. Another power option would be a Sterling cycle engine using crew waste heat. These are all fairly large development efforts individually, saving time/money on systems development was part of the attraction of the Sundancer-Dragon-ERV semi-Direct posts. That setup allows for a comparatively minimal ERV (return heatshield, engines, tankage, crew in small cockpit) compared to the more expansive TSTO ERV Zubrin proposes. An integ <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[gunsandrockets]

Hey Propforce!<br /><br />Come on and post something on this thread, I want to hear your ideas! <br /><br />

 

[gunsandrockets]

[What we are left with is the typical Zubrin ERV, but one that leaves an inflatable habitat with an Earth re-entry capsule parked in LMO. The ERV with a small cabin lands on the Martian surface unmanned and loads up with propellant while awaiting a crew. Later the crew boards the ERV and launches into LMO, where the ERV will rendezvous with the habitat+capsule stack. Then the entire stack is sent on an Earth return trajectory, with the crew living in the inflatable habitat. When the ERV arrives at Earth, the crew boards the re-entry capsule which then separates from the rest of the ERV and lands on Earth.] <br /><br />[I think there is merit in leaving some ERV mass parked in Mars orbit until needed. It does complicate mission design, and it reduces the total useful mass delivered to the Martian surface, but we have already taken that path by choosing an inflatable habitat for the trip home to Earth. If you can't use the inflatable habitat on the Martian surface, why bring it down to Mars? It's more efficient to leave the inflatable in Mars orbit.]<br /><br />[This revised Mars Direct mission architecture ends up as a hybrid between the Zubrin plan and the NASA DRM. The ERV rocket propellant is made on Mars and launched from Mars as per Zubrin, but the ERV habitat is parked in Mars orbit as per the NASA DRM.]<br /><br />I posted this some time ago but much more recently discovered that the hybrid idea is a pretty old one, and in fact Zubrin designates the concept as the 'Hybrid' too since it is a cross between his original Mars Direct and the NASA revision called Mars Semi-Direct.