From LEO to Mars (Chemical propulsion?)

[mikkelrj]

When we are going to mars, the ship will be assembled in LEO most likely. If we are going to use existing technology, that means we will use a chemical propulsion stage for the required deltav mars trajectory. Is that even possible, i mean, how much propellant would be needed for, lets say, a 50metric tons marsship? Is it impossible to carry out a manned expedition to mars with chemical propulsion? I hope some one can enligthen me on this chemical vs nuclear propulsion thingie.

 

[nacnud]

Take a wonder around here:<br /><br />http://www.astronautix.com/craftfam/martions.htm

 

[mikejz]

Well when you say 'going to Mars' what do you mean<br /><br />Any Orbit?<br />Low Mars Orbit<br />Orbit and then retro-fire and land on the surface<br />Direct-Entry<br />etc....<br /><br />What time table are you talking Hohmann or something quicker?

 

[spacester]

Chemical propulsion to Mars is very doable.<br /><br />Complete enlightenment on the subject is going to exceed bandwidth limits. <img src="/images/icons/laugh.gif" /> Perhaps a specific question?<br /><br />Oh wait, you did ask a specific question . . . <img src="/images/icons/wink.gif" /><br /><br />The amount of propellant needed to get a spaceship of a given mass from A to B is going to depend on just two things: the deltaV and the Isp = specific impulse ("fuel efficiency") of your rocket engines. Isp=375 sec is a reasonable number. Calculating the deltaV is rather non-trivial <img src="/images/icons/laugh.gif" /> but can be done.<br /><br />There are different ways to get from A to B, and as noted, you need to be specific about both A and B. The total deltaV is actually the addition of the dV for departure and for arrival. You burn your rocket engines to get out of the gravity well of A, and having done so you are in orbit around the sun and you can just coast. But once you get to B, you need to burn your engines again to get captured in B's gravity well.<br /><br />This all changes from year to year, and because Mars' orbit is rather eccentric, it changes more than a little bit.<br /><br />But I can pull some numbers out of the air based on experience and answer the question.<br /><br />Departure from Earth on a 180 day flight to Mars might require something like 3.5 km/s of dV. Arrival at Mars might require something like 2.5 km/s. That's a total of dv=6.0 km/s<br /><br />The rocket equation is<br /><br />mo/mf = e^(dV/(Isp * g))<br /><br />plug in the numbers:<br />mo/mf = e^(6.0 / (375 * 9.807 / 1000)) = 5.11<br /><br />mo = original mass<br />mf = final mass<br />mo = mf + mp<br />mp = mass of propellant<br />(mf + mp)/mf = 5.11<br />mp / mf = 4.11<br /><br />So, you would need 4.11 times as much mass in propellant as the mass of your unfueled 50 tonne ship.<br /><br />You would need 205.6 tonne of propellant.<br /> <div class="Discussion_UserSignature"> </div>

 

[henryhallam]

Direct entry can reduce that quite a bit...

 

[JonClarke]

So would using a more realistic Isp of 450 using LOX-H2 for LEO departure. <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>

 

[spacester]

Jon, I'm sorry to surmise that you cannot afford a calculator with an e^x key . . . <img src="/images/icons/wink.gif" /><br /><br />How are you going to re-fuel your LH2/LOX engines at Mars? You going to just throw them away? Who's going to finance such a wasteful architecture?<br /><br />I wasn't aware that the question called for a trade study. I guess unless I'm prepared to do so I shouldn't even bother . . . <div class="Discussion_UserSignature"> </div>

 

[JonClarke]

Keh?<br /><br />Every Mars mission study using chemical propellants I have seen, (and I have read at least summaries of dozens) throws away it's TMI stage. So to all the nuclear thermal missions. The only ones that don't throw away the TMI stage are solar electric or nuclear electric systems. So by thowing abway my TMI stage I am going with the majority.<br /><br />As for refueling on Mars, if you have a direct mission architecture you have the option of manufacturing your propellant on Mars. You have several options. Otherwise you take it with you, either as hypergolics or methane-LOX, given the long term storage issues of hydrogen in Mars orbit.<br /><br />Why pick such a low Isp for your earth departure propellant? Just about every chemical proposal of the last 40 years has used hydrogen.<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>

 

[scottb50]

This is why water makes the most sense, easy and safe to transport, non-volital and can be stored long term with minimal complications. Whether you carry water or another fuel and oxidizer your going to have a lot of propellant to carry. Are you going to contain LOX for up to four years or carry something less stable along for the trip?<br /><br />Start building up return propellant enroute and eventually set up an orbital propellant production facility in Mars orbit, water would be shipped in and processed once the cyclers start running on a regular schedule. <div class="Discussion_UserSignature"> </div>

 

[ve7rkt]

The Russians had a dual fuel engine planned for the MAKS mini-shuttle, burning a mix of LH2/LOX/kerosene for the first minute and a half, then LH2/LOX until orbit. The engine was tested, switching between fuels repeatedly. If I understand things right, the first blend was used for its density and high thrust to get the craft out of the atmosphere, and the latter blend was used for a longer burn with its high ISP.<br /><br />So how about an engine that burns LH2/LOX for its high ISP during the TMI burn, then easily stored methane/LOX on MOI, and either stored or locally produced methane/LOX for the trip home? Put the LH2 and part of the LOX in droppable tanks instead of a separate stage. With only one set of engine hardware, you end up with a lower mass leaving Earth to put the same mass at Mars.<br /><br />I'm almost certainly wrong here, so what I want to know is, why am I wrong? Would a dual-fuel engine compromise performance too much?

 

[spacester]

Ah, the tyranny of the majority . . . <img src="/images/icons/laugh.gif" /><br /><br />You have not read my proposal then. My TMI stage achieves HEMO and is refuelled (and re-oxidizered <img src="/images/icons/smile.gif" /> ) from ISRU operations and then goes back to Earth for its next mission. Call me crazy, but IMO if we're going to have a sustainable space architecture, we're going to have to learn to not throw stuff away unless we have to.<br /><br />So I chose to use Isp = 375 sec for LC2H4/LOX in answering the question. <div class="Discussion_UserSignature"> </div>

 

[JonClarke]

The wisdom of the majority should always been listened to, even when you depart from it. Especially when it represents more then 50 of years thinking by professionals.<br /><br />1) You and the OP did not specify "your "architecture, only a generic one.<br /><br />2) No, I have not read it, where is it?<br /><br />3) Nobody has developed a reasonable way of pumping tens of tonnes of cryogenics from one spacecraft into another. Until it is, we are stuck with disposables.<br /><br />4) The siren of resuability has lead better minds than ours onto the rocks of loss of credibility before now. <br /><br />5) The best is the enemy of the good enough. I am implacably opposed to developing any more new technology than is absolutely neccessary for going to Mars (or anywhere else. For Mars, bulk cryogenic refueling, like many other technologies, comes into that category. if of course the technology is already avalaible, that is a very different story.<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>

 

[spacester]

Well shoot, Jon, how are we going to argue if you're going to be all reasonable and logical like that . . . <img src="/images/icons/smile.gif" /> <img src="/images/icons/wink.gif" /><br /><br />When I reject the majority opinion, it's almost never from a standpoint of 'those guys are stoopid'. I do it because I disagree with a basic premise behind their thinking. I like to think I have a talent for identifying unacknowledged, hidden, perhaps invalid assumptions. As a design engineer and by nature, I know that the hidden assumptions are the main guiding force in a design. As my design goal is to ‘speed up space development’, my focus for years now has been to identify the assumptions and bring the ones that are holding us back into the light of day. As you know, one near the top of the list is what I perceive to be a ‘Science First, everything else has to wait’ mantra.<br /><br />Resetting the founding principles allows the design process to ‘take it from the top’ and often you find that the previous ‘conventional wisdom’ is perfectly valid but the little tweaks done to conform to the new principles make all the difference. Unfortunately, in space flight stuff, it is just as likely to call for major recasting of the whole big picture. I wish it weren’t so, I guess it’s the nature of the beast.<br /><br />Well said about the ‘siren of reusability’, very nice phrase, much wisdom there. And I have to admit that I have not heard about anybody working on transferring tons of cryos. Still, if ISRU is an enabling strategy like everyone says it is, it seems to me that it is one of those techs that are in fact ‘absolutely necessary’. Even if transferring in space is another level of difficulty past surface operations. So in this case I have to say that the hidden assumption that we should go before we are prepared to field a sustainable architecture in the one I’m questioning.<br /><br />I outlined my Mars Settlement Architecture in those really long threads – what, you didn’t commit the <div class="Discussion_UserSignature"> </div>

 

[mattblack]

Excellent website I discovered last night: Has tables for Earth-Lunar & Earth-Mars Delta-V and mass/propellant ratios. It is proving useful for the research for my novel!:<br /><br />http://www.mars-lunar.net/index.html<br /> <div class="Discussion_UserSignature"> <p> </p><p>One Percent of Federal Funding For Space: America <strong><em><u>CAN</u></em></strong> Afford it!!  LEO is a <strong><em>Prison</em></strong> -- It's time for a <em><strong>JAILBREAK</strong></em>!!</p> </div>

 

[gunsandrockets]

"Is it impossible to carry out a manned expedition to mars with chemical propulsion?"<br /><br />No. Here is a plan from TRW in 1963. It makes extensive use of aerobraking.<br /><br />http://www.astronautix.com/craft/trwmars.htm<br /><br /><br /><br />"I hope some one can enligthen me on this chemical vs nuclear propulsion thingie."<br /><br />Ah! Nukes make everything better! Here is one comparison. One chemical propulsion manned-mission to Phobos with a total mass budget of 253 tonnes in LEO is reduced to 145 tonnes by using nuclear propulsion.<br /><br />http://uplink.space.com/showthreaded.php?Cat=&Board=missions&Number=392009&page=&view=&sb=&o=<br /><br /><br /><br />

 

[gunsandrockets]

"Excellent website I discovered last night: Has tables for Earth-Lunar & Earth-Mars Delta-V and mass/propellant ratios."<br /><br />Thanx for the link!

 

[gunsandrockets]

"Every Mars mission study using chemical propellants I have seen, (and I have read at least summaries of dozens) throws away it's TMI stage. So [do] all the nuclear thermal missions."<br /><br />Well the Von Braun Mars plan of 1969 didn't throw away the nuclear TMI stages. In fact...<br /><br />"They [Mars ships] were entirely reusable for future expeditions, the only element being expendable being the Mars Excursion Module used to visit the planet's surface. "<br /><br />http://www.astronautix.com/craft/vonn1969.htm

 

[JonClarke]

Reason and logic are SOOO annoying......<br /><br />As you may remember, I did not take much part in your settlement threads for a range of reasons.<br /><br />You may well be right in the long term, but to get there several things need to happen first. ISPP of 10's and then 100's of tonnes of cryogenics must become a reality, engines resuable with a minimum of inspection must be developed, and of course the whole issue of zero G transfer of bulk crysogenics must be solved. Flexible bellows as on Progress aren't going to work with those masses, volumes, and temperatures. Launching disposable tanks might be a worthwhile intermediate technology once the first two issues are addressed.<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>

 

[JonClarke]

More recent plans would include Mars Direct and DRM version 1.0, which also use aerobraking and chemical fuels. The TRW 1963 study was an excellent and epocj making one but of course in 1963 the Martian atmosphere was though to be much more dense than we now know it to be.<br /><br />Nuclear propulsion reduces the mass ratio by perhaps up to 60% over the best chemical propellants. This makes it propulsively more efficient. Whether it makes things easier remains to be seen, given the technical, environmental, and political risks that come with it.<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>

 

[JonClarke]

Please note the tiny qualifier in my statement: "chemical". WvB's 1960 presentation used the NERVA NTR. How reusable it actually was remained to be seen, other NERVA Mars missions - the Boeing 5 NERVA scenario and an earlier NASA 4 NERVA scenario did not have reusability. Neither to most others. The greatly reduced efficiency of NTR after a few firings, because of posioning of the reaction by fission products, make resuability less attarctive. Not to mention the serving an safety issues of returning highly radioactive cores to LEO, and the minor issue of refueling the reaction mass (although disposable tanks was an option).<br /><br />Of course the practicality of this scanrio (which was only a presentation rather than a detailed study) was never fully investigated. Nor was its advisability, with 6 operational multi-megawatt thermal reactors in low earth orbit, all lacking containment and shielding. Yum! Oddly enough the next US Mars missions all returned to chemical propulsion.<br /><br />However, the 1986 Case for Mars cycler architecture had limited reuse of chemical engines, with the same engines used for Mars landing and ascent. <br /><br />Jon<br /><br /> <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>