Requirements for a manned mission to Phobos

[themanwithoutapast]

I like the idea that has been outlined here: http://www.thespacereview.com/article/501/1, a manned mission to Phobos.<br /><br />There are certainly fewer obstacles to just go to Mars orbit (escape velocity for Phobos is 11m/s thus in effect we talking about a Mars orbital mission) than to Mars surface. The question I was asking me is what the minimum requirements of such a mission would be. <br /><br />As far as I see it minimum requirements are set out by a minimum mission duration of at least 24 months and a minimum crew of 4 (a number lower than that seems unrealistic).<br /><br />My questions thus:<br /><br />1. What would be the minimum size in metric tons (without propulsion stage) for a mission module (the lowest estimates I have are: Earth return capsule: 5 tons, habitation and supply module at a minimum-40 tons, instruments and other equipment needed for Phobos exploration: 5 tons). <br /><br />2. A Mars orbital mission is not that different to a Moon orbital mission - the increase in delta-v required for a Mars trajectory is only marginal higher than for the Moon. Thus given the answer to 1. what is the required size in metric tons of a (conventional or non-conventional) propulsion stage.<br /><br />3. Critical mission parts would include the docking of propulsion and mission module in LEO (if necessary or if necessary an array of docking maneuvors), the construction of one part of the mission module as a radiation shelter (however we could leave this out as not constituting a minimum requirement), breaking into Mars orbit (the 6,000km orbit of Phobos) as well as breaking into Earth return trajectory and the airobracking part for return to Earth in Return capsule.<br /><br />So, any ideas how much "simpler" a manned Mars orbit mission would be compared to a Mars surface mission? And any estimates for minimum weight of such a Mars orbit spaceship?

 

[JonClarke]

I am actually working though the numbers for a Mars Transfer Vehicle with a colleague now. It serves as a baseline. Very roughly, for a 4 person crew and a conjunction mission (900 day mars stay), and aerobraking at both ends you end up with<br /><br /> <br /><br />Crew module Mars departure mass (including 5 tonne earth entry capsule) ~ 30 tonnes<br /><br />Fuelled propulsion stage (LOX-methane) ~ 40 tonnes<br /><br />Total mass prior to Mars departure ~70 tonnes<br /><br />Crew + propulsion module (+aeroshell and consumables) in LEO ~100 tonnes<br /><br />Earth departure stage (LOX-hydrogen) ~153 tonnes<br /><br />Total mass LEO ~253 tonnes<br /><br /> <br /><br />Note:<br /><br /> <br /><br />1) This is close to rock bottom mass this and does not include margins. If you have a 20% margin (which is what reviewers have demanded) on the crew module, you end up with a total LEO mass of ~360 tonnes. Either way you are looking at something big with at least some orbital assembly<br /><br />2) This does not include the propellant to travel to the moons, equipment to study them, or robots for landing on Mars.<br /><br />3) You could potentially have these items into a separate exploration vehicle that the MTV must rendezvous with. It could have small LEM-like spacecraft that would fly to the moons, robots to land on the surface of Mars, plus the scientific equipment to study Mars from orbit and process samples.<br /><br />There are two big issues with such missions. One is the fact that the crew will spend the entire time in microgravity (barring use of spin gravity which raises its own problems, the other is you can't make use of martian resources to reduce the mission mass.<br /><br />Jon11111111 <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]

One possible way to cut down on the mass of the JonClarke plan is to divide the Mars mission into an unmanned component and manned component.<br /><br />1) Preposition at Mars the maximum possible amount of equipment and propellent by using an unmanned vehicle.<br /><br />"3) You could potentially have these items into a separate exploration vehicle that the MTV must rendezvous with. It could have small LEM-like spacecraft that would fly to the moons, robots to land on the surface of Mars, plus the scientific equipment to study Mars from orbit and process samples."<br /><br />You send the separate 'exploration module' to Mars with a slowboat nuclear electric rocket (NER) tug. Plus the tug takes with it the Mars departure stage and Earth reentry capsule. The NER tug could take an extra year or more to reach Mars, but that doesn't matter since there is no crew on board and the tug departs for Mars way before any manned vehicle follows.<br /><br />Because of the long flight time, hypergolic fuels might be a better choice for the Mars departure stage than using LOX/CH4.<br /><br />Because the NER tug is nine times more efficient than a chemical rocket Earth Departure Stage (EDS), the overall mass of the mission is cut back tremendously. Rough guess for the total mass in LEO of the cargo preposition mission (unmanned tug+cargo) is 90 tonnes.<br /><br />2) Reduce the mass of the manned component of the mission even further by using a nuclear thermal rocket (NTR) for the EDS instead of a chemical rocket. That would cut in half the needed mass of the EDS.<br /><br />Rough guess of the total mass in LEO of the manned mission (NTR plus habitat and consumables) is 55 tonnes.<br /><br /><br /><br />So the total Mars mission mass (unmanned plus manned) therefore equals 145 tonnes in LEO. The greatest mass savings come from using NER propulsion combined with the unmanned preposition mission. Not so much mass savings for using NTR on the manned component.<br /> <br /><br />

 

[JonClarke]

It's actually very tough to get the mass below what I have given for an MTV. <br /><br />The mass that returns to earth is fixed, you need a return capsule, a certain living volume and a fixed amount of consumables. You can save a bit by higher levels of recyling, especially water, but those numbers assume 80% already. You could have a nuclear thermal propulsion, but the savings are of the order of perhaps 30% of the return stage. So your mass departure mass might be shaved to perhaps 50 tonnes at best, even with 90% recycling and nuclear propusion. Remember that the higher the recycling efficiency the higher the power and mass of plant required to do it, so you end up with diminishing returns.<br /><br />You can preposition supplies at Mars, but which ones? If they are life support consumables you end up with a rendevzous or die situation, and you want to minimise those risks. Even if all the consumables are waiting for you at Mars except those you need to get there, you only save about 18 tonnes off the earth departure mass. This brings the total LEO mass down to about 246 tonnes, which is still a lot (remember we are using chemical propulsion. It is better that the supplies you pre-position are the non mission critical ones, the exploration or science component. This is additional mass over and above what you need to get to and from Mars orbit.<br /><br />It's difficult to explain without a spread sheet. We are trying to get our mass down to 125 tonnes in LEO for the MTV (minus the earth departure stage but plus a 20% margin). This is to match the nominal payload of the SDLV. We are nearly there....<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>

 

[najab]

><i>There are certainly fewer obstacles to just go to Mars orbit (escape velocity for Phobos is 11m/s thus in effect we talking about a Mars orbital mission)....</i><p>Are you considering in this that you still need to brake all the mass sent from Earth into Mars orbit and then escape not just from Phobos but from Mars - Phobos missions cut down on mass since you don't need to have heavy heat shielding, but they don't cut down <b>that</b> much. Consider the fact that almost half the mass of a non-aerobraking Mars orbiter is propellant for MOI.</p>

 

[gunsandrockets]

I think we are having a failure to communicate.<br /><br /><br />"Even if all the consumables are waiting for you at Mars except those you need to get there, you only save about 18 tonnes off the earth departure mass... (remember we are using chemical propulsion."<br /><br />My whole point is to reduce the mass of your Mars plan by ridding chemical propulsion from the Earth Departure Stage (EDS). It's your EDS which forms 60% of your mass budget and from which the greatest mass savings can be achieved.<br /><br />The Nuclear-Electric-Rocket (NER) has the greatest mass advantage over chemical propulsion. The point of dividing your Mars mission by prepositioning cargo is to allow a NER to replace chemical propulsion in the EDS. The slower transit time of NER compared to chemical propulsion mandates an unmanned cargo preposition mission. <br /><br />"The mass that returns to earth is fixed,...You could have a nuclear thermal propulsion, but the savings are of the order of perhaps 30% of the return stage. "<br /><br />I know that, which is why I didn't propose a nuclear-thermal-rocket (NTR) for the return stage. I suggested a very safe, stable and reliable hypergolic fuel rocket for the return stage even though it would be somewhat more massive than a CH4/LOX stage.<br /><br />"You can preposition supplies at Mars, but which ones? If they are life support consumables you end up with a rendevzous or die situation, and you want to minimise those risks."<br /><br />There are more flight risks than just rendezvous risks, it's all a tradeoff. If safety is the primary concern, the lower mass budget of my plan has the margin for redundant modules. Or multiple vehicles. In my opinion the benefits gained from rendezvous outweigh the risks.<br /><br />.

 

[JonClarke]

We made an unfront decision not to use NTR in our mission because the risks (technical, environmental, political) were too great.<br /><br />If you only use NTR for the trip to Mars the MTV remains essentially unchanged. You do save on total LEO mass, however. The higher efficiency of the NTR means the entire vehicle can be assembled from 2 SDLV launches, not 3. Unfortunately the risks (all of them) of using NTR are greatest in LEO, where you need it most.<br /><br />I agree that a RV in Mars orbit is an acceptable risk. It is an essential part of our architecture.<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]

"We made an unfront decision not to use NTR in our mission because the risks (technical, environmental, political) were too great. "<br /><br />Ah well, nothing to be done then. Did you even consider NER as opposed to NTR? <br /><br />If you are writing off nuclear propulsion are you also writing off nuclear power? Even RTGs? If so, I imagine you are using solar power then.<br />

 

[JonClarke]

We wanted to see if a feasible mission was possible without nuclear power, hence this constraint.<br /><br />Although NEP has lower environmental risk than NTR we ruled it out of our study because we deemed the political and technical risks as still too high. We want to keep these, like other risks, to a minimum.<br /><br />We looked at solar electric propulsion briefly for the trip back. The problem was we weren't able to find very well constrained masses for systems of the power needed. So we had to be very conservative and it looked as if the mass savings for the return home were small. This, combined with the high technical risk and the increased complexity of a whole new propulsion system meant we abandoned this line of approach.<br /><br />We ruled out the use of solar electric going to Mars because of mission complexity - you first have to spiral out through the van Allen belts into a highly ellipical orbit, then launch a special high dV craft with your crew to RV with it before heading off. It saves a bit of mass but gets very complex and ugly. NEP for earth departure has the same draw backs plus the political risk.<br /><br />For a 4 person crew and a semidirect architecture like ours you don't need a reactor to provide power for the surface base. You can do it with solar panels provided the landing site is not at too high a latitude. You thus can eliminate the risks associated with taking a reactor to Mars.<br /><br />If you want a larger crew, or a direct mission architecture, or land at higher latitude, then you will need the reactor.<br /><br />RTGs don't deliver enough power to be practical for a surface manned base. You would need several 100 units, to start with. <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>