the Earth Return Vehicle of Zubrin's Mars Direct plan

[scottb50]

An ERV has to have enough performance to send the crew all the way back to Earth, sustain the crew for months, and provide an aerocapture method of stopping at Earth. Tall orders all.<br /><br />Not if all it has to do is ferry people and cargo to LMO and back. It makes a lot more sense to operate from LEO to LMO and back, especially when you would have to ferry all your propellant from LEO to begin with.<br /><br />While Zubrins idea of producing propellant on Mars sounds very reasonable, until it is actually proven to work I would not want to base a mission on it.<br /><br />I also think it would be much easier to have dedicated vehicles rather than do all machines. An Earth to LEO and back to Earth vehicle, an LEO-LMO vehicle and an LMO to Mars and back to LMO vehicle. <div class="Discussion_UserSignature"> </div>

 

[gunsandrockets]

[While Zubrins idea of producing propellant on Mars sounds very reasonable, until it is actually proven to work I would not want to base a mission on it.] <br /><br />[I also think it would be much easier to have dedicated vehicles rather than do all machines. An Earth to LEO and back to Earth vehicle, an LEO-LMO vehicle and an LMO to Mars and back to LMO vehicle.]<br /><br />That may be true, but it is irrelevant to this thread.<br /><br />As I pointed out previously there are many other threads discussing alternative architectures to Mars, and what might be ideal. This thread is intended for discussion of the Mars Direct ERV.

 

[gunsandrockets]

"One final thought that kinda leave me pondering is this. I am not an expert on the Soyuz rocket system or how their spacecraft is fitted out. You have stated 3 tonnes for the Return to Earth Stage. This seems like it is a little heavy for what we need to do."<br /><br />The re-entry vehicle of the manned Soyuz spacecraft is pretty minimalist. Just ask anyone who's ever had to squeeze into one of the Soyuz capsule's three seats! In fact the Soyuz RV is so minimalist, it very well may not be good enough for the Mars ERV.<br /><br />Earth return velocity from Mars is higher than returning from the moon, maybe as high as 14 km/s. G loading of the headlight shaped Soyuz re-entry capsule might be too high under those conditions. Odds are a re-entry vehicle with a higher lift to drag ratio will be required, or perhaps a skip-reentry profile could mitigate the forces of deceleration.<br /><br />The important point is you probably can't get more bare bones than a Soyuz capsule! <br />

 

[scottb50]

As I pointed out previously there are many other threads discussing alternative architectures to Mars, and what might be ideal. This thread is intended for discussion of the Mars Direct ERV.<br /><br />Ithought that's what I was doing. Pointing out how it wouldn't work and offering an alternative. I suppose if the only option allowed to be considered is the one Zubrin proposed and we have to accept it as written in stone then fine. <div class="Discussion_UserSignature"> </div>

 

[gunsandrockets]

" I suppose if the only option allowed to be considered is the one Zubrin proposed and we have to accept it as written in stone then fine."<br /><br />Don't take it hard. I'm just trying to prevent this thread from drifting off topic. Mars Direct is far from my favorite architecture to reach Mars. But I think Mars Direct is interesting enough in and of itself to deserve a thread discussing it in detail.<br /><br />I do not want to shut you down. If you feel strongly about your Mars plan, maybe you should start your own thread for discussion about it?

 

[j05h]

Ok, so what we've got baselined is 46.2 tonnes to Mars (per Zubrin) for getting the crew home. Everyone seems to like the idea of Soyuz and Sundancer. <br /><br />One option for first-mission ISRU is that the first craft on Mars would make prop for only an orbital flight, like the Reference Mission. The first flight (replaces all-in-one ERV) consists of a Sundancer and long-duration Soyuz stacked together inside a large egg-shaped craft's cargo bay, next to 6-10 tons of nuclear plant and an ATHLETE robot. The craft is a multi-role Mars craft per the ERV bulkier, lighter and less capable. It is aerodynamic enough to serve as it's own shroud atop ARES-V or other HLV. At Mars, it aerobrakes, deploys the Sundancer and Soyuz, deorbits and begins base preparation. The orbiters dock and go dormant. On the surface the nuke is deployed and the ATHLETE also can reposition other elements like the ERV. The craft reclaims water (ice, condenser, etc) and cracks it into methane for the later boost to orbit. Not sure if they have enough total delta-V, but the Sundancer and Soyuz would parttake in TEI burn, with whatever mods are necessary including extra stages. Both Soyuz and the ERV can be equipped with ballutes to aid in aerobraking.<br /><br />Following Zubrin's numbers, the basic 46.2 tons (i'm assuming that's metric) in this scenario includes a full Soyuz stack (7t) and Sundancer (10t). Zubrin assumed a 4.5t reactor and H, let's call it even at 10t with the ATHLETE, hold the Hydrogen. I'd actually put prop storage at/near the nuke instead of in the ERV - first filling station on Mars. That leaves 19.2t (call it 18t) for the ERV and any extra Fregat stages (6.5t) or Parom tugs (6.8t). <br /><br /><br />The ERV is going to mass upward of 100t at takeoff, and could possibly provide some delta-v for the Soyuz/Sundancer stack heading home. Total minimum return delta-v is around 6.4km/s, between the various craft. I can't find delta-v info on SD or Fregat, but Soyuz TMA is 900km/s<br /><b></b> <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[gunsandrockets]

Okay, I'm having a little trouble following your revised architecture so bear with me if I misunderstand anything. It appears you want to split the Zubrin ERV functions into a Mars lander and a Mars orbiter. I think you have both vehicles travel together from Earth, which is different from the NASA DRM which uses a separate launch for each vehicle.<br /><br />As for the delta-v requirements, the source you use is very handy and I use it myself all the time. But the total delta-v requirement shown of 6.4 km/s to travel from Mars surface to Earth is for a mimimum energy transfer orbit. I believe that means a travel time of around 240 days. <br /><br />To reduce that time down to six months as the Zubrin plan describes requires more delta-v. According to a table in Zubrin's book, 3 km/s delta-v is needed to return to Earth from Low Mars Orbit (LMO)! That is a total delta-v of 7.1 km/s from the surface of Mars to reach Earth.<br /><br />[I can't find delta-v info on SD or Fregat, but Soyuz TMA is 900 km/s]<br /><br />The Soyuz TMA has only 390 meters per second of delta-v. Far short of 3.0 km/s or even 2.4 km/s. If you have a Soyuz+Sundancer stack in LMO they are going to need a lot more umph to get to Earth than from the propellant they normally carry onboard! <br /><br />The Soyuz TMA masses 7,250 kg of which only 900 kg is propellant. The breakdown of the Sundancer is not available but I think it's safe to assume a similar propellant fraction as the Soyuz TMA. The propellant loads of those vehicles are intended for modest delta-v orbital maneuvers, not the kind of high energy changes needed for travel between the planets. <br /><br />Using the same storable propellants of around 305 seconds ISP performance that the Soyuz uses, the propellant mass required for a 17 tonne Soyuz+Sundancer stack to achieve 3 km/s of delta-v is about 29 tonnes of propellant! Tankage for holding that much propellant would be about 27 cubic meters in volume. You probably would really want

 

[JonClarke]

Drat! Drat! Drat! This is a fantastic discussion on a favourite topic. Well done everyone on their contributions. I have lots I could say but I am going on three weeks holiday tomorrow, so I can't (just as well some people say!).<br /><br />Keep it up.<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]

[...This is a fantastic discussion on a favourite topic...Well done everyone on their contributions...]<br /><br />Thanx!<br /><br />[Drat! Drat! Drat!...I have lots I could say but I am going on three weeks holiday tomorrow, so I can't...Keep it up.] <br /> <br /><br />Plenty more to talk about, we have just scratched the surface.<br />Join in when you can. <br />

 

[j05h]

gNr- You've got most of it down. If i had to rephrase it, it would be like this: "There are better ways to use Zubrin's 46.2 tonnes to Mars." <br /><br />My main attraction to the Sundancer/Soyuz stack is availability. Soyuz exists now, Sundancer is almost guaranteed to fly. I screwed up the Soyuz numbers, too, as you pointed out (900 km/s? I must have been sleep-typing). Anyway, one is off-the-shelf and the other is almost ready. My off-the-cuff answer to delta-V shortfalls is to use Fregat upper stages. I agree that we need another thrust stage for the return trip. <br /><br />I was assuming the ascent vehicle (originally the ERV) could provide some of the push to Earth, then place itself in a highly elliptical Mars orbit, aerobrakes and deorbits to be a "Hopper". If the Mars ascent vehicle needs to be brought all the way back to Earth, I'd consider ditching the Soyuz (alt, you could spin them...) <br /><br />The reason I keep bringing up a separate ascent vehicle is that leaving it in cis-Mars space gives yet another backup for future crews. It provides a second vehicle at the base to be used for exploration, emergency ascent, storage or a second propellant-factory. Give it 4 - 5 tons total and call it Dragon?<br /><br />If you had to bring the ERV back to Earth, could it and Sundancer do a free-return to Mars or someplace useful? Aerobrake with the capsule, then enter HEEO? Thoughts? <br /><br />Any "cruise" stages (TMI, TEI, etc "EDS") could be used as spin-gravity counterweights. <br /><br />Generally, you might be right about not needing an entire Soyuz. However, that assumes that Soyuz separates from Sundancer within "potty break" time, as the Orbital MOdule has the toilet. The service module is probably necessary for long-term storage. You might be able to cut it down some, mass-wise, but it is pretty much an integrated unit. <br /><br />Zubrin's fear of orbital docking/assembly stems from the ISS, I think. If we can't do orbital rendezvous at Mars, we aren't going anywh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[bpfeifer]

"One thing, we've kind of slipped into, is that we are now talking about a 3-crew mission, or two Soyuz."<br /><br />When we first strarted talking about Soyuz, I was using it as an example of the kind of system I wanted. I never intended to suggest using actualy Soyuz hardware. One of the reasons is, as you pointed out, that the soyuz is a 3-man capsule and the mission outline calls for 4 astronauts.<br /><br />Of course there are advantages to using off the shelf hardware, but once you add the other components, is it really the same vehicle any more? This is something that NASA is facing with development of the stick. At that point, we may as well assume we're building something Soyuz-like from scratch, and merely applying the lessons learned from Soyuz development and flight history. <div class="Discussion_UserSignature"> Brian J. Pfeifer http://sabletower.wordpress.com<br /> The Dogsoldier Codex http://www.lulu.com/sabletower<br /> </div>

 

[gunsandrockets]

"When we first strarted talking about Soyuz, I was using it as an example of the kind of system I wanted. I never intended to suggest using actualy Soyuz hardware. One of the reasons is, as you pointed out, that the soyuz is a 3-man capsule and the mission outline calls for 4 astronauts."<br /><br />It wasn't just the tiny Soyuz capsule that is a three crew vehicle, the 10 tonne Sundancer inflatable module is also a three crew vehicle. The combination of the Sundancer and Soyuz capsule equalled 10% MORE mass than the 11.5 tonnes Zubrin assumed for the total mass of the ERV cabin!<br /><br />A reduction of the crew size from four down to three is kind of forced upon us because Zubrin's mass estimates were too optimistic.<br /><br />Now with some of the ideas we are playing around with, maybe we can squeeze more efficiency out of the Zubrin ERV and thereby find the mass margin to enlarge the cabin up to a size four crew again. But if the mass of the cabin is limited to ~11.5 tonnes, It's hard to see how that is enough for a crew of four.

 

[gunsandrockets]

[I was assuming the ascent vehicle (originally the ERV) could provide some of the push to Earth, then place itself in a highly elliptical Mars orbit, aerobrakes and deorbits to be a "Hopper". If the Mars ascent vehicle needs to be brought all the way back to Earth, I'd consider ditching the Soyuz (alt, you could spin them...)]<br /><br />The only practical way to send the habitat back to Earth is with Mars derived propellant, so the ERV must go along it can't stay behind at Mars. And yes, a full Soyuz spacecraft is superfluous and unneeded. The burned out ERV might have enough leftover mass to provide a counterweight for tethered-spin artificial gravity. A lot depends on how much staging the ERV must perform to deliver adequate propulsion for Earth return.<br /> <br />It occurs to me now that if the ERV habitat is left in Mars orbit when the rest of the ERV goes down to Mars, it doesn't really make sense to leave the habitat in a low Mars orbit. Instead the habitat should be left in a very elliptical high orbit. That way less delta-V is needed to send the habitat back to Earth on the return flight.<br /><br />[If you had to bring the ERV back to Earth, could it and Sundancer do a free-return to Mars or someplace useful? Aerobrake with the capsule, then enter HEEO? Thoughts?]<br /><br />The sixth month long return flight to Earth could be on a free-return trajectory that would bring it back to Mars two years after passing by Earth. But what would be the point? The sundancer would be empty of supplies and the ERV stage would be empty of rocket propellant.<br /><br />[The reason I keep bringing up a separate ascent vehicle is that leaving it in cis-Mars space gives yet another backup for future crews. It provides a second vehicle at the base to be used for exploration, emergency ascent, storage or a second propellant-factory.]<br /><br />You're reusable lander is getting at something I will address in a different post. But in general the Zubrin Mars Direct plan is a dispos

 

[gunsandrockets]

[The reason I keep bringing up a separate ascent vehicle is that leaving it in cis-Mars space gives yet another backup for future crews. It provides a second vehicle at the base to be used for exploration, emergency ascent, storage or a second propellant-factory.]<br /><br />What you are describing is very much like a vehicle Zubrin imagines for the later stages of Mars exploration, when a permanent base is established on Mars. He calls it a NIMF -- Nuclear rocket using Indigenous Martian Fuel. It is a manned reusable Mars lander propelled by a Nuclear Thermal Rocket (NTR) engine. It has a built in propellant aquisition system which sucks up Martian air and then liquifies it for replenishing the NIMF propellant tanks. This enables the NIMF to travel dozens of times to Mars orbit and back, or to carry a significant cargo mass on a suborbital hop to almost anywhere on the suface of Mars because the NIMF propellant source is virtually unlimited.<br /><br />The NIMF is an interesting concept worthy of it's own thread, and therefore a detour of this thread which is devoted to the Mars Direct ERV.

 

[gunsandrockets]

[The ERV could have a nose heatshield (eyeballs out or t/space seats) with the rockets in pillars extending out the rear with landing pads (like a mushroom w/ 4 legs).]<br /><br />I've been thinking about your nose mounted heatshield suggestion (which strongly resembles the Kistler K-1 orbital vehicle heatshield) and I'm really warming to the idea.<br /><br />Since the ERV travels to Mars unmanned, we can use ideas that we wouldn't normally include with a manned lander. Eyeballs-out entry isn't a problem because there are no eyeballs aboard! High 10+ g loading entry isn't a problem because there is no crew aboard. That makes the idea of a relatively small nose mounted blunt heatshield a lot more practical.<br /><br />In fact a blunt nose mounted heatshield would perfectly integrate into the horizontal landing ERV concept I brought up earlier. The basically cylindrical ERV would enter nose-first with a nose mounted heatshield, then transition to the sideways landing profile after the deployment of the drogue parachutes and main parachutes. It very much is a copy of the Kistler K-1 orbital vehicle landing profile! Cool.<br /><br />One nice advantage of a nose mounted heatshield is it can be smaller and lighter than a heatshield for the sideways entry profile that I first imagined. Another advantage is the ERV would undergo compressive g loading along the long axis of the vehicle, the same loading axis as during launch from Earth, simplifying and lightening the overall structure. Cool.<br /><br />Great! Unless someone wants to suggest something else, I recommend adopting the nose-mounted heatshield with a cylindrical shaped sideways landing ERV. An almost complete rip-off of the Kistler K-1 orbital vehicle outline!<br /><br />http://www.rocketplanekistler.com/flightprofile/fp04.html

 

[solarspot]

Actually, I don't know wether this has already been suggested in this thread, but what about an aft-mounted heat shield? Last time I tried to design a bare-bones archetecture for getting to mars, the best idea I came up with (that I know) was having the Ascent vehicle descend through the Atmosphere base first with a Heatshield at the base... before landing the Heatshield would jettison soyuz-style and retro rockets would land the thing (maby those could also force off the heatshield? The Section containing the retro rockets would also have the landing gear (legs?) and would serve as the launch platform for the upper section of the Ascent vehicle... when done just start up the main engine on the Ascent stage and head to orbit pointy end first... The vehicle would always be 'accellerating' towards the pointed end of the vehicle, so it isn't impossible for this thing to also carry the crew down to the surface from orbit...<br /><br />Sorry if all that is reduntant to something already said here...

 

[j05h]

The only issue I see with a "K-1" landing profile for the ERV is getting it back into the air. Does it launch sideways from Mars?<br /><br />I think a more general review of Mars Direct is needed, not just the ERV. It might make more sense to land the prop factory with Hab hardware and the ERV separately. Every time I think of the ERV, it seems like it and the Hab should be interchangable - use the same aerobraking and EDL hardware, common interfaces and the ability to land lots of extra gear (ATVs, science kit, extra inflatables). What we need is a general craft for getting cargo & people to and off Mars. Maybe pulling 10Gs makes sense, but if it can be a common system from the start it leverages itself that much quicker.<br /><br />Assume a 10m payload fairing, the craft can reliably be 12m wide (hammerhead), wide enough to fit a standard Sundancer sideways inside, or lengthwise. 10m is fine, too. The nose of the ERV is a sphere-cone heatshield that trails off to a standard cylinder (or exotic egg shape). The craft is bulky, with an extensive cargo bay (for zero-G and landing) and tankage, it is designed to provide general access to cis-Mars space. If it is TSTO (for TEI tug), then the stages must be easily stackable, with a modular crew cabin. The base craft is bulky enough that fairly simple heatshield materials can be used. The initial aerobraking pass(es) could be a proving-ground for a trailing ballute - it could provide a decent COG change, plus added drag. <br /><br />So if you plan to use the ERV for TEI, how about this scenario. The ERV uses a transparational-cooled metal heatshield for a multiple-skip aerobrake, discharging the Sundancer/Capsule between passes. On landing it spends a while making propellant. It is TSTO, the first stage lands to become a hopper, the second stage becomes the tug stage for the capsule/Sundancer's missing 4km/s, providing the needed delta-v to Earth. In this scenario, it might make sense to bring the capsule to the surface, th <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[j05h]

Solar- one thing that GnR and I agree on (i think) is that reusability is important. Jettisoning heatshields makes them hard to put back on for reflight. Base first 'big capsule' lander have been discussed extensively but are usually proposed for crew landers not cargo. I'm advocating a single basic lander of whatever form that can handle modules including a crew cabin. <br /><br />Josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[j05h]

the TSTO ERV/Hopper craft's upper stage (with metal heatshield) functions as an aerobrake in both directions. The upper stage is a 10m "Sled" that holds generic exploration hardware, it can deliver payload to Mar's surface, return to LEO and tug another payload later. A LEO/HEEO tank farm could get filled with residual methane too. <br /><br />The actual heatshield could be one of the demonstrated planetary-probe metals with or without transparational cooling. An all metal hull would be very retro - but I can also picture it as a composite structure. Another option for the main heatshield is a cast ceramic. This could enable your robotic 10G aerobraking. Crews would ideally use the same craft (with another capsule, exploration kit, Sundancer and Nautilus) and follow a more gradual set of aerobraking passes using the same basic vehicle. As the technology evolves the ERV would drop our "Soyuz" lifeboat and become a general transit craft with passengers continuing to Earth on another craft. In an emergency, the transit stage can land on Earth directly but is designed for years of use in space. <br /><br />Once launched the ERV system should build on itself. <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[solarspot]

"reusability is important"<br />Maby, but I've spent somewhat more time studying (albiet not professionally lol) expendable hardware, so I will devout the rest of this post to expendable solutions to the Mars lander / ERV.<br /><br />As for one system for both Crew and Cargo... The system I outlined included an expendable / seperable landing sytem, with the Crew using the upper section. If development costs really do justify this, what about replacing the ascent vehicle with Habitat sections or other cargo? The descent vehicle might not be the exact optimum size for cargo, but it would improve commonality (sp?).

 

[gunsandrockets]

<As for one system for both Crew and Cargo... The system I outlined included an expendable / seperable landing sytem, with the Crew using the upper section. If development costs really do justify this, what about replacing the ascent vehicle with Habitat sections or other cargo? The descent vehicle might not be the exact optimum size for cargo, but it would improve commonality.><br /><br />The goal is to save total mission costs for whatever level of commonality is used between the ERV and the Mars Lab. Using a great deal of commonality could save development costs. But using too much commonality could eliminate those important differences which add to mission capability. Are the savings from commonality worth the price of -- let's say -- reducing the crew by 1/3?<br /><br />Splitting mission mass between the Mars Lab which carries the crew from Earth to Mars, and the ERV which carries the crew from Mars to Earth, isn't just a way to divide up the total mass budget that our launch vehicles must deliver into space. Because of the different nature of each vehicles job they must meet very different design requirements.<br /><br />For example the ERV because it travels to Mars unmanned can take the low-energy/slow-trip to Mars thereby permitting an increased ERV mass. Likewise there is no need for the ERV to operate a life support system or furnish artificial gravity during the trip to Mars.<br /><br />I have a feeling that forcing too much commonality between the ERV and the Mars Lab would compromise both vehicles more than any commonality is worth. The mission requirements for each vehicle are hard enough to meet without worrying about commonality too.<br /><br />I say let's come up with the best possible specialized design first and then look for possible ways to add commonality!

 

[gunsandrockets]

"Actually, I don't know wether this has already been suggested in this thread,..."<br /><br />No it hasn't, and by the way, welcome to the party!<br /><br />The outline you describe is very much a standard and conventional configuration. In fact the skeletal description Zubrin presented for his ERV very much follows your concept for an ERV.

 

[gunsandrockets]

[The only issue I see with a "K-1" landing profile for the ERV is getting it back into the air. Does it launch sideways from Mars?]<br /><br />Exactly! It launches sideways!<br /><br />Exact configuration of the ascent engines and ascent engine propellant tanks is yet to be determined. But the basic profile of the ERV is fairly flat when resting on the surface of Mars. Because the atmospheric pressure on the surface of Mars is about the equivalent of atmospheric pressure 100,000 feet above sea level on Earth, air induced drag is negligable for an ERV lifting off the surface of Mars.<br /><br />But by using a flat un-aerodynamic profile, the ERV has a wider stance and a lower center of gravity making it a much more stable Mars lander. And because the ERV would not tower above the Martian surface, crew access and unloading the ISPP equipment is a lot easier. <br /><br /><br />

 

[gunsandrockets]

[Every time I think of the ERV, it seems like it and the Hab should be interchangable - use the same aerobraking and EDL hardware, common interfaces and the ability to land lots of extra gear (ATVs, science kit, extra inflatables). What we need is a general craft for getting cargo & people to and off Mars. Maybe pulling 10Gs makes sense, but if it can be a common system from the start it leverages itself that much quicker.]<br /><br />My comment under the same subject heading to Solarspot covers my ideas about commonality.<br />

 

[gunsandrockets]

[It might make more sense to land the prop factory with Hab hardware and the ERV separately.]<br /><br />?? Are you suggesting moving the ISPP plant from the ERV to the Mars Lab manned lander? If so, that is asking for trouble, as explained near the bottom of this post.<br /><br />[Assume a 10m payload fairing, the craft can reliably be 12m wide (hammerhead), wide enough to fit a standard Sundancer sideways inside, or lengthwise. 10m is fine, too.]<br /><br />I think the largest payload fairing we can expect is 8.4 meters diameter, the same diameter as the Ares V third stage. Anything larger than that would sacrifice payload mass due to greater air friction.<br /><br />[The nose of the ERV is a sphere-cone heatshield that trails off to a standard cylinder (or exotic egg shape). The craft is bulky, with an extensive cargo bay (for zero-G and landing) and tankage, ...]<br /><br />Okay so far...<br /><br />[... it is designed to provide general access to cis-Mars space. If it is TSTO (for TEI tug), then the stages must be easily stackable, with a modular crew cabin. ]<br /><br />??<br /><br />[So if you plan to use the ERV for TEI, how about this scenario. The ERV uses a transparational-cooled metal heatshield for a multiple-skip aerobrake, discharging the Sundancer/Capsule between passes.]<br /><br />Keep in mind the very first pass through the Martian atmosphere has to change the ERV velocity enough so that the ERV captures into Mars orbit! Unless you want to waste rocket propellant for a propulsive capture.<br /><br />[On landing it spends a while making propellant.]<br /><br />Spends a while, as in months or a year or more;-)<br /><br /><br />[... the second stage becomes the tug stage for the capsule/Sundancer's missing 4km/s, providing the needed delta-v to Earth.] <br /><br />Okay. Though I think the delta-V is quite a bit less than 4 km/s, especially if the Sundancer is parked in a high Mars orbit.<br /><br />[In this scenario, it might make sense to bring the capsule to the surface,