Quote from: Krevsin on 10/14/2015 06:52 amThe fact that SpaceX were looking into SEP could mean they were looking at an all-in-one vehicle with both SEP and chemical propulsion.As was already mentioned in the thread, that would require the large solar arrays not only somehow be retracted into the MCT itself but also be deployed again after re-launch from Mars, and thus be so reliably shielded from re-entry (and issues on the surface like dust) that you can guarantee they will redeploy for the return trip to Earth. There's simply no advantage to justify that complexity.Quote from: Krevsin on 10/14/2015 06:52 amMCT will remain a "land the entire thing" architecture, no matter how disagreeable you find the prospect of it.The original land-the-entire-thing comment was made in the context of the typical multi-stage launch/landing architectures. It doesn't mean that Musk has ruled out having a SEP stage that remains in orbit.And given that he's happy to do EOR refuelling, to avoid the restrictions of monolithic launches that are typical of Mars architectures, I can't see why anyone would think he'd arbitrarily rule out such an option just to satisfy one interpretation of a throw away quote.We might find out in a month or so. Until then, these options are on the table... "No matter how disagreeable you find the prospect."
The fact that SpaceX were looking into SEP could mean they were looking at an all-in-one vehicle with both SEP and chemical propulsion.
MCT will remain a "land the entire thing" architecture, no matter how disagreeable you find the prospect of it.
His original understanding based on the interview he gave was that they would try to eliminate all complexity by doing a mission without any orbital rendezvous: Take off from Earth, go directly to Mars (in 3-4 months), come directly back.
The requirements for an MCT vehicle which does perfect massless aerocapture and refuels only in LEO and on the Martian surface with ~100d transfers are still over 10km/s in that one stage.
Quote from: Burninate on 10/14/2015 08:07 pmThe requirements for an MCT vehicle which does perfect massless aerocapture and refuels only in LEO and on the Martian surface with ~100d transfers are still over 10km/s in that one stage. are still 7.6km/s in that one stage.Are you sure you are adding up the right numbers? Because with the most efficient (not fastest) Hohmann transfer, i get ~9.5 km/s from LEO to the Martian surface, but that is with NO aerobraking at Mars. With aerobraking that should come down to less than 8km/s, right? Yes, a lot of the ideas that Musk have thrown out for "MCT" seem almost impossible (or actually impossible) if one assumes they are all true, at once. Many of them I interpret as "nice to have" for future versions, and one of them I see that way is the "fast transit". I don't expect that to happen until a propellant depot is established in LMO.
The requirements for an MCT vehicle which does perfect massless aerocapture and refuels only in LEO and on the Martian surface with ~100d transfers are still over 10km/s in that one stage. are still 7.6km/s in that one stage.
Quote from: Impaler on 10/10/2015 03:14 amI'm strongly in favor of SEP and see considerable use for it making VERY FAST transit possible.How is this possible? Wouldn't the system need to be huge and have magical power sources and such (insert anything Zubrin has ever said). NO, you can get a Fast transit on the order of 100 days to Mars with a slow, low power SEP system.The trick is you use your SEP to move your Mars bound vehicle with propellants up to high Earth orbit and then then drop by the Earth for a huge Oberth assisted burn. For 2 km/s you should leave Earth with huge escape velocity and reach mars in 100 days (average). Now the problem is capturing at mars, the answer is Magneto-Plasma Aerocapture, this lets us avoid expensive propulsive capture and is then followed by about a week of Plasma assisted aerobraking which lets the eventual EDL be from a gentle 4 km/s. So we get to have both fast transit and easy low speed EDL. The SEP system has not even left Earth yet in this scenario, so you can do either one of two things, bring it back down to LEO for refueling and do it again (basically making it a Cis-lunar tug), or send it to mars by the conventional slow method of spiraling out from the Earths SOI (the SEP is too delicate to take the high thrust of the Oberth maneuver). In the latter case your going to arrive much later then the manned capsule but if this is a conjunction mission the crew will be spending around 600 days on mars so their is plenty of time for the SEP to arrive before it is needed for departure which is what I favor.The MCT would only need to reach low Mars orbit and would then rendezvous with the SEP and head for Earth, this return transit is made reasonably short by the fact the MCT is a completely dry shell now of only 100 mT (75 vehicle mass + 25 return cargo) and the SEP is nearly dry too so power to weight ratios are increased, also were not aiming to match Earth's orbit and capture gently, were going to simply intersect it on an elliptical orbit around the sun, that cuts the DeltaV needed. At Earth we used the Magneto to capture again and bring both SEP and the MCT down to LEO (they probably need to separate to do this as the SEP is more delicate and would slow the process down for the MCT). The crew can be retrieved via a Dragon capsule now, and we need to send another tanker to LEO to put landing propellants into the MCT, if we use enough the MCT can do a lot of retro-propulsion on entry and bring it's entry speed down from the 7.7 km/s of orbit down to the range of 4 km/s which matches it's mars entry speed, so all the thermal protection systems can be designed for this low performance point.IMLEO is estimated at 570 mT of which 100 mT is the cargo load, 75 mT is the MCT dry mass, 200 mT is chemical propellant in the MCT (2 tanker loads of 100 mT each), 155 is SEP propellant, 15 mT is the SEP tank and 22 mT is the SEP hardware which has a power output of 4.5 MW which corresponds to an alpha value of 5 kg/kw.BTW Using a braking system like Magneto Plasma is the only way I can see an Integrated Bi-conic and direct Earth return being viable, without it the entry conditions are too extreme to meet the low dry mass fractions that it's advocates are proposing.Do you have math to back this mission plan up?Edit: For the case of the highly simplified circular Mars / circular Earth orbit rendezvous, based on:http://forum.nasaspaceflight.com/index.php?topic=37536.msg1371984#msg1371984If you start at high Earth orbit and descend to perigee, then you can burn for 2241m/s to raise aphelion to 3.31AU, which drops time of flight to 100.03 days. Then you come in to the Mars approach at 12880m/s, and have to burn off 9275m/s in order to capture into a highly elliptical orbit. This is fairly difficult (capture intensity goes up faster than linearly because you have a shorter chord of atmosphere to cut through, not just less time in that atmosphere), and exceeds the capabilities of any chemical propulsion capacity in this planning exercise by a large measure.
I'm strongly in favor of SEP and see considerable use for it making VERY FAST transit possible.How is this possible? Wouldn't the system need to be huge and have magical power sources and such (insert anything Zubrin has ever said). NO, you can get a Fast transit on the order of 100 days to Mars with a slow, low power SEP system.The trick is you use your SEP to move your Mars bound vehicle with propellants up to high Earth orbit and then then drop by the Earth for a huge Oberth assisted burn. For 2 km/s you should leave Earth with huge escape velocity and reach mars in 100 days (average). Now the problem is capturing at mars, the answer is Magneto-Plasma Aerocapture, this lets us avoid expensive propulsive capture and is then followed by about a week of Plasma assisted aerobraking which lets the eventual EDL be from a gentle 4 km/s. So we get to have both fast transit and easy low speed EDL. The SEP system has not even left Earth yet in this scenario, so you can do either one of two things, bring it back down to LEO for refueling and do it again (basically making it a Cis-lunar tug), or send it to mars by the conventional slow method of spiraling out from the Earths SOI (the SEP is too delicate to take the high thrust of the Oberth maneuver). In the latter case your going to arrive much later then the manned capsule but if this is a conjunction mission the crew will be spending around 600 days on mars so their is plenty of time for the SEP to arrive before it is needed for departure which is what I favor.The MCT would only need to reach low Mars orbit and would then rendezvous with the SEP and head for Earth, this return transit is made reasonably short by the fact the MCT is a completely dry shell now of only 100 mT (75 vehicle mass + 25 return cargo) and the SEP is nearly dry too so power to weight ratios are increased, also were not aiming to match Earth's orbit and capture gently, were going to simply intersect it on an elliptical orbit around the sun, that cuts the DeltaV needed. At Earth we used the Magneto to capture again and bring both SEP and the MCT down to LEO (they probably need to separate to do this as the SEP is more delicate and would slow the process down for the MCT). The crew can be retrieved via a Dragon capsule now, and we need to send another tanker to LEO to put landing propellants into the MCT, if we use enough the MCT can do a lot of retro-propulsion on entry and bring it's entry speed down from the 7.7 km/s of orbit down to the range of 4 km/s which matches it's mars entry speed, so all the thermal protection systems can be designed for this low performance point.IMLEO is estimated at 570 mT of which 100 mT is the cargo load, 75 mT is the MCT dry mass, 200 mT is chemical propellant in the MCT (2 tanker loads of 100 mT each), 155 is SEP propellant, 15 mT is the SEP tank and 22 mT is the SEP hardware which has a power output of 4.5 MW which corresponds to an alpha value of 5 kg/kw.BTW Using a braking system like Magneto Plasma is the only way I can see an Integrated Bi-conic and direct Earth return being viable, without it the entry conditions are too extreme to meet the low dry mass fractions that it's advocates are proposing.
-snip- (see original post) -snip-The tradeoff is: you can keep the BFR as a 4.5km/s vehicle instead of making it a (3.2km/s earth escape) + (2.4km/s Impaler Short Transit injection) + (~2km/s or whatever you need for EDL) ~= a 7.6km/s vehicle. You're also going to need comparable efforts to get the thing home on a fast transit, though.
My question is, why not send the crew up in traditional capsules to dock and load into the MCT. If the MCT has to refuel in LEO. The refueling station could be where the crew transfers in. No need to put launch escape system on MCT.
Quote from: spacenut on 10/14/2015 01:27 pmMy question is, why not send the crew up in traditional capsules to dock and load into the MCT.Because that'd be way expensive cumulatively than launching the MCT itself, and cost is a factor.
My question is, why not send the crew up in traditional capsules to dock and load into the MCT.
Now, you may argue that this is me reading too deeply into quotes
But nowhere did I find any indication that they are stepping away from a monolithic MCT and looking into having dedicated in-space elements, which means the original statement by Elon that their design goal for the MCT is "land the whole thing" remains a valid criterion in ascertaining whether or not a marsbound architecture can be considered an MCT architecture or not.Now, you may argue that this is me reading too deeply into quotes and that any design posted in this thread is, as it were, cut from whole cloth. While that is all true (Especially the former, as I love reading too deeply into things), the purpose of this thread is to design a vehicle and accompanying architecture based entirely on criteria extracted from the fragments of information we are able to scrounge up.I like Impaler's proposed architecture. However, I disagree on it being a viable MCT.
It is 'grossly improportional' if it also cripples your architecture. And I still don't think you are understanding how capable your abort capsule would have to be to avoid black zones on Mars. And parachutes won't help much for something this size. (they don't scale up well, and Mars atmosphere is thin to begin with) Consider this... Delta-V to reach low Mars orbit is over 3 km/s. So if you have a problem when you are *almost* in orbit, you now have ~2(?) km/s of propulsive capability that this capsule needs to have in order to land softly - and you will land far over the horizon, on the other side of the planet. Any capsule capable of 2+ km/s that is large enough to hold 100(!) people is going to be massive. MASSIVE. Your simple capsule just won't cut it. Sometimes abort capability is impractical, and you mitigate risks other ways. You make your vehicle capable of abort, by having engine out capabilities and redundant systems. But you cannot remove all risks. Airliners still sometimes fall out of the sky, yet we don't have escape pods or parachutes. But I'm tired of arguing these points over and over. For some, launch abort seems to be an essential that they just cannot comprehend going without.
Quote from: Lars-J on 10/14/2015 08:23 pmQuote from: Burninate on 10/14/2015 08:07 pmThe requirements for an MCT vehicle which does perfect massless aerocapture and refuels only in LEO and on the Martian surface with ~100d transfers are still over 10km/s in that one stage. are still 7.6km/s in that one stage.Are you sure you are adding up the right numbers? Because with the most efficient (not fastest) Hohmann transfer, i get ~9.5 km/s from LEO to the Martian surface, but that is with NO aerobraking at Mars. With aerobraking that should come down to less than 8km/s, right? Yes, a lot of the ideas that Musk have thrown out for "MCT" seem almost impossible (or actually impossible) if one assumes they are all true, at once. Many of them I interpret as "nice to have" for future versions, and one of them I see that way is the "fast transit". I don't expect that to happen until a propellant depot is established in LMO.Quote from: Burninate on 10/11/2015 08:56 amQuote from: Impaler on 10/10/2015 03:14 amI'm strongly in favor of SEP and see considerable use for it making VERY FAST transit possible.How is this possible? Wouldn't the system need to be huge and have magical power sources and such (insert anything Zubrin has ever said). NO, you can get a Fast transit on the order of 100 days to Mars with a slow, low power SEP system.The trick is you use your SEP to move your Mars bound vehicle with propellants up to high Earth orbit and then then drop by the Earth for a huge Oberth assisted burn. For 2 km/s you should leave Earth with huge escape velocity and reach mars in 100 days (average). Now the problem is capturing at mars, the answer is Magneto-Plasma Aerocapture, this lets us avoid expensive propulsive capture and is then followed by about a week of Plasma assisted aerobraking which lets the eventual EDL be from a gentle 4 km/s. So we get to have both fast transit and easy low speed EDL. The SEP system has not even left Earth yet in this scenario, so you can do either one of two things, bring it back down to LEO for refueling and do it again (basically making it a Cis-lunar tug), or send it to mars by the conventional slow method of spiraling out from the Earths SOI (the SEP is too delicate to take the high thrust of the Oberth maneuver). In the latter case your going to arrive much later then the manned capsule but if this is a conjunction mission the crew will be spending around 600 days on mars so their is plenty of time for the SEP to arrive before it is needed for departure which is what I favor.The MCT would only need to reach low Mars orbit and would then rendezvous with the SEP and head for Earth, this return transit is made reasonably short by the fact the MCT is a completely dry shell now of only 100 mT (75 vehicle mass + 25 return cargo) and the SEP is nearly dry too so power to weight ratios are increased, also were not aiming to match Earth's orbit and capture gently, were going to simply intersect it on an elliptical orbit around the sun, that cuts the DeltaV needed. At Earth we used the Magneto to capture again and bring both SEP and the MCT down to LEO (they probably need to separate to do this as the SEP is more delicate and would slow the process down for the MCT). The crew can be retrieved via a Dragon capsule now, and we need to send another tanker to LEO to put landing propellants into the MCT, if we use enough the MCT can do a lot of retro-propulsion on entry and bring it's entry speed down from the 7.7 km/s of orbit down to the range of 4 km/s which matches it's mars entry speed, so all the thermal protection systems can be designed for this low performance point.IMLEO is estimated at 570 mT of which 100 mT is the cargo load, 75 mT is the MCT dry mass, 200 mT is chemical propellant in the MCT (2 tanker loads of 100 mT each), 155 is SEP propellant, 15 mT is the SEP tank and 22 mT is the SEP hardware which has a power output of 4.5 MW which corresponds to an alpha value of 5 kg/kw.BTW Using a braking system like Magneto Plasma is the only way I can see an Integrated Bi-conic and direct Earth return being viable, without it the entry conditions are too extreme to meet the low dry mass fractions that it's advocates are proposing.Do you have math to back this mission plan up?Edit: For the case of the highly simplified circular Mars / circular Earth orbit rendezvous, based on:http://forum.nasaspaceflight.com/index.php?topic=37536.msg1371984#msg1371984If you start at high Earth orbit and descend to perigee, then you can burn for 2241m/s to raise aphelion to 3.31AU, which drops time of flight to 100.03 days. Then you come in to the Mars approach at 12880m/s, and have to burn off 9275m/s in order to capture into a highly elliptical orbit. This is fairly difficult (capture intensity goes up faster than linearly because you have a shorter chord of atmosphere to cut through, not just less time in that atmosphere), and exceeds the capabilities of any chemical propulsion capacity in this planning exercise by a large measure.Quote from: Burninate on 10/13/2015 09:53 am -snip- (see original post) -snip-The tradeoff is: you can keep the BFR as a 4.5km/s vehicle instead of making it a (3.2km/s earth escape) + (2.4km/s Impaler Short Transit injection) + (~2km/s or whatever you need for EDL) ~= a 7.6km/s vehicle. You're also going to need comparable efforts to get the thing home on a fast transit, though.Yeah, I made an error there (I added escape velocity twice). 7.6km/s to get to Mars from LEO in 100 days assuming that perfect aerobraking accepts all excess velocity for free. But you go too far on the other point: Without aerobraking you'rs looking at 9.3km/s just for capture, making the total stage capability more like 16.9km/s. If you assume a direct descent to a Vacuum Mars Analog, the figure is 12.9km/s plus gravity losses, making for a total stage capability of 20.5km/s.These numbers are so high because 100 day transfers require lots of extra energy that 200 day transfers do not. If you're content with 200 day transfers...-numbers to come, WIP-
Burninate,Thanks for those numbers. The wet to dry mass numbers are fascinating as well. I certainly favor the slower, more delta-V conservative approaches. And I think it is the best hope of making an "MCT" work, without a LMO propellant depot.
Quote from: Lars-J on 10/14/2015 10:58 pmBurninate,Thanks for those numbers. The wet to dry mass numbers are fascinating as well. I certainly favor the slower, more delta-V conservative approaches. And I think it is the best hope of making an "MCT" work, without a LMO propellant depot.Oh, I think there's still room for an LMO prop depot even in the most conservative case. There's still a question of whether slow-transit, LEO depot, plus LMO depot is *enough* for a vehicle with integrated ISRU capacity on Mars to do a first mission to a new site reusably. A lot of it falls on what the mass return ratio for the ISRU system ends up being.If it's not high enough, you either *have* to resort to a nested MAV (and return the big'un next synod), or you just have to throw away a whole MCT for every site you land at, as a permanently landed asset.
Quote from: Burninate on 10/14/2015 11:04 pmQuote from: Lars-J on 10/14/2015 10:58 pmBurninate,Thanks for those numbers. The wet to dry mass numbers are fascinating as well. I certainly favor the slower, more delta-V conservative approaches. And I think it is the best hope of making an "MCT" work, without a LMO propellant depot.Oh, I think there's still room for an LMO prop depot even in the most conservative case. There's still a question of whether slow-transit, LEO depot, plus LMO depot is *enough* for a vehicle with integrated ISRU capacity on Mars to do a first mission to a new site reusably. A lot of it falls on what the mass return ratio for the ISRU system ends up being.If it's not high enough, you either *have* to resort to a nested MAV (and return the big'un next synod), or you just have to throw away a whole MCT for every site you land at, as a permanently landed asset.That has always been my assumption - that every new site would have not just one, but several unmanned MCT land to set up ISRU and other equipment. Most of those initial unmanned "pathfinder" MCT would not be returned, I would expect - They would instead become the first outpost habitats, storage sheds, and MCT spare part depots. This is not flags and footprints that are planned here. The goal is to create a permanent manned outpost. But the first crews would not be launched until the next launch window after ISRU production is well underway.
Quote from: Lars-J on 10/14/2015 11:15 pmThat has always been my assumption - that every new site would have not just one, but several unmanned MCT land to set up ISRU and other equipment. Most of those initial unmanned "pathfinder" MCT would not be returned, I would expect - They would instead become the first outpost habitats, storage sheds, and MCT spare part depots. This is not flags and footprints that are planned here. The goal is to create a permanent manned outpost. But the first crews would not be launched until the next launch window after ISRU production is well underway.The threshold you have to pass, then, is being able to set up and run an ISRU water-mining workflow without any human assistance, perhaps without any realtime telerobotics, if we don't go for a landing.
That has always been my assumption - that every new site would have not just one, but several unmanned MCT land to set up ISRU and other equipment. Most of those initial unmanned "pathfinder" MCT would not be returned, I would expect - They would instead become the first outpost habitats, storage sheds, and MCT spare part depots. This is not flags and footprints that are planned here. The goal is to create a permanent manned outpost. But the first crews would not be launched until the next launch window after ISRU production is well underway.
You also need it to last a decade.
Burninate, I still see a potential for SEP instead of the magneto hydrodynamic breaking in what you describe.Imagine going with say a 120 day transfer to a point where Mars would have been about 20 days before the craft gets there, but for the full length of the 150 day flight SEP is running at 0.4 mm/s2 to alter the orbit so that it actually arrives at Mars later, a little further along on Mars orbit, but at almost the same speed for a relatively low energy capture (say around 1km/s for capture).and yes that is just roughed out, I don't have Andy Weir's continuous thrust orbit model software. So really it is a matter of optimizing for the mass of the SEP system including solar arrays against how much faster than a standard half ellipse Hohmann you want to fly.
That has always been my assumption - that every new site would have not just one, but several unmanned MCT land to set up ISRU and other equipment. Most of those initial unmanned "pathfinder" MCT would not be returned, I would expect - They would instead become the first outpost habitats, storage sheds, and MCT spare part depots.
Quote from: Impaler on 10/13/2015 05:35 amQuote from: meekGee on 10/13/2015 05:07 amI'll also add that in addition to the trip duration, there's the "land the whole thing" mentality. If you're talking about ISS-esque solar array and radiator array, the idea of stowing them before EDL is not practical.If instead you have a "Hermes"-type orbit-to-orbit ship in mind, then now SEP starts making sense.But again, this is about MCT.We KNOW SpaceX is considering SEP for Mars, and as it is impossible to land a SEP, that mean IPSO FACTO that they are have ALSO considered a Semi-Direct architecture of a transit-vehicle and a separate landing vehicle. And that such a vehicle trades very well against a single massive direct vehicle.It's not, in fact, impossible to land an SEP on Mars. Just kind of odd and mass inefficient. But with a sufficiently good solar array, you could do it. I should point out that an "ISS-esque" solar array is, in fact, retractable!
Quote from: meekGee on 10/13/2015 05:07 amI'll also add that in addition to the trip duration, there's the "land the whole thing" mentality. If you're talking about ISS-esque solar array and radiator array, the idea of stowing them before EDL is not practical.If instead you have a "Hermes"-type orbit-to-orbit ship in mind, then now SEP starts making sense.But again, this is about MCT.We KNOW SpaceX is considering SEP for Mars, and as it is impossible to land a SEP, that mean IPSO FACTO that they are have ALSO considered a Semi-Direct architecture of a transit-vehicle and a separate landing vehicle. And that such a vehicle trades very well against a single massive direct vehicle.
I'll also add that in addition to the trip duration, there's the "land the whole thing" mentality. If you're talking about ISS-esque solar array and radiator array, the idea of stowing them before EDL is not practical.If instead you have a "Hermes"-type orbit-to-orbit ship in mind, then now SEP starts making sense.But again, this is about MCT.
This is my preference as well. Absent a good characterization of MAC, let's keep its use at a manageably small level. We have a good characterization of SEP, on the other hand, and SEP can be used for much of capture. The simplified case that's most conservative on MAC would be to go with the 180-day version I calculated above, but plan on burning for about 2 months while approaching aphelion. There are other benefits to using modest amounts of SEP; It makes things like Mars-orbit habs and propellant much easier to deal with.