Lobo makes a good case for a bi-conic shape,
and really when were debating between a Dragon style capsule with a 15 degree wall slope and a bi-conic with perhaps 10 degrees the difference is really small. It's more a debate between the orientation of the vehicle during entry, either bottom-forward or side-forward. Given the similar volumes discussed the final diameter and shape of the MCT is really splitting hairs.
I think the disagreement is more centered on what the vehicle will be CAPABLE of largely due to disagreements over the dry mass estimate. I simply don't find dry masses under 100 mT credible given what MCT needs to do and the volume it will have. Comparisons to Staurn V stages don't seem credible when these were expendable rocket stages that are hold nothing but propellents and are not capable of EDL on Mars.
I feel that only extrapolations from manned craft that carry cargo and perform atmospheric re-entry are logical (Dragon, Orion, Shuttle Orbiter all of these would make more sense), simply saying that MCT will do the ADDITIONAL task of being a second stage doesn't make it have the mass/volume ratio of a traditional 2nd stage, it just makes it a harder vehicle to engineer as it has more demands put upon it.
Also I find some flaws in the extrapolations from the Saturn 2nd Stage.
Lobo states this stage completes the delivery to LEO, it dose not, at 2nd stage engine cut off the vehicle is only going 7 km/s and a burn of the 3rd stage is necessary to actually reach LEO, though it is a modest one of just under 1 km/s.
Lastly Lobo states the INT-21 would have delivered 140 mT to orbit, but it was actually rated at 115 mT.
All together this just seems a bridge too far for me to believe the MCT can be combined with it's 2nd stage, though I think it could do considerable 3rd-stage duty and thus significantly reduce the ultimate Delta-V burden on the 2nd stage.
Someone asked up thread how big the tanks have to be to return a 60 ton dry mass MCT to Earth's surface.
Quote from: CyclerPilot on 04/29/2015 04:27 amSomeone asked up thread how big the tanks have to be to return a 60 ton dry mass MCT to Earth's surface.That was me. Thanks!603mt doesn't seem too prohibative. And 603mt will weigh about 200mt on the Mars surface.
A 500klb Raptor is 226mt of thrust. If MCT was 60mt dry, that makes a surface mass of about 220mt (equivalent). Which would mean a single Raptor could about do the job by itself.
Nit, why does everyone insist the MCT will go into Mars orbit and then do a TEI burn? I think there is a significant savings (10-20%?) in DV by going directly from Mars surface to Earth. And with the exponential nature of the rocket equation, doesn't this translate into even bigger savings in fuel? It just seems to me that there is absolutely no reason to loiter in Mars orbit before returning to Earth. Sorta like me taking off on a cross country trip, but first circling my city a few times :-) CyberPilot, how much better do your numbers look if they do a direct launch from Mars to Earth?
As for solar energy on Mars, Having fold-down petals on the outside of the MCT can be quickly used for both in space and on Mars. If SpaceX does what I am proposing they would need to figure out all the details that you mentioned. They may need to also add ground solar.
Why couldn't the MCT actually be a two piece lander, that the bottom half stays on Mars as habitat and/or water, methane, or oxygen storage? Either engines could be mounted on the sides or in the center that would stay with the upper half returning to earth. The bottom half could be around the returning center. It could be sealed over after returning ship detaches for internal living quarters. Outside previous methane and oxygen tanks could be refilled with ISRU made fuel and oxygen which would offer some radiation protection. If the lander is high enough off the ground, it could also be sealed around the outside bottom for underneath living areas, vehicle storage and compression/decompression chamber for EVA's on the surface to mine for water. Why not just bring the center or top half of the spacecraft back to earth to be reused? Less building would be required, just modification of the bottom or outer ring of the spacecraft. Less fuel needed for return.
correct weight would be ~2460 MN. So that's more than the ~2256 MN stated by elon as the raptor thrust.So it has to weight something like <50 tonnes instead of 60 tonnes to use only one raptor for liftoff, or raptor thrust has to be greater than elon's last comment about it.
Agreed. If MCT were a minimally sloped capsule shape, it would look more like a traditional biconic, rather than a biconic nosed cylinder. Certainly a lot of side entry biconics that actually do look like that (see below).However, I go with the biconic nosed cylinder becuase it's easier to make big tanks in a cylindrical shape, than it is to make biconic shaped fuel tanks.
I have no idea what the dry mass of MCT might really be like. I'm using the Saturn S-II as a reference that would be an upper stage in the ball part of what I think the MCT spacrcraft/upper stage might be like. And to show that MCT itself doesn't necessarily have to be some ginormous BFR putting near 300mt inot LEO and have 15Mlbs of thrust and 30+ Raptor engine.So, let's say today, we could make an S-II stage that's say 35mt dry, rather than 45mt dry, with modern manufacturing techniques and materials...for example. To that we need to add:1) crew cabin2) landing legs3) TPS system on the side4) ECLSS5) Sabatier reactor6) LH2 fed stock or equipment that can mine water vapor out of the air and then electrolyze it for H2 feed stock. (there's some interesting research into that).7) Cago bay (this is basically empty space, but the alloy housing it would add a bit of overall mass. It could be unpressurized to reduce the housing mass required.8 ) Deployable/retractable solar array(s) for power during transit)9) Anything else I can't think of right now).
If we start with 35mt base for the "stage", how much extra mass will all of this add? I have no idea. If about 30mt, then MCT would dry mass around 65mt, and be in our ballpark.I don't think the ECLSS system or sabatier reactor would be all that heavy. Ditto for a cargo area. The crew cabin would essentially be empty space too and not mass much itself. The crew accomodations like bathrooms, seats, controls, computers, etc would be where the mass for that comes from. That leaves landing legs, TPS, and the solar array.Given the much larger surface area, I think they could use a TPS that's lighter per square ft than the Pica-X they use on Dragon. The fragile ceramic tiles of the Shuttle should probably be avoided in favor of something a little more robust that needs less maintenance to turn around after a round trip.What are some options that are out there to consider? Anyone know what NASA would have used for their Aeroshells in DRM 5.0?Can we keep this down to around 65mt, give or take? There's the question.The problem with using those for analogs, is they are just crew (or payload) housings. They aren't rocket ships. MCT will need to be a rocket stage/ship. Even if it's lofted to LEO by a 2-stage booster, it still has considerable dV requirements to provide itself that no other spacecraft has.I think a better analog would be Rocketplane Kistler's K-1. That's probably about as close to this as has been legitimately designed. But it did a ballistic nose entry rather than a biconic side entry, and it landed on airbags on it's side. So it's not a great analog, but one of the closest I think. See below.
So how did Skylab get into it's orbit? There was no 3rd stage on it.
You certainly could be right. There might be a 2-stage booster under the MCT vehicle. But I still think there are more advantages to having MCT as the 2nd stage. One being you only pay the mass penalty for the upper stage(s) once, not twice. Otherwise you are pushing the dry mass of the 2nd stage along with the dry mass of MCT to LEO. A dedicated 2nd stage would be resuable, so you'd pay for the additional dry mass of it's reusability hardware twice. The mass of a TPS on both the 2nd stage and MCT, landing legs on both, etc. With just one stage, you only "pay" for all of that once. So there's advantages to it. And if you are refueling in LEO anyway, why not?Not to mention it's one less development to pay for, and one less piece to have to remotely deorbit and land. This way, you only are bringing back the actual MCT (which can be a LEO taxi configuration, or tanker configuration) back from orbit. Rather than both an MCT and a dedicated 2nd stage.Of course, this is all predicated on the assumption that an integrated upper stage/MCT -can- be made to work feasible. If not, then there will be a dedicated 2nd stage.
The question I was trying to answer for myself for solar power, is it enough for a unmanned ISRU MCT to generate 470MT of CH4 & LOX in 25 months fully autonomously. Peak solar power of 40KW and working with the daily temperature variations on Mars may be enough, but I do not have the time or the knowledge to know that with any certainty. ISRU of propellant on Mars is the pivotal technology to make the MCT system work. My guess is the first MCT to land on Mars will be a ISRU MCT. With about 1000 m3 of volume to utilize. The first MCT will have 2-3 Sabatier reactors and possibly 4 sources of hydrogen to experiment with. 1) High pressure Hydrogen brought from Earth, 2) Water brought from Earth, 3) Water from the atmosphere, 4) Water dug from the surface. 1-2 Rovers with back-hoes will also be included for scouting the area and digging trenches to look for water to deposit in the MCT. If all goes to plan, the first MCT will have enough fuel for the second MCT to land to return to Earth and validate the MCT for the Earth return part of the mission.Autonomously refueling between the two MCT should be a fun. A long hose, precision landing and reliable and capable rovers.
If someone has a more accurate calculation of DeltaV from Mars surface to Earth return trajectory thous would be very useful. Their may be some saving, but I suspect it will be minimal because a planetary surface and Low orbit are basically the same with regard to the gravity well and Oberth effect.Of particular interest would be transit times in a direct return, how low can you go and how much dose it cost?
Quote from: Impaler on 04/30/2015 01:41 amIf someone has a more accurate calculation of DeltaV from Mars surface to Earth return trajectory thous would be very useful. Their may be some saving, but I suspect it will be minimal because a planetary surface and Low orbit are basically the same with regard to the gravity well and Oberth effect.Of particular interest would be transit times in a direct return, how low can you go and how much dose it cost?It is a bit more to it since you dont have to raise the periapsis of the escape trajectory above the horizon. Still, the spacecraft needs to achieve escape velocity. So the savings are the "shortcut" that the spacecraft takes in a velocity triangle. However, the escape trajectory from surface has to be more "up" than "sideways", indicating higher gravity losses. I cant imagine that the savings will be significant. I can be totally wrong on this one though, its just my personal, feeling based opinion. As is btw. the opinion of other NSF members that the direct return trajectory saves a lot of fuel. Unless someone can show me the math, I dont believe that a direct return trajectory is better than going to orbit first.
But even if the savings is 2% (and I think it's quite a bit more), what purpose is served by going to orbit first?
If the biconic has a reasonably low slope to the side wall then a cylindrical tank (or 2) can fit inside of a conic without wasting much room. The outer form of the vehicle don't have to be the shape of the tanks inside unless it is like a rocket stage in which these are one in the same, but the thermal protection systems necessary for reentry dictates that the outer surface can't be the tank anyways.
I think we need to minimizing integration of superfluous systems into the vehicle to get as low a mass as possible. I'd drop the Sabatier reactor and H2 equipments (atmospheric moisture extractors most likely) immediately, that is a system that makes absolutely ZERO sense to bring back to Earth as its ONLY usable on the surface of Mars and we desperately want to get as high a volume of propellent production going as possible so every bit of ISRU equipment sent to Mars needs to stay to build up production capacity.
How about we try this strategy, start with your basic rocket-stage mass and fractions (and I'd recommend using F9 upper stage as an analog rather then Saturn V hardware, it's hydrocarbon based an much more modern), and then for the 'other' stuff try to derive that extra mass from vehicle that had thouse things like capsules or the shuttle orbiter, taking into account the target volumes devoted to these purposes. That splits the difference between considering the vehicle as 'all stage' or 'all capsule'.
Quote from: Lobo on 04/29/2015 04:23 pmSo how did Skylab get into it's orbit? There was no 3rd stage on it.According to what I've read Skylab had a mass of only 75 mT, so their was plenty of performance in the first 2 stages to reach LEO without the 3rd stage, or so I presume.
Staging is ALWAYS going to give performance, not take it away. Having a 2nd stage that did the full Delta-V to LEO and simply dropped the MCT there would indeed be inefficient in the sense that the empty stage mass is mass that could have been vehicle, but I'm expecting the MCT to have some propellent at launch allowing it to do 1-2 km/s after separating from the second stage, that 2nd stage has not in fact been brought to LEO. A 2nd stage like this that is short of orbital speed like this might still go around the Earth once to allow a RTLS, otherwise downrange recovery may be needed.
...even if the savings is 2% (and I think it's quite a bit more), what purpose is served by going to orbit first?
Consumables cached in parking orbit for the return journey: food, batteries, medicine, clothing, spare parts, water, anything and everything that you need for the return but that you don't want to expend the ΔV/impulse for downmass/upmass. You'd need to do a trade to measure taking all that to the surface and back up again as opposed to leaving it in parking orbit and picking it back up on the return. Even if you've recycled or made water on the surface, its heavy to bring up from the surface; leave your water there and pick up this water going uphill. Leave old worn out clothes on the surface (except for what you're wearing); pick up fresh ones from parking orbit. Maybe the trades would say better just to take it all to the surface and back on direct return. Maybe they's show better to park that in orbit and pick it up on the way back. I dunno, but examining the trades is the best way to decide.
Except that it is probably much more fuel effective to land and use the atmosphere for most of the breaking instead of using fuel to enter an orbit.