Author Topic: MCT Speculation and Discussion Thread 3  (Read 348204 times)

Offline Semmel

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Re: MCT Speculation and Discussion Thread 3
« Reply #320 on: 04/21/2015 09:45 pm »
One more question regarding the MCT: How do engines and a heat shield work together? I can only come up with bad ideas:

1. The engines are the heat shield. They need to be active during reentry, otherwise they would not survive.

2. The engines are mounted like the superdracos on Dragon 2. But that would probably conflict with the large engine bells that are required.

3. There is a deployable heat shield of some sort.

4. The engines are deployable and sit on arms that move outwards from the centre body of the MCT. Adds a lot of failure modes.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 3
« Reply #321 on: 04/21/2015 11:26 pm »
It fully depends on the overall design of the MCT itself but it COULD be as simple as the top being a modified DragonV3 or some such.


The "Ship within a ship" concept that has been mentioned before is certainly a possibility.  I tend to doubt it for a few reasons, but there's not technical reason it couldn't be so that I can thing of. 
Mainly I think that almost any abort scenario that the ship within a ship concept can handle, having the MCT itself be the lifeboat, and build extra redundancy into it, also can handle.  Neither can save the crew from an exploding fuel tank on Mars ascent.   So I just don't know that you really gain much from it. 
Perhaps moreso for Earth ascent, but not much for Mars ascent....that I can see anyway.


They are "hover-slamming" a recoverable booster stage... Again it depends on what the MCT design is but it could be as simple as parachutes and a set of solid retro rockets to cushion the landing shock ala Soyuz. It's not that complicated really.

 You'd need far more than Soyuz's meager retro burn to keep the crew alive.  Probably better just to go all propulsive rather than mess with a chute at all in this case.  I don't think a chute will supply enough dV to be worth having.  The crew will die one way or other if the landing engines don't work, but with a chute, the crew will die of either the parachute -or- landing engines don't work.  Probably better just to put more fuel in the engines and land fully propulsively


Engine out capability I would figure as a given as it's been SpaceX policy since the failures of the Falcon-1 so I don't see a reason it wouldn't apply to the MCT. (And every reason that it would in fact) I would reject the notion that the MCT would go to TEI if had a failure though. Any engine failure and I highly doubt THAT MCT will be going to Mars or Earth. They will abort and try again when the engines are fixed. It would make more sense (even early on) to abort back to the surface of Mars as a more survivable option. It's the failure where the MCT can not in fact land itself where some sort of LAS would come into play. IF you assume the entire MCT is something like a giant Dragon capsule then a life boat LAS makes little sense of course but that also assumes that the MCT CAN act as its own LAS which is not clear given a need to re-propellant on-orbit unless its using up it's own abort propellant to GET to orbit. (Both at Earth and Mars)

I think you missed my point here some.  If there was an engine out during Earth ascent. MCT could still make it to LEO.  An "abort to orbit".  That would be much more desirable than trying to land somewhere downrange, because they can they orbit back to a reentry window that will take them back to the launch site.  But I'm sure if an MCT engine failed before LEO, they would scrub the mission.
However, after that, they are committed.  If an engine fails during the TMI burn, they could perhaps correct to a free return trajectory and come back, but it might be better to go to the surface and make use of an existing contingency MCT already waiting there (ala Mars Direct).
If an engine fails during EDL, they also can then transfer to the contingency MCT on the surface for the return home.  If an engine fails during Mars ascent, you could abort back to the surface if early enough, and then transfer to the contingency MCT, but once you are too far up I'd think you'd just keep buring all the way through TEI, as you still have sufficient operational engines, and with a cluster of 5, actually could withstand another engine failure.


Continuing the 'logic' we note that the DragonV2 can use its propulsion to land on Earth IF it doesn't abort, now carry that through to MCT. If it has to abort from the BFR, yet normal operations require it to be refueled in Earth orbit, will it be able to land BACK on Earth if has to use its on-board propellant to abort? Or are we going to assume a change in SpaceX policy and figure they will just "write-off" the crew and passengers if a failure occurs?

??
First, I'm talking of sending up a crew on an MCT that's outfitted specifically for crew transfer.  That means, it wouldn't get to LEO empty, as the Mars MCT loaded with cargo would.  It would get to LEO with enough propellant to land itself again.  Same with the tanker MCT's that would fill the MARs-MCT up in orbit.  They all need enough residual propellant to land...or else they'd be expendable.  Fotuantely, that won't require that much propellant. 
So let's say MCT aborts while on the booster.  The booster is shut down, and probably legs deployed to help slow it down, MCT fires it's engine early and does an emergency separation.  As the booster will probably typically stage about the time F9 does now, MCT could do a RTLS abort up until staging.    In this abort mode, MCT may "fly" itself around a bit to burn off enough propellant so that it doesn't land too heavy as it would have a full propellant load during such a booster abort.

Once the booster is staged normally, an MCT engine failure after that could be tolerated (with engine out capability) and MCT aborts to orbit, as the shuttle did on STS-51F.  At that point it would orbit around to the proper return window and return to launch site. 

Why does any of this involve SpaceX just "writing off" the crew and passengers?  MCT would always have enough propellant to land.  I'm not sure what's unclear?


In my mind there's no way that the entire MCT is going to have a LAS but that the MCT can act as its own LAS is possible as is a portion of the vehicle being a "life-boat" type escape vehicle. That would make good sense in any situation where the abilities of the MCT to reach Earth or Mars orbit come into effect. (In most of THOSE cases there is not saving the MCT anyway and your only option is to abandon ship) The trade comes in figuring where those scenarios come to the point where your crew has time to take action or not and figuring from there.


I agree, I just don't think that any sort of separate lifeboat could really protect against an exploding booster or MCT.  -Maybe- an exploding booster as there may be enough distance between the booster and lifeboat, but if the MCT tanks explode, the crew, even in a lifeboat on top, will still be very close to them.  (but, as always, I could be wrong.  ;-)  )
And if it can't protect against an exploding MCT, then why have it?  Just make MCT the lifeboat, as I outlined.  It would be like STS, only safer in that your TPS system wouldn't be subject to debris shedding from the ET.  You could still have a Challenger like incident, but not a Columbia like one.  At least not one caused by tank debris shedding. 

This would assume that there's no pad abort options, again, like STS.  Maybe a full up lifeboat could protect against something like that.  An Antares like failure where it lifts off but then fails to achineve enough lift and stalls.  It would be pretty hard to give MCT enough power to get itself up and away from that situation.  But then again, is a lifeboat launch compartment in MCT the best way to handle that?  Or just have a separate crew launch vehicle?  I can't see early missions having more than 5-10 people.  Dragon v2 could put 7 people on a Mars-MCT, and it'd already have an LAS system capable of pad abort, and exploding-booster abort.  Two Dragons could put a crew of 14 on an MCT bound for Mars. 
A decade or two later when there might actually be colonization, maybe they either make a Dv3 that can take colonists up to MCT en masse?  Or they make a version of MCT that actually has a full propulsive LAS system capable of doing a pad abort?  Put enough engines on it and it'll get itself up and off the booster, and be designed to have enough propellant left to land somewhere near the pad, away from the explosion.  Make a "super dragon" version of MCT for mass passenger transport?


Offline Lobo

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Re: MCT Speculation and Discussion Thread 3
« Reply #322 on: 04/21/2015 11:32 pm »
One more question regarding the MCT: How do engines and a heat shield work together? I can only come up with bad ideas:

1. The engines are the heat shield. They need to be active during reentry, otherwise they would not survive.

2. The engines are mounted like the superdracos on Dragon 2. But that would probably conflict with the large engine bells that are required.

3. There is a deployable heat shield of some sort.

4. The engines are deployable and sit on arms that move outwards from the centre body of the MCT. Adds a lot of failure modes.

Agreed.  So you resolve this by not having the engines in the heat shield at all.  That gets back to the biconic aeroshell with engines at the aft.  Essentially a wingless Spaceshuttle that lands on it's SSME's. 


Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 3
« Reply #323 on: 04/22/2015 12:19 am »
You need an LAS when you launch from Mars as much (or as little) as you do when launching from Earth. Sending crew up separately is thus pointless.

???

How exactly would an LAS system operate/beenfit MCT when launching itself off the surface of Mars?  I don't get that.  If it suffers a catesphrophic explosion, there's no way to get the crew away as it's all one spacecraft....
One just lacks a certain imaginative creativity if one can't figure out any way to do it.

Anyway, I wasn't trying to say a LAS should be used for Mars ascent, but if you're dead set on using one for Earth ascent, you might as well get it to work for Mars ascent, too. Yes, it is possible. The thin atmosphere of Mars and the fact that it's fairly easy to just cut the engines on a liquid rocket make it possible. Everyone should probably be suited up, though.
« Last Edit: 04/22/2015 12:34 am by Robotbeat »
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 3
« Reply #324 on: 04/22/2015 12:24 am »
...
Also when leaving Mars, what is the point of having a LAS? When the rocket fails, it and all its resources will be lost. How can the people survive on Mars without it? So to have a LAS on Mars to make sense, the colony infrastructure must have the margin to support the aborted colonists until a resupply ship arrives and they can be brought back with the next MCT. Is that realistic? Especially in the beginning?
In the context of a colony, it's absolutely realistic.

If SpaceX is serious about large scale Mars settlement (and every indication is that they will be), then except for the first few synods, a single extra set of MCT passengers wouldn't be even the slightest burden. What you're doing here is carrying over thinking from when we used to only think about Mars exploration in terms of a half dozen or so people on a base. That isn't relevant except in the slimmest corner case for the MCT.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 3
« Reply #325 on: 04/22/2015 12:27 am »
...However, I've never seen a Mars plan which would have any different result on Mars Ascent.  NASA's DRM 5.0 plans don't have any sort of LAS system on the MAV, nor did Zubrin's Mars Direct or Semi-Direct (at least that I recall). 
...
Of course not, because no other detailed Mars plan was seriously considering full scale colonization. MCT isn't being built for mere exploration, it's being built to shuttle people (and equipment) to and from a colony. It's right in its name.
« Last Edit: 04/22/2015 12:28 am by Robotbeat »
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Offline spacenut

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Re: MCT Speculation and Discussion Thread 3
« Reply #326 on: 04/22/2015 12:32 am »
Earth's LEO is about 100-120 miles or so.  What is Mars LEO?  If one can achieve orbit on Earth in say 5-10 minutes or so, Mars should be about 40 miles and 2-5 minutes.  Might not really need a LAS if it is a fairly quick launch. 

Offline Elmar Moelzer

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Re: MCT Speculation and Discussion Thread 3
« Reply #327 on: 04/22/2015 03:11 am »
One more question regarding the MCT: How do engines and a heat shield work together? I can only come up with bad ideas:

1. The engines are the heat shield. They need to be active during reentry, otherwise they would not survive.

2. The engines are mounted like the superdracos on Dragon 2. But that would probably conflict with the large engine bells that are required.

3. There is a deployable heat shield of some sort.

4. The engines are deployable and sit on arms that move outwards from the centre body of the MCT. Adds a lot of failure modes.
There are plenty of base first reentry concepts for SSTOs as well. Gary Hudson would be able to tell about those {Phoenix SSTO comes to mind).
One could also arrange them away from the base as a kind of aerospike/truncated spike/plug. There were concepts for that (Aerospike Test Vehicle).

Offline Hyperion5

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Re: MCT Speculation and Discussion Thread 3
« Reply #328 on: 04/22/2015 03:16 am »
One more question regarding the MCT: How do engines and a heat shield work together? I can only come up with bad ideas:

1. The engines are the heat shield. They need to be active during reentry, otherwise they would not survive.

Can't say this seems like a good idea, though obviously with enough testing it might be mastered. 

2. The engines are mounted like the superdracos on Dragon 2. But that would probably conflict with the large engine bells that are required.

Well considering the vehicle will be landing on Mars, you actually don't need large engine nozzles to get great Isp out of staged combustion methane-oxygen engines in a near vacuum.  Best example?  The closest known engine design to the Raptor is an under-design (seemingly perpetually so) engine from KBKhA called the RD-0162.  It would put out 226 tf of thrust in a vacuum with an Isp of 356 seconds.  Scaling up to a Raptor gets you about a 1.75 m nozzle.  Luckily there are two things going on that make this much more manageable. 

1) You don't need that large of a nozzle, as a SC methalox engine can produce 345-350 seconds of Isp out of a more compact nozzle. 
2) The MCT is going to be a very wide craft (14-17 meters in diameter is very plausible).  Proportionately 8 Raptor engines with SL-optimized nozzles should fit just fine into the sides of such a vehicle. 

3. There is a deployable heat shield of some sort.

This is a bad idea?  Aside from one of our L2 experts being enamored of the idea, you'll have to explain to me why NASA engineers and other visionaries have used this approach multiple times in their concepts.  See the attached pictures. 

4. The engines are deployable and sit on arms that move outwards from the centre body of the MCT. Adds a lot of failure modes.

Can't say I'd go for this.  The winning option is either #3 on its own or a combination of options 2 & 3. 


Agreed.  So you resolve this by not having the engines in the heat shield at all.  That gets back to the biconic aeroshell with engines at the aft.  Essentially a wingless Spaceshuttle that lands on it's SSME's.

Given the number of designs with engines embedded behind deployable heat shield doors, I would think the "wingless" Space Shuttle design is far from given.  The simplicity of landing with a Dragon-style vertical lander on Mars is not something to be given up lightly. 
« Last Edit: 04/22/2015 03:41 am by Hyperion5 »

Offline sanman

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Re: MCT Speculation and Discussion Thread 3
« Reply #329 on: 04/22/2015 03:53 am »
So will MCT use "hoverslam" when landing on Earth and Mars? Musk said it's going to be using "many engines", with the same throttleability limits in both cases.

But while Mars has lower gravity than Earth, it also has greater terminal velocity. So is it automatically the case that for Mars landing fewer engines will be used, and for Earth landing more of those engines would be used?

Since MCT will be bigger and heavier, will the thrust-to-weight ratio be > 1 under the minimum throttle value?

Are vehicular mass constraints going to dictate the engine configuration, or will engine configuration constraints dictate the vehicular mass?


Offline sanman

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Re: MCT Speculation and Discussion Thread 3
« Reply #330 on: 04/22/2015 04:02 am »
Earth's LEO is about 100-120 miles or so.  What is Mars LEO?  If one can achieve orbit on Earth in say 5-10 minutes or so, Mars should be about 40 miles and 2-5 minutes.  Might not really need a LAS if it is a fairly quick launch.

Can you even have LAS for SSTO? I thought MCT on Mars is going to be SSTO.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 3
« Reply #331 on: 04/22/2015 05:15 am »
Earth's LEO is about 100-120 miles or so.  What is Mars LEO?  If one can achieve orbit on Earth in say 5-10 minutes or so, Mars should be about 40 miles and 2-5 minutes.  Might not really need a LAS if it is a fairly quick launch.

Can you even have LAS for SSTO? I thought MCT on Mars is going to be SSTO.
Unpressurized escape pod.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 3
« Reply #332 on: 04/22/2015 05:24 am »

2) The MCT is going to be a very wide craft (14-17 meters in diameter is very plausible).  Proportionately 8 Raptor engines with SL-optimized nozzles should fit just fine into the sides of such a vehicle. 

This is what my money is on, 4 pairs of 2 engines just like Dragon, this gives huge thrust for escaping an exploding booster (at 2,300 kN per raptor, MCT could mass up to 370 MT while still having the T:W of ~5 like Dv2), huge redundancy for landing on any pair of opposing engines and keeps the interior volume of the vehicle clear and the heat-shield monolithic.  Power to weight ratios of SpaceX engines are high and this means the performance cost of high and even surplus engines is mitigated substantially, at 100:1 T:W these 8 raptors only mass 18 MT.

Note that Lobo's earlier 5 engine configuration in which a central engine penetrates the heat-shield and is used for landing isn't viable because that central engine can't gimble, which is absolutely necessary for controlling the hover-slam.  The engine-through heat-shield concept is only viable for high altitude deceleration and even then it's though that the engines will need to be angled to the side so the vehicles remains stable and doesn't 'wobble' on it's own plume, when landing you simply must have a gimbling engine or multiple engines with adjustable thrust.

Lastly several people have commented that no abort is needed for 'cargo' variant MCT because their are no people on board to save.  But this ignored that SpaceX would VERY MUCH like to save the MCT itself from being destroyed, so it can you know be REUSED (provided that is is a whole vehicle escape system as with Dragon).  Having no LAS for a cargo carrying rocket is the normal thing in expendable rocketry because they are equally 'gone' if the launch is a success or failure, but it dose not hold that it's superfluous upon a reusable vehicle.  On Earth we have lots of systems to to keep trains from derailing and we use these systems on both passenger and freight trains because we want to reuse both types of trains many times, so it seems reasonable that we have one variant that hauls everything and has a single abort system.

Offline hkultala

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Re: MCT Speculation and Discussion Thread 3
« Reply #333 on: 04/22/2015 06:04 am »

2) The MCT is going to be a very wide craft (14-17 meters in diameter is very plausible).  Proportionately 8 Raptor engines with SL-optimized nozzles should fit just fine into the sides of such a vehicle. 

This is what my money is on, 4 pairs of 2 engines just like Dragon, this gives huge thrust for escaping an exploding booster (at 2,300 kN per raptor, MCT could mass up to 370 MT while still having the T:W of ~5 like Dv2),

No, it does not. It gives practically no thrust for escaping exploding booster. It only gives huge thrust couple of seconds too late.

High-pressure staged combustion has just too slow thrust response to be useful as abort motor.

Pressure fed or solids are good for abort motors. (also hybrids would work but they seems to have other problems)
« Last Edit: 04/22/2015 06:06 am by hkultala »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 3
« Reply #334 on: 04/22/2015 07:35 am »
I mentioned the spin up time issue on an earlier post, should have mentioned it again.  Some modification of the engine would be need to get the start time of a pressure fed system, I'm thinking small pressure tanks to feed the engine (bypassing the turbo-pump) for a second or two until the spin up of the pump is complete.  Yes this is speculative engineering on my part.

Offline R7

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Re: MCT Speculation and Discussion Thread 3
« Reply #335 on: 04/22/2015 12:53 pm »
I mentioned the spin up time issue on an earlier post, should have mentioned it again.  Some modification of the engine would be need to get the start time of a pressure fed system, I'm thinking small pressure tanks to feed the engine (bypassing the turbo-pump) for a second or two until the spin up of the pump is complete.  Yes this is speculative engineering on my part.

If you suddenly raise the pressure inside combustion chamber by pressure feeding (requires additional injectors for LOX in ORSC) and igniting propellants how do you stop the combusting gases from flowing upstream via regular fluid routes to pump and turbine outlets? And where do you direct all TPA pump outlets and turbine exhaust during spin up, those don't want to go into the combustion chamber its pressure is greater?
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Offline JasonAW3

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Re: MCT Speculation and Discussion Thread 3
« Reply #336 on: 04/22/2015 01:43 pm »

2) The MCT is going to be a very wide craft (14-17 meters in diameter is very plausible).  Proportionately 8 Raptor engines with SL-optimized nozzles should fit just fine into the sides of such a vehicle. 

This is what my money is on, 4 pairs of 2 engines just like Dragon, this gives huge thrust for escaping an exploding booster (at 2,300 kN per raptor, MCT could mass up to 370 MT while still having the T:W of ~5 like Dv2), huge redundancy for landing on any pair of opposing engines and keeps the interior volume of the vehicle clear and the heat-shield monolithic.  Power to weight ratios of SpaceX engines are high and this means the performance cost of high and even surplus engines is mitigated substantially, at 100:1 T:W these 8 raptors only mass 18 MT.

Note that Lobo's earlier 5 engine configuration in which a central engine penetrates the heat-shield and is used for landing isn't viable because that central engine can't gimble, which is absolutely necessary for controlling the hover-slam.  The engine-through heat-shield concept is only viable for high altitude deceleration and even then it's though that the engines will need to be angled to the side so the vehicles remains stable and doesn't 'wobble' on it's own plume, when landing you simply must have a gimbling engine or multiple engines with adjustable thrust.

Lastly several people have commented that no abort is needed for 'cargo' variant MCT because their are no people on board to save.  But this ignored that SpaceX would VERY MUCH like to save the MCT itself from being destroyed, so it can you know be REUSED (provided that is is a whole vehicle escape system as with Dragon).  Having no LAS for a cargo carrying rocket is the normal thing in expendable rocketry because they are equally 'gone' if the launch is a success or failure, but it dose not hold that it's superfluous upon a reusable vehicle.  On Earth we have lots of systems to to keep trains from derailing and we use these systems on both passenger and freight trains because we want to reuse both types of trains many times, so it seems reasonable that we have one variant that hauls everything and has a single abort system.

Interesting thought here;  If the MCT lander is configured similar to the Dragon V2 in engimne layout, it may be possible to hang a low mass cargo carrier with a one shot heat shield under the main craft, in many ways, like the Dragon trunk.

     If so, should a MCT lander have to abort to orbit, it could eject the cargo module, allowing it to either crash or attempt a parachute descent independant of the main craft itself, thus freeing up fuel to abort to orbit.

     Should the MCT lander and cargo module successfully land, one could detach the Lander, loft it up a few hundred feet and land nearby.  The cargo could then be offloaded and the module could be sealed up and used as a habitat module.  (This also assumes a "stand off" distance above the martian surface as well as the use of the one shot heat shield as an insulating layer between the surface and the module itself).

     The Lander would then be refueled from IRSU sources, for it's flight back to Earth.
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Offline spacenut

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Re: MCT Speculation and Discussion Thread 3
« Reply #337 on: 04/22/2015 02:25 pm »
In order for SpaceX to avoid having to build multiple launch pads for BFR and MCT, I think the MCT maximum diameter should be 12m if they are to use Kennedy and the BFR maximum 12 million lbs thrust.  Otherwise a lot of expense would have to go out to build launch facilities to handle multiple launches in a narrow window of time.  NASA might agree to build pad 39C that was originally planned or give up their SLS pad for dual launches from Kennedy. 

So I think MCT will be 12 m and not like a capsule but like the wingless shuttle idea combining 2nd stage with MCT.  This would require possibly refueling in LEO with fuel being launched to a large station able to control boil off during the off months when not going to Mars and back.

Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 3
« Reply #338 on: 04/22/2015 03:51 pm »
3. There is a deployable heat shield of some sort.

This is a bad idea?  Aside from one of our L2 experts being enamored of the idea, you'll have to explain to me why NASA engineers and other visionaries have used this approach multiple times in their concepts.  See the attached pictures. 

I think its probably a bad idea - but maybe it turns out to be the least bad option.

Single point of failures are bad, even a free return trajectory would not help as the heatshield would be needed at Earth as well.

Perhaps with multiple redundant actuators and deployment early enough so that there is time to sort out problems (in extreme by a spacewalk) it would be acceptable.

But even if it is acceptable on its own as a feature, it would preclude other options, e.g. using the main engines for landing.

Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 3
« Reply #339 on: 04/22/2015 03:57 pm »
2. The engines are mounted like the superdracos on Dragon 2. But that would probably conflict with the large engine bells that are required.

Well considering the vehicle will be landing on Mars, you actually don't need large engine nozzles to get great Isp out of staged combustion methane-oxygen engines in a near vacuum.  Best example?  The closest known engine design to the Raptor is an under-design (seemingly perpetually so) engine from KBKhA called the RD-0162.  It would put out 226 tf of thrust in a vacuum with an Isp of 356 seconds.  Scaling up to a Raptor gets you about a 1.75 m nozzle.  Luckily there are two things going on that make this much more manageable. 

1) You don't need that large of a nozzle, as a SC methalox engine can produce 345-350 seconds of Isp out of a more compact nozzle. 
2) The MCT is going to be a very wide craft (14-17 meters in diameter is very plausible).  Proportionately 8 Raptor engines with SL-optimized nozzles should fit just fine into the sides of such a vehicle. 

Engines mounted on the sides would use differential thrust for steering. A SC engine has a rather slow throttle response and so would be difficult to use in that situation.

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