Author Topic: MCT Speculation and Discussion Thread 4  (Read 505196 times)

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1340 on: 01/13/2016 05:05 PM »
How does the MCT escape module land on Mars w/o killing everyone as it slams into the regolith?  Chutes ain't gonna git 'er done there....
...they will if you add a retro-rocket to them, like this:


They're used every crew mission to the ISS, on the Soyuz lander.

Offline dror

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Re: MCT Speculation and Discussion Thread 4
« Reply #1341 on: 01/13/2016 05:18 PM »
This is my idea of MCT, focusing on survival on all critical parts of Mars Journey, since there will multiple MCT on the way and they will not probably use 100 people before base is build and they could help each others.
That reminds me of the idea which was to fly a F9 reusable 2nd stage together with a dragon 2 as one spaceship.
How does the crew transfer between the escape pod and the hab module through the heat shield?
Is the heat shield necessary for an aerobreak?
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Online docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #1342 on: 01/13/2016 06:02 PM »
This is my idea of MCT, focusing on survival on all critical parts of Mars Journey, since there will multiple MCT on the way and they will not probably use 100 people before base is build and they could help each others.
That reminds me of the idea which was to fly a F9 reusable 2nd stage together with a dragon 2 as one spaceship.
How does the crew transfer between the escape pod and the hab module through the heat shield?
Is the heat shield necessary for an aerobreak?

Gemini-B would have had a hatch in the heat shield.
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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1343 on: 01/13/2016 09:05 PM »
...they will if you add a retro-rocket to them, like this...

That chute will help some after the heat shield reaches terminal velocity, but in an atmosphere that's 0.0059 X the density of Earth's atmosphere at the surface, you're still going to have a lot more velocity to overcome with retrorockets than that pallet has. Your gravity losses will be less, but you still have the inertia to kill off.
Smaller objects have lower terminal velocity and can reach terminal velocity quick enough. And yeah, you need more rocket impulse, but rockets scale well, as long as you're below like 100m/s.
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1344 on: 01/14/2016 02:53 AM »
I would ask that you guys focus for a bit on a problem that's been nagging me: the physical arrangement of mass on an MCT, versus the entry method.   During reentry and descent, the vehicle is going to shuttlecock - the center of mass will end up being in front of the rest of the vehicle.  During launches, shuttlecocking may or may not be an important element of stability, but much less so than during hypersonic entry.  Mass may be shifted around to control orientation, but it's going to need to be a *lot* of mass, and whatever faces forward needs heatshielding.  You can't heatshield engines.

The vehicle may have gliding entry, or may not.  Gliding entry is certainly not as useful during aerocapture.

The biggest deal is getting the engines pointed towards the ground in a retroburn, on the same vehicle that does the aerocapture and entry, without throwing anything away... when the center of mass was supposed to be on the other end of the vehicle.

And then there's the implicit assymmetries: full tanks will dominate mass considerations, but empty ones will shift mass to one end or the other.  Cargo unloading or Matryoshka-staging a subvehicle will drastically change center of mass.  The vehicle has to work in both orientations.

Where does that leave cargo & crew unloading?

I think this might end up being a greater concern than "But then the astronauts would be up in the sky!";  Ladders and lifts are trivial hacks relative to this issue.
« Last Edit: 01/14/2016 03:20 AM by Burninate »

Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #1345 on: 01/14/2016 03:22 AM »
Can it really not do Mars entry engines first? Rocket engines already deal with pretty intense heat...

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1346 on: 01/14/2016 03:43 AM »
Can it really not do Mars entry engines first? Rocket engines already deal with pretty intense heat...

If it was feasible to do so, why did we ever invent ablative heatshields?  Rocket engines deal with heat either ablatively or with regenerative cooling - heating up the propellant - in which case you need to expend the propellant steadily.

The guidance from the SSTO guys is that for Mars, direct retro burns basically overwhelm any drag benefit of the rest of the spacecraft;  You're left with a mass-intensive propulsive descent, as if Mars had no atmosphere.
« Last Edit: 01/14/2016 03:53 AM by Burninate »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1347 on: 01/14/2016 05:37 AM »
My earlier thoughts had been towards a vehicle with cargo in the bottom, tanks above and heat-shields on the nose of an overall conical/bi-conical vehicle.  The entry is nose first with a a considerable lift component, the vehicle is stabilized by flaps at it's base that extend out mechanically, these create a lot of drag and move the center of drag to the read behind the center of mass and allow the vehicle to be steered and adjust to atmospheric conditions without moving mass internally.  After decelerating to under 1 km/s the vehicle flips from horizontal to vertical and lands on retro-rockets.

But I've begun to favor a far large umbrella of carbon-fiber fabric, a concept called ADEPT.  The vehicle would have a similar internal arrangement but the shell opens to become the center of the heat shield and the umbrella which is stowed underneath opens to a huge area for entry in a traditional 70 degree cone, adjusting the arm of the umbrella would give an off sent mass and a lift vector adjustable in real-time during entry.  The vehicle would again make a turn at a lower speed and would retain the umbrella in an open position but inverted so that it becomes an upward cone, some additional decent with deceleration purely from drag would occur and then finally rocket landing.

Offline RanulfC

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Re: MCT Speculation and Discussion Thread 4
« Reply #1348 on: 01/14/2016 05:40 PM »
Can it really not do Mars entry engines first? Rocket engines already deal with pretty intense heat...

If it was feasible to do so, why did we ever invent ablative heatshields?  Rocket engines deal with heat either ablatively or with regenerative cooling - heating up the propellant - in which case you need to expend the propellant steadily.

The guidance from the SSTO guys is that for Mars, direct retro burns basically overwhelm any drag benefit of the rest of the spacecraft;  You're left with a mass-intensive propulsive descent, as if Mars had no atmosphere.

That's because the exhaust creates a virtual "aerospike" which reduces the drag on the vehicle, but has nothing to do with the question. He didn't ask about using the engines but if they could stand the reentry temperatures and therefore have an "engines-forward" entry.

Engineers working on the SERV concept originally estimated that they would need TPS doors to cover the engine nozzles during reentry but subsequent testing showed that especially in SERVs case the overall base diameter was sufficient to reduce the heating load so that no TPS doors or special insulation was required. That's Earth though and Mars is a bit different but you should be able to, if the base is wide enough, to enter engines first as long as they are designed properly and integrated into the base of the vehicle. At worst you can probably cold-flow some propellant through the engine system during the period of highest heating, dumping it through the nozzle and overboard.

The benefit is once you're down far enough you only have to add in the other propellant to the engine and ignite it for the powered landing.

Randy
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Offline JasonAW3

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Re: MCT Speculation and Discussion Thread 4
« Reply #1349 on: 01/14/2016 06:05 PM »
Randy, Speaking of the SSTO designs of the past, has it occured to anyone that the current design for the Dragon V2 could easily be the same basic design, externally, for the MCT?

    If SpaceX were to use a asymetrical aerospike system for each of the quads, the design would be very similar to many of teh SSTO designs of the 1960's and 70's.  I meantion this idea as what most people know is that exhaust effeciency at various altitudes changes as you get higher, resulting in teh need for a radically different exhaust bell than you'd have at sea level.  (Part of the reason we stage rockets).

     Aerospikes don't suffer this issue and asymetric aerospikes, using mostly Raptor components, should prove most effecient for Landings on both earth and Mars and launches from Mars.
     From what I've heard, it seems that the Raptor engines are primarily for the BFR first stage, while the same plumbing could be used for the MCT.  This should also give the advantage of less debris being kicked up under the MCT, as the exhaust would be spread and off to the sides, rather than concentrated underneith the craft, and having to make holes in the TPS.

     Making holes in the TPS adds mass for the hing and closing systems, (which would also have to be shielded against teh exhaust of engines cutting through the TPS) plus it complicates the landing sequence more than needed.

     I have further thoughts on the MCT design that I'll post later.
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Offline RanulfC

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Re: MCT Speculation and Discussion Thread 4
« Reply #1350 on: 01/14/2016 07:33 PM »
Randy, Speaking of the SSTO designs of the past, has it occured to anyone that the current design for the Dragon V2 could easily be the same basic design, externally, for the MCT?

Probably, and probably quite often as well but not mentioned because calling something (or comparing it to, as well) "SSTO" carries a lot of baggage that can and often does get in the way :)

I've seen numerous "guesstimates" of the MCT/BFR design that basically simply scale up the Dragon V2 because it's a known design. (And it's easier to play-with-paint if someone else has made a picture of the design anyway :) )

Quote
    If SpaceX were to use a asymetrical aerospike system for each of the quads, the design would be very similar to many of the SSTO designs of the 1960's and 70's.  I meantion this idea as what most people know is that exhaust effeciency at various altitudes changes as you get higher, resulting in the need for a radically different exhaust bell than you'd have at sea level.  (Part of the reason we stage rockets).

     Aerospikes don't suffer this issue and asymetric aerospikes, using mostly Raptor components, should prove most effecient for Landings on both earth and Mars and launches from Mars.

Well I have to say that one of the first things "I" thought of when we began speculating on the design of the BFR and MCT was the German "BETA" SSTO design and variations thereof. But while true on Earth it's not as important on Mars and landing an MCT on Earth after a mission it's probably even less important. The cosign losses might overwhelm any benefit. On the other hand part of the draw of an aerospike type engine, (specifically any "plug" variety thereof) is an ability to use the "engine" as part of the heat-shield so you don't have two separate systems (and the mass that entails) on the vehicle. You get more advantage with locating the "plug" nearer the center of the heat shield than at the edges.

Still... You can actually use pretty much any engine as the basis of an aerospike or plug nozzle engine as the combustion chamber is the main component with most of the support systems (turbopumps, plumbing, controls, etc) as common parts. Hence we've had designs for J2, RL10, Atlas booster and sustainer, and even F1 engines as the basis for such engines.

Quote
     From what I've heard, it seems that the Raptor engines are primarily for the BFR first stage, while the same plumbing could be used for the MCT.  This should also give the advantage of less debris being kicked up under the MCT, as the exhaust would be spread and off to the sides, rather than concentrated underneith the craft, and having to make holes in the TPS.

     Making holes in the TPS adds mass for the hing and closing systems, (which would also have to be shielded against teh exhaust of engines cutting through the TPS) plus it complicates the landing sequence more than needed.

Actually my point was that the holes in the TPS weren't found to be that much of a problem and no doors were needed. The debris thing could go either way as I've seen comments on arrangements like the Dragon V2 SDs actually kicking debris farther and the more concentrated "blast" actually being easer to adjust for.

Quote
     I have further thoughts on the MCT design that I'll post later.

It's been like 10 minutes already, aren't you done yet? ;)

One of the things that comes to mind when you mentioned this as I noted was some of the work on the BETA SSTO and there was a "more-near-term" version that was actually an "assisted" SSTO using a zero-stage with engines such as you are talking about. However it wrapped a duct around the engines and created a air-augmented rocket system that boosted the thrust and ISP from zero to around Mach-2 where the augmentation dropped off till staging at around Mach-4/6. The augmentation more than covered the mass of the ducts and the ducting helped with landing the stage back at base as well. Of course at this point it begins to look like nothing we've been discussing :)

Randy
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1351 on: 01/14/2016 11:52 PM »
Can it really not do Mars entry engines first? Rocket engines already deal with pretty intense heat...

If it was feasible to do so, why did we ever invent ablative heatshields?  Rocket engines deal with heat either ablatively or with regenerative cooling - heating up the propellant - in which case you need to expend the propellant steadily.

The guidance from the SSTO guys is that for Mars, direct retro burns basically overwhelm any drag benefit of the rest of the spacecraft;  You're left with a mass-intensive propulsive descent, as if Mars had no atmosphere.

That's because the exhaust creates a virtual "aerospike" which reduces the drag on the vehicle, but has nothing to do with the question. He didn't ask about using the engines but if they could stand the reentry temperatures and therefore have an "engines-forward" entry.
I'm pointing out that the cooling mechanism on a regeneratively cooled engine, the thing that keeps it from melting during normal operation, is cold propellant flowing through it rapidly on the way to combustion.  You can't do that without having the engine turned on.  Having the engines directly in the flow poses perhaps the worst aerothermodynamic problem: a parachute-shaped facing surface with sharp frontal edges.
Quote
Engineers working on the SERV concept originally estimated that they would need TPS doors to cover the engine nozzles during reentry but subsequent testing showed that especially in SERVs case the overall base diameter was sufficient to reduce the heating load so that no TPS doors or special insulation was required. That's Earth though and Mars is a bit different but you should be able to, if the base is wide enough, to enter engines first as long as they are designed properly and integrated into the base of the vehicle. At worst you can probably cold-flow some propellant through the engine system during the period of highest heating, dumping it through the nozzle and overboard.

The benefit is once you're down far enough you only have to add in the other propellant to the engine and ignite it for the powered landing.

Randy

You're talking about engines arrayed in a ring around the edge of a large blunt heatshield, whose bow shockwave separation shields them.  These engines would have to have small bell nozzles (lower Isp), be tightly spaced, and have a limited thrust versus a vehicle with record-setting mass per cross-sectional area.
« Last Edit: 01/14/2016 11:54 PM by Burninate »

Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #1352 on: 01/15/2016 03:35 AM »
Can it really not do Mars entry engines first? Rocket engines already deal with pretty intense heat...

If it was feasible to do so, why did we ever invent ablative heatshields?

Well, a small dense capsule is a rather different case from a huge low-density rocket stage. And early reentry vehicles didn't have large rocket engines to use, either.


Quote
Rocket engines deal with heat either ablatively or with regenerative cooling

Radiative cooling also exists. It seems to be mostly nozzles, which suggests it's fairly limited. The question is how the heat flux on the rocket nozzle for a Mars entry compares to the heat flux in use.

Quote
- heating up the propellant - in which case you need to expend the propellant steadily.

How much propellant does it take if you are only cooling and not caring about thrust? Does film cooling change that?

I don't know the answers to any of these questions...

Offline Lampyridae

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Re: MCT Speculation and Discussion Thread 4
« Reply #1353 on: 01/15/2016 06:13 AM »
Can it really not do Mars entry engines first? Rocket engines already deal with pretty intense heat...

If it was feasible to do so, why did we ever invent ablative heatshields?  Rocket engines deal with heat either ablatively or with regenerative cooling - heating up the propellant - in which case you need to expend the propellant steadily.

The guidance from the SSTO guys is that for Mars, direct retro burns basically overwhelm any drag benefit of the rest of the spacecraft;  You're left with a mass-intensive propulsive descent, as if Mars had no atmosphere.

That's because the exhaust creates a virtual "aerospike" which reduces the drag on the vehicle, but has nothing to do with the question. He didn't ask about using the engines but if they could stand the reentry temperatures and therefore have an "engines-forward" entry.
I'm pointing out that the cooling mechanism on a regeneratively cooled engine, the thing that keeps it from melting during normal operation, is cold propellant flowing through it rapidly on the way to combustion.  You can't do that without having the engine turned on.  Having the engines directly in the flow poses perhaps the worst aerothermodynamic problem: a parachute-shaped facing surface with sharp frontal edges.

I know it's probably been mentioned many, many times now but it's something which SpaceX have already accomplished, several times now?

https://www.nasa.gov/press/2014/october/new-commercial-rocket-descent-data-may-help-nasa-with-future-mars-landings/
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1354 on: 01/15/2016 07:21 AM »
Can it really not do Mars entry engines first? Rocket engines already deal with pretty intense heat...

If it was feasible to do so, why did we ever invent ablative heatshields?  Rocket engines deal with heat either ablatively or with regenerative cooling - heating up the propellant - in which case you need to expend the propellant steadily.

The guidance from the SSTO guys is that for Mars, direct retro burns basically overwhelm any drag benefit of the rest of the spacecraft;  You're left with a mass-intensive propulsive descent, as if Mars had no atmosphere.

That's because the exhaust creates a virtual "aerospike" which reduces the drag on the vehicle, but has nothing to do with the question. He didn't ask about using the engines but if they could stand the reentry temperatures and therefore have an "engines-forward" entry.
I'm pointing out that the cooling mechanism on a regeneratively cooled engine, the thing that keeps it from melting during normal operation, is cold propellant flowing through it rapidly on the way to combustion.  You can't do that without having the engine turned on.  Having the engines directly in the flow poses perhaps the worst aerothermodynamic problem: a parachute-shaped facing surface with sharp frontal edges.

I know it's probably been mentioned many, many times now but it's something which SpaceX have already accomplished, several times now?

https://www.nasa.gov/press/2014/october/new-commercial-rocket-descent-data-may-help-nasa-with-future-mars-landings/
No, SpaceX have been sidestepping reentry heating by starting off slow (I'd guess ~2-4km/s at separation instead of ~8km/s from LEO to ~12-14km/s Mars-Earth Transfer) and then spending reentry reducing speed propulsively behind the exhaust plume down to some velocity at which heating is not an issue.
« Last Edit: 01/15/2016 08:13 AM by Burninate »

Offline Falcon H

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Re: MCT Speculation and Discussion Thread 4
« Reply #1355 on: 01/15/2016 02:53 PM »
This is my idea of MCT, focusing on survival on all critical parts of Mars Journey, since there will multiple MCT on the way and they will not probably use 100 people before base is build and they could help each others.
That reminds me of the idea which was to fly a F9 reusable 2nd stage together with a dragon 2 as one spaceship.
How does the crew transfer between the escape pod and the hab module through the heat shield?
Is the heat shield necessary for an aerobreak?

Gemini-B would have had a hatch in the heat shield.
The Space Shuttle also had hatches in it's heat shield for landing gear, I believe. Since the Space Shuttle has flown 133 successful times, I think heat shields like this are very well proven.
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Offline JasonAW3

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Re: MCT Speculation and Discussion Thread 4
« Reply #1356 on: 01/15/2016 03:18 PM »
I'm a bit confused guys...

     When did SpaceX say that the MCT was going to be two sections, a main ship and an escape module?

     While, on the surface, this may seem like a good idea, the extra mass of a second heat shield would add tons of mass that likely wouldn't really be needed, as the MCT itself IS its' own escape craft.
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Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1357 on: 01/15/2016 03:47 PM »
I'm a bit confused guys...
When did SpaceX say that the MCT was going to be two sections, a main ship and an escape module?

You are right, they have not said so.

However, there is a vast number of people who can't conceive of a crewed spacecraft without an abort module, so they believe it must be so.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1358 on: 01/15/2016 05:42 PM »
In my preferred architecture, the tip is not a launch abort precisely, but an Earth return capsule, a barebones backup MAV (which rendevous' with an LMO lifeboat), and maybe other things depending on configuration.  It may also serve as an independent crew launch vehicle to bring crew up to an unmanned MCT;  I'm not finished investigating which mission options are compatible with this, but there are enough of them that it makes a lot of sense to assume a nested capsule up there.

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Re: MCT Speculation and Discussion Thread 4
« Reply #1359 on: 01/15/2016 09:51 PM »
An abort system would have a very limited advantage on Mars descent, ascent and earth reentry. I don't believe they would carry such a system all the way to Mars and back. If it is seen necessary on earth ascent, which I don't, then they would build a separate crew ascent vehicle that can cram 100 people in and get them to safety if necessary. Any excess lifting capability could go to a final top off for MCT.

It would be a lot less useless mass on the way to Mars and back.  So probably not even an additional launch because less fuel is needed.

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