Author Topic: MCT Speculation and Discussion Thread 4  (Read 623936 times)

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #620 on: 10/07/2015 04:40 AM »
In Early exploration any abort to surface is fatal because their is zero rescue infrastructure on the surface and no conceivable capsule could carry sufficient supplies to see them through more then a few days.  So I consider this a pointless abort. By the time you have an infrastructure to do surface rescue your passenger count is much too high for the small capsule your proposing, it would need to be a large vehicle comprising a significant portion of the whole mass of the vehicle and would present great difficulty in landing as you going to be falling on a ballistic trajectory from a high altitude and need massive retro-propulsion to not impact the surface, it in no way resembles the kind of un-powered capsule landing that can be done on Earth.

This. You put it better than I could have. This is why IMO the way to increase safety for the MCT is to make he whole thing abort capable, through added redundancies in propulsion and systems.

If a MCT has to abort during a Mars ascent and land far down-range, the crew can survive for an *extended* period in the MCT. Not in a small capsule where everyone is squeezed into. Designing in a pointless separable abort capsule leads you down the path of terrible engineering trade-offs.
« Last Edit: 10/07/2015 05:49 AM by Lars-J »

Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #621 on: 10/07/2015 05:35 AM »
I don't know if you saw it earlier but I did a mass brake down of my MCT concept.  I'd like to see what the IBMCT comes out at.


Thermal protection at 5 kg/ m^2 over an area of 650 m^2:  5 mT

Tanks and Plumbing 5% of 300 mT propellant mass:  15 mT

Landing legs, 10% of touch down mass: 18 mT

4 Raptor Engines at 150:1 T:W ratio:  6 mT

Vernier Engines that can hover on landing at 100:1 T:W ratio:  1 mT

Solar, Radiator and computer systems:  5 mT

Structural skeleton, 1/3rd of dry mass: 25 mT

Total 75 mT dry mass

How do you intend to return to Earth with that?

In fact, how do you get 100t payload to Mars with it?

« Last Edit: 10/07/2015 05:38 AM by Oli »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #622 on: 10/07/2015 05:54 AM »
A SEP transit vehicle that takes you to high earth orbit followed by a perigee burn near Earth to send you to Mars, the SEP vehicle flies independently to mars and you rendevoue with it on low mars orbit to return to a high Earth orbit where crew disembark on a Dragon capsule.  The bi-conic just dose mars assent with a 25 mT habitat inside pluss a modest landing propellent reserve. The intent is to ultimatly be able to do a rapid cycle between mars surface and low orbit, loading cargo and orbit and unloading on the surface.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #623 on: 10/07/2015 04:26 PM »
In Early exploration any abort to surface is fatal because their is zero rescue infrastructure on the surface and no conceivable capsule could carry sufficient supplies to see them through more then a few days.  So I consider this a pointless abort. By the time you have an infrastructure to do surface rescue your passenger count is much too high for the small capsule your proposing, it would need to be a large vehicle comprising a significant portion of the whole mass of the vehicle and would present great difficulty in landing as you going to be falling on a ballistic trajectory from a high altitude and need massive retro-propulsion to not impact the surface, it in no way resembles the kind of un-powered capsule landing that can be done on Earth.

This. You put it better than I could have. This is why IMO the way to increase safety for the MCT is to make he whole thing abort capable, through added redundancies in propulsion and systems.

If a MCT has to abort during a Mars ascent and land far down-range, the crew can survive for an *extended* period in the MCT. Not in a small capsule where everyone is squeezed into. Designing in a pointless separable abort capsule leads you down the path of terrible engineering trade-offs.

I'm not sure I agree with that.

Ok, so let's say you have a 12.5m wide IBMCT (our working diameter for it and the booster).  A separable nose would be 12.5m wide at the base, tapering down.  So you'd have quite a lot of volume there for a small crew returning from Mars if they had to abort and land down range.  For 100 people, yea, it's not going to keep them for very long, but for a crew of 5-7?  Should be just fine.  In fact, that may be the only hab space they need/have for exploration missions, with everything below for surface cargo.  That's a volume twice as wide as Skylab op the base, and probably about the same height as the Skylab pressurized volume.  There's no squeezing involved.
They would have provisions and supplies sufficient for the 4-6 month transit back to Earth, so they should be ok for quite awhile.

Then you have the question of what sort of contingency plan do you want to have in place to deal with them at that point.  That's really a separate discussion.  Maybe a remote operated large pressurized rover that could drive itself over to the lifeboat, to give them transportation to a supply cache somewhere pre-positioned for such a contingency?

In a situation where they are transporting 100 people to Mars, obviously 100 people won't be coming back home, so there will be far fewer people on it.  Probably just some SpaceX employees or NASA personnel returning home after a tour of serving at the colony, and a few people who have either become more ill than can be treated on Mars, or have changed their minds and want to go home.   But even if it were more people, with a colony on Mars, rescue could be dispatched anywhere on the globe, it's just a matter of how long it would take to get there, so the lifeboat would need to be set up to support X number of people of Y length of time needed to get rescue there.

As far as whole vehicle abort goes, there really is no such thing for Mars ascent.  If the MCT MPS explodes, a separable lifeboat can save the crew.  Whole vehicle abort would only work for Earth ascent. 
If there's a non explosive failure, like an engine out, that's when having a redundant engine comes in. 
So it's really all or nothing if you don't have a separable design.  And that's ok, the LAS lifeboat is mainly for Earth ascent so the crew can get away from an exploding booster where you cannot abort the whole fueled stage.  But with that comes the ability to abort on Mars if necessary.  But there would need to be contingency plans to for the marooned crew obviously.

In Early exploration any abort to surface is fatal because their is zero rescue infrastructure on the surface and no conceivable capsule could carry sufficient supplies to see them through more then a few days.  So I consider this a pointless abort. By the time you have an infrastructure to do surface rescue your passenger count is much too high for the small capsule your proposing, it would need to be a large vehicle comprising a significant portion of the whole mass of the vehicle and would present great difficulty in landing as you going to be falling on a ballistic trajectory from a high altitude and need massive retro-propulsion to not impact the surface, it in no way resembles the kind of un-powered capsule landing that can be done on Earth.

Not necessarily.  It would only be a smaller portion of the whole weight of the vehicle (Maybe 1/3 total dry mass or so?...but more importantly is it leaves all the propellant mass behind with just the LAS/landing propellant on board).  It would leave behind the main tanks, engines, most of the TPS covering, etc.  It wouldn't be insignificant, but it would be certainly less than the whole vehicle.  The LAS engines and tanks would need to be sized not only for abort, but for propulsive landing.
Also it would be a biconic shape.  So it can do a biconic EDL rather than ballistic.  It would still need a large retro propulsion as any vehicle would, but again, that would have to be designed into the LAS system if you wanted it.

Again, this a concept in reaction to some who feel strongly the LAS is necessary.  (I argued against an LAS, but was out voted, heh)  It could very easily be left off, with no Mars ascent abort option (for an explosive event, you can abort to orbit with just an engine out), and a separate LEO-taxi with LAS for Earth ascent.  For those that favor that, I think that's viable too.
It would probably still need landing thrusters of some sort to land on Earth, which would likely be pressure fed for fast reaction control and reliability.  Otherwise a means of landing on Earth with a vacuum Raptor nozzle would need to be figured out.  Something like a retractable nozzle extension, or a jettisonable nozzle extension, so that the Raptor thrust isn't too over expanded for sea level.  I'm not an engine expert, but have been told by several that vacuum engines with large vacuum nozzles like M1D-Vac and RL-10B cannot operate at sea level due to their large high efficiency nozzles.
But Raptor would still have to be capable of quickly responsive throttle in order to be able to land, which it may not be being a big pump fed staged combustion main propulsion engine.  Otherwise you are back to landing thrusters.





Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #624 on: 10/07/2015 04:41 PM »
Isn't LOX heavier than Liquid methane? Also it will take twice the lox to burn the methane.  So shouldn't the Lox tank be on bottom with the Methane on top?   

That was discussed.  I can't remember the exact discussion but it was determined the LOX would be better placed on top.  But weight isn't always the reason for tank placement.  LOX tanks are on top of LH2 in the Shuttle ET and D4 booster.   And the mass of the LOX in the Shuttle ET was about 6X that of the mass of LH2.  D4 is probably a similar mass ratio.
The guy who did the renderings originally placed the LCH4 on top, but then after discussion, changed it so the LOX was on top.

Volumetrically, I think the LOX and LCH4 tanks will be similar in size.  At least more similar than kerolox and hydrolox tanks.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #625 on: 10/07/2015 04:52 PM »

I've said all along that an elongated 2nd stage would do tanker duty, and depots are in my opinion unnecessary, the MCT will act as it's own depot taking on propellants from visiting 2nd stages until it is full.  As MCT must depart Earth with some propellant for EDL at mars and must then hold significant amounts while on the martian surface (which while cold is still warmer then cryogenic LOX), so the MCT will have to have significant long-term cryo-storage capabilities likely through a combination of insulation and cryo-coolers, thus it makes an excellent depot. 

These cryo-systems along with radiators and solar arrays are the only systems that I would integrated into the vehicle.  Human habitats placed into the MCT are simply plugged into these utilities much like an RV.


That'd be personal preference.  That was my original thought too.  Except, what do you want loitering in LEO while it's being filled up?  A reusable unmanned depot, or your crewed MCT?  I think the preference would be to not have MCT floating around up there any longer than necessary.  Even if you sent the crew up later once it was tanked up, it's just that much more time in LEO to get struck by MMOD.  Just that many more miles on the odometer before the mission even really starts.  Plus you have multiple docking events from the tankers rather than just one from a depot.

And the depot would just be an MCT without the hab section in it.  It's not really -another- vehicle.  Once it's filled up an MCT bound for Mars it can come back down, be refitted, and go back up for the next mission.

As for an active cryo system, MCT with a depot probably wouldn't need one.  It wouldn't be loitering in LEO, and it would burn most of it's propellant shorter after debarking from the depot for the TMI burn.  And it could carry enough residuals to account for boiloff...which shouldn't be too bad with medium cryos like LOX and LCH4. 

Again, it's personal preference.  Neither is right or wrong.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #626 on: 10/07/2015 04:58 PM »

One significant factor that needs to be taken into account for a biconic vehicle (which I advocate) is a the mass distribution. It needs to be have the proper balance when almost empty (normal atmospheric entry & landing), and it needs to be able to also have the proper balance for a near full propellant load. (Earth or Mars abort) Also, when near empty of propellant the vehicle must also be properly balanced for a full cargo load vs empty.

To handle this range, the layout that makes most sense (IMO) is to put the cargo/crew in the middle of the vehicle - with the LOX tank above, and Methane tank below. This would allow a balanced biconic sideways reentry with ANY cargo load, and ANY propellant load.

EDIT: See image below for how that might look, in this DC-Y(?) drawing:

The IBMCT would have most of the dry mass in the nose and MPS as the aft (Engines, thrust structure, etc).  Between the two would empty main propellant tanks, a cargo deck, and volumous, but relatively light Hab area.   The cargo deck would be between the hab volume and the tanks, and may have a fairly heavy mass when loaded with surface cargo.  So for Mars EDL, that would be about in the middle.  So you have your greatest mass areas in the nose, in the tail, and [roughly] in the middle.  So it shouldn't be too bad.  There will be some residuals in the tanks to power Raptor from terminal velocity to hover.

I don't know you need two separate tanks top and bottom like in this concept.
« Last Edit: 10/07/2015 04:59 PM by Lobo »

Online RonM

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Re: MCT Speculation and Discussion Thread 4
« Reply #627 on: 10/07/2015 05:05 PM »
Again, this a concept in reaction to some who feel strongly the LAS is necessary.  (I argued against an LAS, but was out voted, heh)  It could very easily be left off, with no Mars ascent abort option (for an explosive event, you can abort to orbit with just an engine out), and a separate LEO-taxi with LAS for Earth ascent.  For those that favor that, I think that's viable too.

I think SpaceX will be forced to have a LAS on MCT or a LEO-taxi with LAS because of liability issues.

The LEO-taxi also means the 100 passengers won't have to wait in orbit for refueling or any other delays, but it would add extra cost using another vehicle design.

Guys, keep up the great work. There are a lot of interesting MCT concepts in this thread.

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #628 on: 10/07/2015 05:45 PM »

One significant factor that needs to be taken into account for a biconic vehicle (which I advocate) is a the mass distribution. It needs to be have the proper balance when almost empty (normal atmospheric entry & landing), and it needs to be able to also have the proper balance for a near full propellant load. (Earth or Mars abort) Also, when near empty of propellant the vehicle must also be properly balanced for a full cargo load vs empty.

To handle this range, the layout that makes most sense (IMO) is to put the cargo/crew in the middle of the vehicle - with the LOX tank above, and Methane tank below. This would allow a balanced biconic sideways reentry with ANY cargo load, and ANY propellant load. Or to balance cargo above and below propellant tanks.

EDIT: See image below for how that might look, in this DC-Y(?) drawing:

The IBMCT would have most of the dry mass in the nose and MPS as the aft (Engines, thrust structure, etc).  Between the two would empty main propellant tanks, a cargo deck, and volumous, but relatively light Hab area.   The cargo deck would be between the hab volume and the tanks, and may have a fairly heavy mass when loaded with surface cargo.  So for Mars EDL, that would be about in the middle.  So you have your greatest mass areas in the nose, in the tail, and [roughly] in the middle.  So it shouldn't be too bad.  There will be some residuals in the tanks to power Raptor from terminal velocity to hover.

I don't know you need two separate tanks top and bottom like in this concept.

No. If you are truly scaling this to be able to deliver 100t of cargo, you really need to have it in the middle. Not up front with the hab volume. When you do atmospheric entry, the propellant tanks will be mostly empty, so then by placing the cargo up top you are now forcing yourself to have to have a substantial minimum cargo load or the thing won't fly right.

Think about it. 100t. And it could be there, or it could be empty. If you do a sideways re-entry, that DOES constrain you to a center placement of cargo. OR you need to split the cargo into two balanced areas, one below and one above propellant tanks.

Think of MCT as a cargo aircraft. The cargo AND propellant must be balanced properly. So you either have to put the cargo in the middle of two propellant tanks, or the propellant in the middle of two cargo bays.
« Last Edit: 10/07/2015 07:02 PM by Lars-J »

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #629 on: 10/07/2015 05:52 PM »
Except, what do you want loitering in LEO while it's being filled up?  A reusable unmanned depot, or your crewed MCT?  I think the preference would be to not have MCT floating around up there any longer than necessary.  Even if you sent the crew up later once it was tanked up, it's just that much more time in LEO to get struck by MMOD.

There will have to be plenty of loiter time in LEO for MCTs. Why? Because of launch windows to Mars. You will likely want to launch a fleet of them in very close succession, and this will require lots of loitering to place, refuel, & prepare the MCT's in orbit. Having a couple of weeks system checkouts in the relative safety of LEO is also advantageous.

Offline RocketmanUS

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Re: MCT Speculation and Discussion Thread 4
« Reply #630 on: 10/07/2015 07:03 PM »
Except, what do you want loitering in LEO while it's being filled up?  A reusable unmanned depot, or your crewed MCT?  I think the preference would be to not have MCT floating around up there any longer than necessary.  Even if you sent the crew up later once it was tanked up, it's just that much more time in LEO to get struck by MMOD.

There will have to be plenty of loiter time in LEO for MCTs. Why? Because of launch windows to Mars. You will likely want to launch a fleet of them in very close succession, and this will require lots of loitering to place, refuel, & prepare the MCT's in orbit. Having a couple of weeks system checkouts in the relative safety of LEO is also advantageous.
And above the weather for TMI burn.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #631 on: 10/07/2015 07:05 PM »


No. If you are truly scaling this to be able to deliver 100t of cargo, you really need to have it in the middle. Not up front with the hab volume. When you do atmospheric entry, the propellant tanks will be mostly empty, so then by placing the cargo up top you are now forcing yourself to have to have a substantial minimum cargo load or the thing won't fly right.

Think about it. 100t. And it could be there, or it could be empty. If you do a sideways re-entry, that DOES constrain you to a center placement of cargo. OR you need to split the cargo into two balanced areas, one below and one above propellant tanks.

It's 100mt of payload.   That may be 100mt of pure cargo down the road at some point, but for quite some time it will be mixed cargo and crew/hab. 

So you have your IBMCT.  It has it's fairly heavy MPS on the bottom.  Above that you have two stacked cylindrical tanks that go up to about the center of the overall vehicle (when measured from tip to tail)...up a little over half way up the cylindrical portion.  Above that you'll have a cargo deck for mixed flights that is maybe 3 meters tall.  As the cargo will have to be lowered to the surface, none of it can be -too- large in one piece, but it will be heavy overall.  Above that you will have a pressurized hab volume filling the rest of the cylinder, and then another tapered pressurized hab volume in the nose.  If an LAS is required, the nose will have LAS/landing engines it it as well, along with small pressurized tanks to fuel them.   Overall, it will have a fair amount of mass in it.
So again, you'll have a mass area in the tail, amidships, and in the nose. 

With 100mt of pure cargo, and no hab area at all, that may be a little more tricky if all that 100mt is between the nose and the tanks.  They'd probably stow the heaviest pieces just above the tanks, with lighter and lighter pieces above that to help with weight distribution.

But, it's obviously something that'd have to be looked at in more detail by actual SpaceX engineers during the actual design process to see how the real weight distribution will interact with the EDL profile.

This divided concept would have the problem of being nose-light.  The mass will be concentrated in the middle, and at the aft in the MPS.  But given it's tapered overall OML, maybe that still makes for a feasible distribution for EDL?  It's a little above my area.  :-)

With the SDMCT, obviously that's not a problem because it's always vertical.
« Last Edit: 10/07/2015 07:10 PM by Lobo »

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #632 on: 10/07/2015 07:28 PM »
You are still not getting what I'm trying to say. Let me try again.

The MCT must be able to launch, fly, and land with full cargo/crew. But ALSO when no cargo/crew is present.
The MCT must be able to launch, fly, and land with full propellant load. But ALSO with tanks nearly empty.

That places severe constraints on the placement of these elements on a biconic entry vehicle, and you can't just hand-wave that away by a "100t payload vs 100t cargo" semantic discussion. It doesn't matter.
« Last Edit: 10/07/2015 07:33 PM by Lars-J »

Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #633 on: 10/07/2015 07:33 PM »
A SEP transit vehicle that takes you to high earth orbit followed by a perigee burn near Earth to send you to Mars, the SEP vehicle flies independently to mars and you rendevoue with it on low mars orbit to return to a high Earth orbit where crew disembark on a Dragon capsule.  The bi-conic just dose mars assent with a 25 mT habitat inside pluss a modest landing propellent reserve. The intent is to ultimatly be able to do a rapid cycle between mars surface and low orbit, loading cargo and orbit and unloading on the surface.

I think SEP from LEO to HEO is no-go if you want to fly MCT every synod. How much time do you think will be left after returning to Earth? (looking at all opportunities).

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #634 on: 10/07/2015 08:05 PM »
I'm rather surprised that you aren't aware of the Pendulum Fallacy
But I don't know that my stated bit about inherent stability is incorrect.  Just not exactly as I stated.
In that, would it not be similar to a helicopter?  Why is a helicopter more stable

Helicopters aren't gravitationally stabilised. If you roll to one side and then let go of the controls, helicopters do not come back to level. They are perfectly happy to roll over and fly straight into the ground.

than something like the LLRV?

Remember that the Flying Bedstead was not being fully controlled by the pilot. Instead the jet engine was to simulate partial gravity for a system intended for 1/6th g (which had never actually been flown and was being controlled by flight systems that were one step above clockwork.) Once it tilted too far off centre, the jet was no longer reducing gravity, it was driving you to one side while you fell to the ground. And none of the "lunar" thrusters were powerful enough to actually offset a full-gravity without that jet. Give the designers another five years and a half-dozen version, and I'm sure they would have come up with something safer... But, of course, that wasn't its purpose.

By contrast, the LEM also had its engines underneath, but was much more stable. By your argument, it should have been as twitchy as the LLRV, but it wasn't.

And in this way I think they'd act more like the F9 core grid fins, which are placed up high to provide force (via air resistance rather than a jet of thrust) to help stabilize the core for landing.

The grid fins are at the top for the opposite reason. The rocket is descending engines-first, hence the grid-fins are at the "back" of the rocket during that phase of flight, essentially playing the role of a "tail" on a conventional aircraft. However, like a tail on a plane, the grid-fins don't result in the rocket having any preferred "neutral" orientation. Ie, it won't "hang down" from the grid-fins, you have to actively control it to keep the rocket vertical.

The lifeboat would be [....] A flight deck in the nose with a bulkhead and hatch between it and the rest of the hab.  Not much more to it that that.  Not quite like a separate spacecraft.

It has to be a separate spacecraft, or it can't operate as a escape vehicle. Such a double-vehicle would become hideously complex. Particularly if you are trying the use the escape vehicle's engines as landing engines for the entire MCT. The forces on the connectors, which must be instantly separable during launch-abort, would be ridiculous.

For early exploration missions Dragon 2 would be sufficient, no separate system needed as crews will only be maybe 6-7.

I assume you mean that the early crews would ferry up to an MCT in LEO using D2? (Since D2 can't launch back off the surface of Mars. It will never be used for human missions to Mars.)

In which case, you are proposing an entirely different kind of MCT just for the first few missions. Your escape vehicle can't be retro-fitted to an existing MCT design. You can't just cut through a few joins connecting the flight-deck to the rest of the MCT and add some pyro-bolts. You have the design the entire MCT around the separation mechanism. That's not going to be an afterthought or upgrade.

Others thought it needed it, as later during colonization they'd want to launch all 100 people on the MCT rather than have a separate ferry.
I argued against an LAS, but was out voted, heh

Sounds like there's some group-think going on. Alternative suggestions are shouted down by a few dominant voices and their followers.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #635 on: 10/07/2015 08:12 PM »
10 or 100 people per flight.
10 people means 10 times the number of manned flights. Which means 10 times the likelihood of a flight with crew loss. Will people say, oh well, it is only 10 people, not 100, that's OK? I doubt it.

However, with extra crew ships on the same route, if there is a failure of a major system on one of the ships, those 10 people can be spread amongst the remaining 9 ships. Additionally, during the emergency, they'd have nearby external help operating out of safe, fully functional ships, rather than trying to save themselves from within the failing ship using failing systems (with the nearest advice operating behind several minutes comms lag.)

With a single crew, no chance of rescue, any major failure means LOM/LOC.

The advantage of that 9-fold backup, IMO, is worth its weight in diamonds.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #636 on: 10/07/2015 08:18 PM »
10 or 100 people per flight.
10 people means 10 times the number of manned flights. Which means 10 times the likelihood of a flight with crew loss. Will people say, oh well, it is only 10 people, not 100, that's OK? I doubt it.

However, with extra crew ships on the same route, if there is a failure of a major system on one of the ships, those 10 people can be spread amongst the remaining 9 ships. Additionally, during the emergency, they'd have nearby external help operating out of safe, fully functional ships, rather than trying to save themselves from within the failing ship using failing systems (with the nearest advice operating behind several minutes comms lag.)

With a single crew, no chance of rescue, any major failure means LOM/LOC.

The advantage of that 9-fold backup, IMO, is worth its weight in diamonds.

You are assuming the major risk is in the cruise phase. I assume it is during launch, TMI and landing,

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #637 on: 10/08/2015 12:20 AM »

I assume you mean that the early crews would ferry up to an MCT in LEO using D2? (Since D2 can't launch back off the surface of Mars. It will never be used for human missions to Mars.)

In which case, you are proposing an entirely different kind of MCT just for the first few missions. Your escape vehicle can't be retro-fitted to an existing MCT design. You can't just cut through a few joins connecting the flight-deck to the rest of the MCT and add some pyro-bolts. You have the design the entire MCT around the separation mechanism. That's not going to be an afterthought or upgrade.


First, it won't be just a few missions.  Probably all the missions for the first couple decades.  There will be many exploration missions with just a small crew before they can possibly think about actual colonization with large numbers of colonists.  You have to explore various potential location looking for a promising site with favorable conditions for a colony.  Then you have to test out the new systems which the colony will use, and test out resource collection, etc.  Not to mention I think it highly likely NASA would jump in bed with them as soon as it were to look likley they could land people on Mars.  They'll provide fund which will be beneficial for SpaceX, but they'll have their own agenda of places they want to go too. 

And you can only fly out every 2 years.  So, after maybe 20 years of Mars mission, you are ready to put 100 colonists on an MCT, what do you do then?
That's the question.

And I think you are viewing this like the Space Shuttle.  There are just a handful ever built.  Any change would require it to be broken apart and remodeled.  But I think there would be more than that built for MCT.  So it's certainly possible to have a "Block 2" MCT which would have the LAS systems installed and some design changes to facilitate it. 
SpaceX isn't modifying D1's to make them D2's, they are building new D2's with the new systems in them.  SpaceX is no more tied to their first MCT design than they are to their first Dragon design.

So an LAS doesn't have to be in the exploration-class design, if deemed unwanted.  It could be added to a Block 2 Colony Transport.  Old Block 1 MCT's could continue to be used for Cargo service.

Or a separate LEO-Taxi version could be developed specifically around an Earth launch abort system, so an LAS wouldn't need to be added to MCT. 
Or maybe after 20 years of operation, the booster is just deemed reliable enough that with 1 redundant engine for an engine out contingency, it's just deemed an LAS system is not critical enough to account for.
My original preference was for the IBMCT was one extra engine on the booster, and an additional engine on the IBMCT, so that you are covered for an engine out on the booster and/or on the IBMCT after staging.  Along with other redundant systems, the crew can safely get to orbit in most failure modes other than explosive failure.  And deem this "safe enough" as by then, there will be some 20 years of flight history to get it reliable.

Options abound.  Again, it was a way to address concerns of those who feel strongly that MCT must have an LAS system on an integrated MCT design.  If you don't like it, then you can favor the SDMCT or NIBMCT which launch empty on a dedicated S2 and abort the whole vehicle...or something else entirely.  That means you have two spacecraft to design from the start rather than one, but that's certainly an option.  And an option originally favored by the team.  My IBMCT kinda grew on them over time.  :-)


Offline UberNobody

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Re: MCT Speculation and Discussion Thread 4
« Reply #638 on: 10/08/2015 12:31 AM »
Just as a reminder, we have a thread full of direct quotes about MCT.  Most everything recent is there. 

http://forum.nasaspaceflight.com/index.php?topic=37839.0
« Last Edit: 10/08/2015 11:37 PM by UberNobody »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #639 on: 10/08/2015 01:00 AM »
You are still not getting what I'm trying to say. Let me try again.

The MCT must be able to launch, fly, and land with full cargo/crew. But ALSO when no cargo/crew is present.
The MCT must be able to launch, fly, and land with full propellant load. But ALSO with tanks nearly empty.

That places severe constraints on the placement of these elements on a biconic entry vehicle, and you can't just hand-wave that away by a "100t payload vs 100t cargo" semantic discussion. It doesn't matter.

I've given some though to mass distribution in my design.  First off my some what naive understanding of a bi-conic (or any entry vehicle) is that you want the center of mass forward of the center of aerodynamic lift or drag.  In essence we have a shuttlecock and we drag is primarily in the rear and the mass in the front so it remains pointed properly into the wind and doesn't disastrous tumble over.  This was one of the biggest dangers in Shuttle re-entry because it was heavy in the rear and often had nothing in the cargo bay making it unstable.

First thing is that I did was give the vehicle a considerable cylindrical hollow 'skirt' below the two conic frustums, these would be a perimeter of hinged body flaps which allow a lot of drag to be generated far in the rear of the vehicle and most importantly allow the amount of drag to be adjusted in flight based on mass distribution, atmospheric conditions and as a general decelerator the the lower martian atmosphere.

Second propellant tankage is divided between a forward tank  set in the first frustum and saddle tanks in the second lower frustum of the vehicle which flank the cargo hold.   All tanks are full only for the assent from mars when the vehicle is a standard assent rocket, during any entry the propellant loads will be small enough for significant mass shifting of mass probably preferably placing it towards the tip of the vehicle.



I'm not sure I agree with that.

Ok, so let's say you have a 12.5m wide IBMCT (our working diameter for it and the booster).  A separable nose would be 12.5m wide at the base, tapering down.  So you'd have quite a lot of volume there for a small crew returning from Mars if they had to abort and land down range.  For 100 people, yea, it's not going to keep them for very long, but for a crew of 5-7?  Should be just fine.  In fact, that may be the only hab space they need/have for exploration missions, with everything below for surface cargo.  That's a volume twice as wide as Skylab op the base, and probably about the same height as the Skylab pressurized volume.  There's no squeezing involved.
They would have provisions and supplies sufficient for the 4-6 month transit back to Earth, so they should be ok for quite awhile.

Then you have the question of what sort of contingency plan do you want to have in place to deal with them at that point.  That's really a separate discussion.  Maybe a remote operated large pressurized rover that could drive itself over to the lifeboat, to give them transportation to a supply cache somewhere pre-positioned for such a contingency?

In a situation where they are transporting 100 people to Mars, obviously 100 people won't be coming back home, so there will be far fewer people on it.  Probably just some SpaceX employees or NASA personnel returning home after a tour of serving at the colony, and a few people who have either become more ill than can be treated on Mars, or have changed their minds and want to go home.   But even if it were more people, with a colony on Mars, rescue could be dispatched anywhere on the globe, it's just a matter of how long it would take to get there, so the lifeboat would need to be set up to support X number of people of Y length of time needed to get rescue there.

As far as whole vehicle abort goes, there really is no such thing for Mars ascent.  If the MCT MPS explodes, a separable lifeboat can save the crew.  Whole vehicle abort would only work for Earth ascent. 
If there's a non explosive failure, like an engine out, that's when having a redundant engine comes in. 
So it's really all or nothing if you don't have a separable design.  And that's ok, the LAS lifeboat is mainly for Earth ascent so the crew can get away from an exploding booster where you cannot abort the whole fueled stage.  But with that comes the ability to abort on Mars if necessary.  But there would need to be contingency plans to for the marooned crew obviously.

In Early exploration any abort to surface is fatal because their is zero rescue infrastructure on the surface and no conceivable capsule could carry sufficient supplies to see them through more then a few days.  So I consider this a pointless abort. By the time you have an infrastructure to do surface rescue your passenger count is much too high for the small capsule your proposing, it would need to be a large vehicle comprising a significant portion of the whole mass of the vehicle and would present great difficulty in landing as you going to be falling on a ballistic trajectory from a high altitude and need massive retro-propulsion to not impact the surface, it in no way resembles the kind of un-powered capsule landing that can be done on Earth.

Not necessarily.  It would only be a smaller portion of the whole weight of the vehicle (Maybe 1/3 total dry mass or so?...but more importantly is it leaves all the propellant mass behind with just the LAS/landing propellant on board).  It would leave behind the main tanks, engines, most of the TPS covering, etc.  It wouldn't be insignificant, but it would be certainly less than the whole vehicle.  The LAS engines and tanks would need to be sized not only for abort, but for propulsive landing.
Also it would be a biconic shape.  So it can do a biconic EDL rather than ballistic.  It would still need a large retro propulsion as any vehicle would, but again, that would have to be designed into the LAS system if you wanted it.

Again, this a concept in reaction to some who feel strongly the LAS is necessary.  (I argued against an LAS, but was out voted, heh)  It could very easily be left off, with no Mars ascent abort option (for an explosive event, you can abort to orbit with just an engine out), and a separate LEO-taxi with LAS for Earth ascent.  For those that favor that, I think that's viable too.
It would probably still need landing thrusters of some sort to land on Earth, which would likely be pressure fed for fast reaction control and reliability.  Otherwise a means of landing on Earth with a vacuum Raptor nozzle would need to be figured out.  Something like a retractable nozzle extension, or a jettisonable nozzle extension, so that the Raptor thrust isn't too over expanded for sea level.  I'm not an engine expert, but have been told by several that vacuum engines with large vacuum nozzles like M1D-Vac and RL-10B cannot operate at sea level due to their large high efficiency nozzles.
But Raptor would still have to be capable of quickly responsive throttle in order to be able to land, which it may not be being a big pump fed staged combustion main propulsion engine.  Otherwise you are back to landing thrusters.


It seems to me that this abort capsule is nearly the size of the entire Separate Bi-conic vehicle I'm imagining.  In essence the split is right along the lines of the 2nd stage and the Bi-conic where I'd just have two separate vehicles.

One of the main issues I see with mars abort capsules is that they need to have basically full mars EDL capability, it needs thermal protection, it needs flight control, it needs lots of retro-propulsion, it needs landing legs and radar to acquire the ground and soft-land.  In fact because the abort capsule is on and unplanned trajectory it's angle of entry into the atmosphere is uncontrolled and atmospheric entry angle is critical to making the process survivable. 

This is a KEY consideration when comparing a direct Earth return vehicle to one ascending to LMO.  The direct return vehicle doesn't arc over, it just goes strait up trying to achieve mars escape.  If you abort from that before escape velocity your now just falling strait down on mars and that is an un-survivable entry due to g-forces alone, the lift provided by a bi-conic shape is perpendicular the the surface and just shifts your point of impact on the surface, it's only useful if your coming in basically horizontal to the surface and the lift is countering gravity and allowing you to bleed speed horizontally.  A vertical entry isn't survivable on Earth either, only by going on a mostly horizontal trajectory intended for orbit do we manage to survive aborts in capsules as we re-enter at a glancing angle. 

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