Author Topic: MCT Speculation and Discussion Thread 4  (Read 567241 times)

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #500 on: 08/30/2015 07:05 AM »
If the return delta-v is that high, the vehicle would only ever be fully fueled on Mars, as it doesn't need nearly as much delta-v to get to Mars.

It would likely be fully fueled in both directions. However the payload back to earth would be much lower to achieve the higher delta-v with the same amount of fuel.

Offline symbios

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Re: MCT Speculation and Discussion Thread 4
« Reply #501 on: 08/30/2015 09:12 AM »
If the return delta-v is that high, the vehicle would only ever be fully fueled on Mars, as it doesn't need nearly as much delta-v to get to Mars.

It would likely be fully fueled in both directions. However the payload back to earth would be much lower to achieve the higher delta-v with the same amount of fuel.

Also you could utilize the extra delta-v to make the booster a little smaller and maybe make the trip to Mars  take less time.

It is the leg that requires most delta-v that steers the parameters of the rest.
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #502 on: 08/30/2015 05:07 PM »
If the return delta-v is that high, the vehicle would only ever be fully fueled on Mars, as it doesn't need nearly as much delta-v to get to Mars.

It would likely be fully fuelled in both directions. However the payload back to earth would be much lower to achieve the higher delta-v with the same amount of fuel.

Also you could utilize the extra delta-v to make the booster a little smaller and maybe make the trip to Mars  take less time.

It is the leg that requires most delta-v that steers the parameters of the rest.

Actually, the dry weight of the MCT (and total mass on a return flight) also is very important in this equation since the ΔV budget of a fully fuelled craft could be significantly different when it departs LEO from when it departs Mars surface.
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Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #503 on: 08/30/2015 05:43 PM »
If the return delta-v is that high, the vehicle would only ever be fully fueled on Mars, as it doesn't need nearly as much delta-v to get to Mars.

It would likely be fully fueled in both directions. However the payload back to earth would be much lower to achieve the higher delta-v with the same amount of fuel.

Maybe, but the return payload has to be large enough to accommodate people + life support according to Musk, and less fuel means fewer tanker launches needed to fuel up the vehicle going to Mars.

Assuming vacuum* Isp of 380 s it needs a mass ratio of 8.57 for 8 km/s, vs. about 5.01 for 6 km/s.

*Mars' atmosphere is close enough to vacuum that the numbers shouldn't be too far off.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #504 on: 08/30/2015 06:10 PM »
It would likely be fully fueled in both directions. However the payload back to earth would be much lower to achieve the higher delta-v with the same amount of fuel.

Maybe, but the return payload has to be large enough to accommodate people + life support according to Musk, and less fuel means fewer tanker launches needed to fuel up the vehicle going to Mars.

Yes, people need to go back, but it would not be 100 like on the flight out. The 100 is a value for colonization and going back is the exception. Recently from SpaceX a number was given of 20 or 25t return cargo.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #505 on: 08/31/2015 05:25 AM »
If the return delta-v is that high, the vehicle would only ever be fully fueled on Mars, as it doesn't need nearly as much delta-v to get to Mars.

It would likely be fully fuelled in both directions. However the payload back to earth would be much lower to achieve the higher delta-v with the same amount of fuel.

Also you could utilize the extra delta-v to make the booster a little smaller and maybe make the trip to Mars  take less time.

It is the leg that requires most delta-v that steers the parameters of the rest.

Actually, the dry weight of the MCT (and total mass on a return flight) also is very important in this equation since the ΔV budget of a fully fuelled craft could be significantly different when it departs LEO from when it departs Mars surface.

This, the dry weight is too small and encloses too large of a volume (spreading it very thin) to be able to survive re-entry on Earth which is a minimum of 11 km/s.  Even once we take into account for the 75% reduction in cargo on the Earth return leg.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #506 on: 08/31/2015 05:57 AM »
This, the dry weight is too small and encloses too large of a volume (spreading it very thin) to be able to survive re-entry on Earth which is a minimum of 11 km/s.  Even once we take into account for the 75% reduction in cargo on the Earth return leg.

The volume would be pressurized so is stable when reentering head on. Only when it has slowed down a lot it would pivot over for flying engines first for landing. I have speculated before that they may pressurize for more than 1000 millibar for reentry.

Going for good mass fraction prohibits massive walls like on capsules.

And one of my standard arguments. :) The designers at SpaceX are certainly aware of that problem and have at least tentatively a solution. A Falcon 9 first stage cannot do it because it is too long and slender. A second stage or a MCT has different proportions.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #507 on: 08/31/2015 07:12 AM »
This, the dry weight is too small and encloses too large of a volume (spreading it very thin) to be able to survive re-entry on Earth which is a minimum of 11 km/s.  Even once we take into account for the 75% reduction in cargo on the Earth return leg.

The volume would be pressurized so is stable when reentering head on. Only when it has slowed down a lot it would pivot over for flying engines first for landing. I have speculated before that they may pressurize for more than 1000 millibar for reentry.

Going for good mass fraction prohibits massive walls like on capsules.

And one of my standard arguments. :) The designers at SpaceX are certainly aware of that problem and have at least tentatively a solution. A Falcon 9 first stage cannot do it because it is too long and slender. A second stage or a MCT has different proportions.
This was proposed on one of these threads, but I have no data on its plausibility.  Can you provide any references on the concept of superpressure as lighter-weight substitute for structural strength during reentry?
« Last Edit: 08/31/2015 07:13 AM by Burninate »

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #508 on: 08/31/2015 07:25 AM »
This was proposed on one of these threads, but I have no data on its plausibility.  Can you provide any references on the concept of superpressure as lighter-weight substitute for structural strength during reentry?

We do know pressurization greatly increases strength. It is done for launch loads on tanks routinely. Why would it not apply during reentry? I have not done engineering calculations on it.

Edit: it is my own idea. I did not get it from anywhere. So it may not be practical but I am quite confident.
« Last Edit: 08/31/2015 07:27 AM by guckyfan »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #509 on: 08/31/2015 08:30 AM »
Internal pressure doesn't prevent you from burning up which is my main concern.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #510 on: 08/31/2015 08:43 AM »
Internal pressure doesn't prevent you from burning up which is my main concern.

That's what PicaX is for. At its size I think MCT would be lighter per volume and per surface than a Dragon capsule. Especially with a small cargo entering earth's atmosphere at high speed. The shield would be thick and strong on the tip but could be thin at the sides.

Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #511 on: 09/01/2015 01:14 AM »
I'm sure MCT will have significant PICA-X TPS.

How heavy is a Dragon heat shield?

Googling I find 848 kg for Apollo heat shield, and PICA-X is supposed to be better than previous materials.

Would it necessarily flip over though? The engines are cooled during firing aren't they... maybe it will have engines sticking out of a PICA-X base and enter tail first?

Online docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #512 on: 09/01/2015 03:10 AM »
>
How heavy is a Dragon heat shield?

Googling I find 848 kg for Apollo heat shield, and PICA-X is supposed to be better than previous materials.
>

This old pdf lists the original PICA-X as having a density of 0.27g / cm^3, and cork is about 0.24. Dragon 2 uses PICA-X version 3.
« Last Edit: 09/01/2015 03:22 AM by docmordrid »
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #513 on: 09/01/2015 04:52 AM »
Internal pressure doesn't prevent you from burning up which is my main concern.

That's what PicaX is for. At its size I think MCT would be lighter per volume and per surface than a Dragon capsule. Especially with a small cargo entering earth's atmosphere at high speed. The shield would be thick and strong on the tip but could be thin at the sides.

Volume density is irrelevant, it is the Ballistic coefficient which matters and it would almost certainly be higher then Dragon capsule, show me some calculations to the contrary if you have done some.

It sounds like your proposing a tubular body doing some kind of ballistic plunge directly into the atmosphere like the F9 first stage.  That would result in almost inconceivable dynamic pressure and crushing forces when it reaches the lower atmosphere or Earth.  The star-dust entry capsule for example entered at 11 km/s and experienced ~40g.  And if you tried a similar entry on Mars you'd probably impact the surface.  A slender body vehicle has to use it's sides to get enough drag and that means thermal protection is heavy on the side/s (potentially just one side as with the shuttle).  This trajectory will be longer and actually result in more integrated heat load then the ballistic entry.

PICAX is nice stuff but no one even knows how to seal seems in it yet so it can only be used in monolithic pieces, assuming this limitation is over come their are other issues.  While it is low density it is ablative and will need to be able to withstand 2 entries which means making it thicker as the erosion rate looks to be around 1 - 0.5 centimeters on each entry from interplanetary speeds.  The bonding of all ablatives is basically done with adhesives and isn't designed to be modified so if the vehicle survived and lands on Earth it would mean a complete disassemble of the outer skin and replacement with a new one, a process likely to kill any chance at quick turnaround times.

http://www.academia.edu/10188019/Defining_Ablative_Thermal_Protection_System_Margins_for_Planetary_Entry_Vehicles

This paper looks to be the latest work on modeling for PICA based heat-shields and the necessary safety margins, it looks like new modeling had significantly reduced the need from the conservative Stardust mission.  Estimates are now at 3.5 cm thickness for a comparable single-use mission.  But note that this is not the only mass contributor, their are adhesives, back skins and aluminum honeycombs so total mass will be greater then the ~10 kg/ m2 that the PICA alone accounts for, by how much I don't know.

Offline sublimemarsupial

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Re: MCT Speculation and Discussion Thread 4
« Reply #514 on: 09/01/2015 06:02 AM »

PICAX is nice stuff but no one even knows how to seal seems in it yet so it can only be used in monolithic pieces, assuming this limitation is over come their are other issues. 

I guess these don't count as seams then? SpaceX figured out how to seal PICAX seams on their very first cargo dragon flight back in 2010.

Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #515 on: 09/01/2015 06:07 AM »
Peak heat rates and heat loads of 5g-limited trajectories for various entry velocities and vehicle shapes. One can see that heat loads/peak rates are massively higher for Moon/Mars entry than LEO entry. On the other hand, their estimates for TPS mass do not seem to increase much with velocity ("the TPS mass is derived by assuming a constant thickness forebody heatshield sized to the stagnation-point heating environment, a conservative assumption"). Needless to say the TPS won't be reusable.

From: Entry System Options for Human Return from the Moon and Mars

Note that Mars entry is a lot less demanding in comparison, closer to LEO entry.
« Last Edit: 09/01/2015 06:37 AM by Oli »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #516 on: 09/01/2015 06:14 AM »
Based on http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20000062016.pdf

It looks like metallic TPS would have a mass of 5 kg/m2 in low temperature areas and 10 kg/m2 in high temperature area and this is a fully inclusive mass down to where the TPS meets an aluminum based vehicle structure or tank wall.

My vehicle concept would have a surface area of 650 m^2 of which 150 should be high and 500 low temperature.  Total mass would be 4 mT which is then padded to 5.  This would be the same average density as shuttle but less total mass due to lower surface area, and remember this is intended for an entry speed half that of the shuttle.  The vehicles total entry mass of 215 mT would yield a 2.5% TPS mass which is consistent with the Viking lander.

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #517 on: 09/01/2015 06:21 AM »

PICAX is nice stuff but no one even knows how to seal seems in it yet so it can only be used in monolithic pieces, assuming this limitation is over come their are other issues. 

I guess these don't count as seams then? SpaceX figured out how to seal PICAX seams on their very first cargo dragon flight back in 2010.

Yes, I'm not sure where this idea comes from. PICA-X does not have to be monolithic. Statements like this makes me question your other assertions, Impaler.
« Last Edit: 09/01/2015 06:21 AM by Lars-J »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #518 on: 09/01/2015 07:22 AM »

PICAX is nice stuff but no one even knows how to seal seems in it yet so it can only be used in monolithic pieces, assuming this limitation is over come their are other issues. 

I guess these don't count as seams then? SpaceX figured out how to seal PICAX seams on their very first cargo dragon flight back in 2010.

Yes, I'm not sure where this idea comes from. PICA-X does not have to be monolithic. Statements like this makes me question your other assertions, Impaler.

It comes from the fact that NASA rejected PICA for the use of the Orion capsule due to not being able to make the gap fillers work and they have said this in publications discussing their design choices, they may be suffering from 'not-invented-here' syndrome but it is something they say which is where the idea comes from. 

I expected it to be a solvable problem in any case so if you want to nitpick this tangential remark rather then looking at the core points go right ahead.  I've provided the links to the relevant papers so you can question the assertions of the authors who wrote them as I'm not asserting anything without a source.

No one advocating for these huge MCT concepts had done any kind of TPS mass estimates, they simply chant PICAX and think they are done, even though they can't even site a mass for the Dragon capsule heat-shield.
« Last Edit: 09/01/2015 07:30 AM by Impaler »

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #519 on: 09/01/2015 04:22 PM »
Here's a fully fueled 180mT MCT outbound from LEO (100mT cargo) compared with the launch back from Mars' surface with "only" 25 mT cargo

MCT Dry Wt & Cargo   180   mT
S2 Mass w/MCT   1025   mT  LEO departure.  Mars Return is 75 mT less
S2 Mass w/MCT   2.3   Million LBS
Stage 2 Km/sec    6.48   Km/sec Rocket Equation  LEO
S2 Mars Return 25mT Cargo  8.5Km/sec Rocket Equation

Exponentials help when you reduce the mass.  Just refuel with less propellant to reduce "excess" Km/sec.
The technical challenge is a lightweight MCT vehicle able to withstand Earth re-entry if that is the goal rather than return to some high Earth/moon orbit.

« Last Edit: 09/01/2015 04:23 PM by philw1776 »
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