Author Topic: MCT Speculation and Discussion Thread 4  (Read 503083 times)

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #820 on: 10/17/2015 05:37 AM »
...
I don't think we will actually see 100 day transits mainly do to capture g-force limitations...
What, exactly, are you talking about? Humans can withstand 6 gees for at least 10 minutes, which is 36km/s of delta-v, usually while still doing simple tasks (unnecessary in this case, since capture/entry/reentry would be automated). Such a long period of fairly constant deceleration is possible using a lifting (re)entry, actually including negative lift. Once captured, your next trip through the atmosphere can be for landing (possibly with a skip). So you just need to get rid of your hyperbolic velocity on the first pass because the next pass will take care of actual (re)entry.

I don't think this is any more radical than anything else SpaceX is doing. And isn't more radical than Shuttle's reentry. Sure, we need a good estimate of Mars' atmospheric state, but that is also a solvable problem.

36km/s of hyperbolic velocity is much more than enough for a 100 day transfer.


...lifting (re)entry is well within the state-of-the-art for both Mars and Earth. SpaceX already does this fairly regularly with Dragon. SpaceX also has a very good heatshield material (PICA-X) that they're very familiar with. I don't have any clue why they would shoot themselves in the foot by not leveraging these things to their full potential.

I never said crew was the limit.  The limits are mostly on the vehicle and it's structure, remember that our aerocapture g's in a bi-conic vehicle are negative, aka nose bleed g's so they represent a 2nd direction that the vehicle needs to tolerate, and the more lift their is the more lateral g you get.

Aerocapture of a large mass at mars from a hohmann trajectory (low incoming speed and low DeltaV needed) would be possible with a bi-conic shape, but 100 day transits result in MUCH larger deceleration needs which means you simply need more surface area then the vehicle itself can provide.  So some kind of 'expander' is needed, either an inflatable device (which is disposable) or the magneto which looks to be fully reusable.  The limitation I see with the magneto is that it doesn't produce lift.  That makes the g-forces much higher because your doing a single short burst of drag.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #821 on: 10/17/2015 05:45 AM »
[Dragon's trunk] It is 2/3 of the way to a lifeboat on a biconic MCT.

This is 2/3rds of the way to MCT?



Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #822 on: 10/17/2015 05:49 AM »
and yet people here easily accept that MCT will be capable of landing dozens of people in the same vehicle that was used for the interplanetary transfer, or up to 100 tons of cargo?

There are at least 4 threads on just this topic, in just this section, of people not merely accepting that.

Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #823 on: 10/17/2015 06:20 AM »
The limitation I see with the magneto is that it doesn't produce lift.  That makes the g-forces much higher because your doing a single short burst of drag.

I recall from the relevant thread that the possibility was mentioned that producing lift by making the magnetic braking device unsymmetric should be possible. In that case it even might be able to do a lot of steering and adjusting to different conditions of the local atmosphere.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #824 on: 10/17/2015 07:55 AM »
The limitation I see with the magneto is that it doesn't produce lift.  That makes the g-forces much higher because your doing a single short burst of drag.

I recall from the relevant thread that the possibility was mentioned that producing lift by making the magnetic braking device unsymmetric should be possible. In that case it even might be able to do a lot of steering and adjusting to different conditions of the local atmosphere.

The same thought had occurred to me, but I realized that the combined vehicle would simply pivot to bring the center of mass on the end of the tether into line with the center of drag.  The effect is analogous to deploying two different sized round non-lift producing parachutes at the same time, they would create different amounts of drag but your line angles would simply be skewed such that the load is directly under the center of drag and you would descend vertically.  Or at least this is my mental picture of what would happen when the plasma is not deflecting any air laterally which by definition this can't do because it's drag is entirely from ion collisions which scatter in all directions and end with the incoming gas just being swallowed into the existing plasma vortex, without deflection you can't have lift because you would be violating conservation of angular momentum.

The only means I can see to get lift is the use the primary vehicle air frame as a kind of canard, angling it strongly off the axis of motion and using it's normal lift properties but perhaps at an angle and lift beyond what would be possible if were also the center of drag.  But this is all really counter to the magneto concept which is all about braking at such a high altitude that the vehicle feels almost no heat or dynamic pressure, once your in dense enough air to get lift your also getting all the nasty effects of near re-renty conditions and the vehicle will not be saving as much mass as it could by being built to lower entry profile margins.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #825 on: 10/17/2015 08:06 AM »
and yet people here easily accept that MCT will be capable of landing dozens of people in the same vehicle that was used for the interplanetary transfer, or up to 100 tons of cargo?

There are at least 4 threads on just this topic, in just this section, of people not merely accepting that.

I would like to point out that my own involvement in these threads should not be taken as an acceptance of any of Musk or SpaceX goals being attainable (particularly in 'Musk-time').  I simply lay out what I think would be the most LIKELY means of succeeding at them given the resources they have available.  I use numerous means to try to reduce the required vehicle performance metrics as much as possible as I believe one of the biggest dangers in the design phase of a vehicle is boxing yourself into a design that needs more performance then the underlying technologies and physics allow.

Offline MP99

Re: MCT Speculation and Discussion Thread 4
« Reply #826 on: 10/17/2015 08:35 AM »


The limitation I see with the magneto is that it doesn't produce lift.  That makes the g-forces much higher because your doing a single short burst of drag.

I recall from the relevant thread that the possibility was mentioned that producing lift by making the magnetic braking device unsymmetric should be possible. In that case it even might be able to do a lot of steering and adjusting to different conditions of the local atmosphere.

The same thought had occurred to me, but I realized that the combined vehicle would simply pivot to bring the center of mass on the end of the tether into line with the center of drag.  The effect is analogous to deploying two different sized round non-lift producing parachutes at the same time, they would create different amounts of drag but your line angles would simply be skewed such that the load is directly under the center of drag and you would descend vertically.  Or at least this is my mental picture of what would happen when the plasma is not deflecting any air laterally which by definition this can't do because it's drag is entirely from ion collisions which scatter in all directions and end with the incoming gas just being swallowed into the existing plasma vortex, without deflection you can't have lift because you would be violating conservation of angular momentum.

Yes, that was my takeaway from that thread, also.

Cheers, Martin

Offline Pipcard

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Re: MCT Speculation and Discussion Thread 4
« Reply #827 on: 10/17/2015 11:38 AM »
and yet people here easily accept that MCT will be capable of landing dozens of people in the same vehicle that was used for the interplanetary transfer, or up to 100 tons of cargo?

There are at least 4 threads on just this topic, in just this section, of people not merely accepting that.
I'm sorry, but where exactly would those be? All I seem to see are people speculating on how MCT/BFR would work, and taking such a system for granted (i.e. they don't seem to acknowledge that EDL for heavy payloads is very hard, unlike every other discussion on Mars mission architectures)
« Last Edit: 10/17/2015 03:19 PM by Pipcard »

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #828 on: 10/17/2015 01:59 PM »
...
I don't think we will actually see 100 day transits mainly do to capture g-force limitations...
What, exactly, are you talking about? Humans can withstand 6 gees for at least 10 minutes, which is 36km/s of delta-v, usually while still doing simple tasks (unnecessary in this case, since capture/entry/reentry would be automated). Such a long period of fairly constant deceleration is possible using a lifting (re)entry, actually including negative lift. Once captured, your next trip through the atmosphere can be for landing (possibly with a skip). So you just need to get rid of your hyperbolic velocity on the first pass because the next pass will take care of actual (re)entry.

I don't think this is any more radical than anything else SpaceX is doing. And isn't more radical than Shuttle's reentry. Sure, we need a good estimate of Mars' atmospheric state, but that is also a solvable problem.

36km/s of hyperbolic velocity is much more than enough for a 100 day transfer.


...lifting (re)entry is well within the state-of-the-art for both Mars and Earth. SpaceX already does this fairly regularly with Dragon. SpaceX also has a very good heatshield material (PICA-X) that they're very familiar with. I don't have any clue why they would shoot themselves in the foot by not leveraging these things to their full potential.

I never said crew was the limit.  The limits are mostly on the vehicle and it's structure, remember that our aerocapture g's in a bi-conic vehicle are negative, aka nose bleed g's so they represent a 2nd direction that the vehicle needs to tolerate, and the more lift their is the more lateral g you get.

Aerocapture of a large mass at mars from a hohmann trajectory (low incoming speed and low DeltaV needed) would be possible with a bi-conic shape, but 100 day transits result in MUCH larger deceleration needs which means you simply need more surface area then the vehicle itself can provide.  So some kind of 'expander' is needed, either an inflatable device (which is disposable) or the magneto which looks to be fully reusable.  The limitation I see with the magneto is that it doesn't produce lift.  That makes the g-forces much higher because your doing a single short burst of drag.
You're still speaking entirely in generalities. What sort of g-load do you believe is a limit? You can't make a claim like you're making (that g-loads are prohibitively high for 100 day transits) without giving some numbers.

As a side note, a Falcon 9 first stage already has 8 deployed aerosurfaces that generate drag. You think SpaceX will not be able to have ANY deployed aerosurfaces for MCT?

SpaceX, on the other hand, has never mentioned magnetohydrodynamic drag devices. You are basically handicapping them by disallowing any of their current ways of solving the problem so you can introduce your own pet solution.
« Last Edit: 10/17/2015 02:02 PM by Robotbeat »
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Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #829 on: 10/17/2015 03:21 PM »
If magnetic deceleration can be used it would help a lot with reusability. If a first pass can achieve capture then a second or third pass can achieve landing. Replacing the heatshield after every flight will cost time and money. I don't see magnetic as a requirement but i hope for it. A metallic heatshield will not do especially for EDL back on earth.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #830 on: 10/17/2015 04:43 PM »
If magnetic deceleration can be used it would help a lot with reusability. If a first pass can achieve capture then a second or third pass can achieve landing. Replacing the heatshield after every flight will cost time and money. I don't see magnetic as a requirement but i hope for it. A metallic heatshield will not do especially for EDL back on earth.
Even a PICA-X shield can take multiple entries (Musk said 100, which is an overstatement but is a lot more than MCT will need). And yes, a metallic shield would work, you just have to do it gently enough. But I suspect SpaceX is looking at PICA-X. A tanker going to LEO, most certainly, could use a metallic TPS without any hard limits on numbers of reuses (thousands are possible).
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Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #831 on: 10/17/2015 06:06 PM »
If magnetic deceleration can be used it would help a lot with reusability. If a first pass can achieve capture then a second or third pass can achieve landing. Replacing the heatshield after every flight will cost time and money. I don't see magnetic as a requirement but i hope for it. A metallic heatshield will not do especially for EDL back on earth.
Even a PICA-X shield can take multiple entries (Musk said 100, which is an overstatement but is a lot more than MCT will need). And yes, a metallic shield would work, you just have to do it gently enough. But I suspect SpaceX is looking at PICA-X. A tanker going to LEO, most certainly, could use a metallic TPS without any hard limits on numbers of reuses (thousands are possible).

A metallic heatshield for reentry from interplanetary speeds? I doubt it. PicaX reusable yes. But from interplanetary speeds? To Mars and back, yes, thats two uses, the second very heavy. But again? Maybe two return flights, maybe not.

Edit: I hope I am wrong.

Metallic heatshield for the tankers, that would help a lot, I agree. Many tank flights without refurbishing the heatshield. But it would mean a completely different design for the tanker. The tanker coming back from LEO can probably make a significant number of flights before refurbishing with PicaX.
« Last Edit: 10/17/2015 06:08 PM by guckyfan »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #832 on: 10/17/2015 10:49 PM »
You're still speaking entirely in generalities. What sort of g-load do you believe is a limit? You can't make a claim like you're making (that g-loads are prohibitively high for 100 day transits) without giving some numbers.

As a side note, a Falcon 9 first stage already has 8 deployed aerosurfaces that generate drag. You think SpaceX will not be able to have ANY deployed aerosurfaces for MCT?

SpaceX, on the other hand, has never mentioned magnetohydrodynamic drag devices. You are basically handicapping them by disallowing any of their current ways of solving the problem so you can introduce your own pet solution.

Partly because I'm still doing research on high speed aerocapture, but I'm looking at 2 g's as a limit for both the aerocapture and the EDL to keep the structural masses under control, the terminal landing burn might go up to 4 because this is in the launch orientation and the vehicle must be able to survive that to abort.

Also my design in fact uses considerable deployed aerosurfaces around the base of a bi-conic which form a kind of shuttlecock effect and act as both control flaps during entry and as a decelerator prior to retro-propulsion being applied.  I believe they will be very useful in shifting the center of drag forward (by retracting the flaps) and allowing a flip over maneuver to bring the engines forward to fire. These flaps might prove useful in aerocapture as well but I do not think that even with this technique the vehicle can create enough surface area to do the very high speed aerocapture your looking for.

My preferred trajectory of 150 days still requires shedding 2.786 km/s in aerocapture and has a velocity at mars perigee of 7.3 km/s which nearly that of Earth re-entry.  With 2 minutes in the atmosphere the force would be 2.3 g's.  The Earth return is even worse in terms of velocity, 11.9 km/s a speed I think is only survivable with the magneto taking virtually all the forces so the vehicles metallic skin isn't experiencing significant heating as around 7 km/s looks to be the limit of entry speed for metallic thermal protection systems, any higher and they just get too hot and melt.  The larger radius of the Earth means we should have an easier time making a longer cord through the atmosphere and that should give more time to shed the relatively low 1160 m/s to achieve capture though we may wish to shed more to keep our apogee below the inner Van Allen belt.

By comparison a 100 day transit has a velocity at mars perigee of 13.7 km/s and must shed 9.27 km/s.  Even if I had unlimited size and unlimited thermal protection at that speed the time in the martian atmosphere is on the order of ~1 minute and you would experience 15 g's for that time to get the necessary deceleration.  So not only is the crew a red smear on the back of the spacecraft, the front of the spacecraft is likewise a smear on the back of the spacecraft.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #833 on: 10/18/2015 12:12 AM »
2 gees is way too low. You're essentially sand-bagging aerocapture and (re)entry. Apollo did over 6 gees (Apollo 16 did 7.19).

You could do both an aerocapture and then a(n) (re)entry on both ends, giving you the ability to stretch your deceleration over many minutes and control your peak deceleration. You're going to need to bring a heatshield along with anyway, it would be stupid not to take full advantage of it.

15 gees is not realistic for a lifting aerocapture followed by a lifting entry. In both cases, lift allows you to spread the deceleration over several minutes. Again, you're sand-bagging it.
« Last Edit: 10/18/2015 12:15 AM by Robotbeat »
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #834 on: 10/18/2015 02:38 AM »
I am already assuming separate aerocapture and entry at both ends and the DeltaV I stated were the minimum to capture, aka you go into a big elliptical orbit.

Given the size of the MCT and the mass it has to work with I think 2 gs's is the best that can be reasonably hopped for and it will still in fact be hard to make the vehicle that strong.  Your not taking into account the square-cube law that makes a large object proportionally weaker then a small one.


Apollo was a tiny capsule just a 3.3 m tall and with a mass of 5.5 mT, at 6 g's that would be a compression force of 323400 N.  Now lets just approximate the vehicles structural mass as a cylindrical rod of aluminum (600 MPa strength, 2.8 g/cm^3) running along the axis of compression.  It would need a rod with a mass of just 5 kg, but in reality the capsule had a structural mass of 1,560 kg (28%) because the real limits tend to be the buckling strengths of the much larger structure along with loads from other directions but it gives us a basis for comparison.

Now lets look at a MCT, it will likely have an entry mass of 200 mT when full of cargo, and will be some 20 m long minimum.  The force at 6 g's is 11760000 N.  The abstracted rod of aluminum to support this force over this distance is now 1100 kg!!!  A factor of 220x over Apollo and if it were subject to the same multiplier as in Apollo to get the real structural mass we would be 50% over the whole entry mass just for structure.  Switching to carbon composites (1500 MPa strength, 1.75 g/cm^3) cuts the abstract rod mass down to 274 kg a big improvement but it would still translate to a total structural mass of 85 mT with the 313x multiplier from the Apollo example. 

If the g-forces are reduced to 2 g's the mass comes down proportionally to 28 mT, very close to my vehicle estimated structural mass of 25 mT which would be 33% of the vehicles dry mass and 16% of the entry mass.  So you see that using our best materials can just barely get a vehicle of this size to survive a very modest g loads and why I consider forces as great at 6 g's impossible as they would cause such growth in structural mass that the vehicle expand beyond any possibility of launching it.

Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #835 on: 10/18/2015 07:23 AM »
NASA did studies for Inspiration Mars. They came to the conclusion that direct reentry from Mars at earth return would be the way to go assuming PicaX as a heatshield material. A skip reentry would increase heatshield stress.

I was considering using high pressure to stabilize MCT during reentry. Maybe 40 to 50 PSI or 3 times earth atomospheric pressure. This pressure during the minutes of reentry will not cause problems to crew given what we know from diving.

Offline MP99

Re: MCT Speculation and Discussion Thread 4
« Reply #836 on: 10/18/2015 12:32 PM »


I am already assuming separate aerocapture and entry at both ends and the DeltaV I stated were the minimum to capture, aka you go into a big elliptical orbit.

Given the size of the MCT and the mass it has to work with I think 2 gs's is the best that can be reasonably hopped for and it will still in fact be hard to make the vehicle that strong.  Your not taking into account the square-cube law that makes a large object proportionally weaker then a small one.

MCT will surely undergo greater g forces during launch to LEO.

Cheers, Martin

Offline Dante2121

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Re: MCT Speculation and Discussion Thread 4
« Reply #837 on: 10/18/2015 01:01 PM »
NASA did studies for Inspiration Mars. They came to the conclusion that direct reentry from Mars at earth return would be the way to go assuming PicaX as a heatshield material. A skip reentry would increase heatshield stress.

I was considering using high pressure to stabilize MCT during reentry. Maybe 40 to 50 PSI or 3 times earth atomospheric pressure. This pressure during the minutes of reentry will not cause problems to crew given what we know from diving.

Is there any concept of a reverse gravity slingshot?- e.g. Use the gravity of the moon to slow down before re-entering earth's atmosphere. 

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #838 on: 10/18/2015 02:19 PM »


Is there any concept of a reverse gravity slingshot?- e.g. Use the gravity of the moon to slow down before re-entering earth's atmosphere.

First the moon's orbit is both slow & long and worse yet inclined off plane so the chance of the moon being where you need it to be is tine.

But given that the moon's mass (1/80th Earth) is too small to shed any meaningful velocity.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #839 on: 10/18/2015 03:07 PM »
NASA did studies for Inspiration Mars. They came to the conclusion that direct reentry from Mars at earth return would be the way to go assuming PicaX as a heatshield material. A skip reentry would increase heatshield stress.
Yes, it's true that if you're just trying to minimize heatshield mass using an ablative heatshield, you often just want to go hot and fast. But in our discussion, it's peak acceleration that we're attempting to minimize, not heatshield mass. 6 gees is a pretty good peak acceleration to use. 5 gees has also been used. Nobody that I'm aware of has used 2 gees in any serious analysis. Heck, a /regular/ reentry uses more than 2 gees. So I find that disingenuous.

Quote
...I was considering using high pressure to stabilize MCT during reentry. Maybe 40 to 50 PSI or 3 times earth atomospheric pressure. This pressure during the minutes of reentry will not cause problems to crew given what we know from diving.
Clever idea. I like it. This gives an important point: there is room for innovation in improving structural mass for a given load. Better materials and better manufacturing techniques are developed regularly. I don't think it's wise to sand-bag peak acceleration to something ridiculously low like 2 gees.
« Last Edit: 10/18/2015 03:08 PM by Robotbeat »
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