Author Topic: MCT Speculation and Discussion Thread 4  (Read 621798 times)

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #40 on: 06/16/2015 02:33 AM »
I do expect the tanker to be different. No payload or crew quarters. Just stretched main tanks. That's a lot more mass efficient. But early on for the first few missions or in a test phase they may use MCT for that purpose too.

Why stretched main tanks?

The lighter that configuration of MCT would be without any crew accomodations, the more propellant it has left when arriving in LEO.  No need to stretch the tanks.  Plus better to keep the basic tank/skin/structure common over all variants, I would think.

The cargo should be shorter, except for light high volume payloads.  A bit like the joined image
« Last Edit: 06/16/2015 02:34 AM by lamontagne »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #41 on: 06/16/2015 03:28 AM »
Why do you differentiate between Earth reentry and Mars reentry? I like most people assume it is one and the same. Some disagree, I know.

No these aren't comparable at all, in fact their are perhaps 6 different entry scenarios.  http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040086716.pdf


Mars from LMO                           3.5 kms

Mars from Interplanetary Slow      6 kms
Earth from LEO                           7.5 kms
Mars from Interplanetary Fast      8 kms

Earth from Interplanetary Slow    12 kms !
Earth from Interplanetary Fast     14 kms !!


Any Mars Entry also requires significant rocket retro-propulsion to not crash, where as on Earth you can basically just do very small amounts of touchdown retro-propulsion because terminal falling velocity in the lower atmosphere is subsonic.

That brutal 12-14 kms entry to Earth is something everyone who is talking about this direct Earth return is glossing over, that is beyond Apollo speeds, the only thing that can survive that kind of heat, dynamic pressure and g-force is a dense capsule with thick heavy ablatives.

This is why it is not valid to design spacecraft by only looking at Delta-V and tank sizes and imagining that a giant 2nd stage can do the job of direct Earth return from Mars surface just because it can hold the propellents to launch to Earth.  It would literally be crushed like an empty beer can against ones forehead when it hits the Earth's atmosphere.

A 2nd stage that can return from Earth orbit is a vastly simpler thing to do because the speed is half (and the energy is a quarter), and it is fairly easy to slow down the 2nd stage by several kms with residual propellents, and to employ disposable things like parachutes because it only needs to perform ONE landing before servicing rather then two, and lastly it can be made much less reliable in landing because it's unmanned, no one dies horribly if it crashes or burns up on reentry unlike MCT.
« Last Edit: 06/16/2015 03:32 AM by Impaler »

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #42 on: 06/16/2015 03:41 AM »
Why do you differentiate between Earth reentry and Mars reentry? I like most people assume it is one and the same. Some disagree, I know.

No these aren't comparable at all, in fact their are perhaps 6 different entry scenarios.  http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040086716.pdf


Mars from LMO                           3.5 kms

Mars from Interplanetary Slow      6 kms
Earth from LEO                           7.5 kms
Mars from Interplanetary Fast      8 kms

Earth from Interplanetary Slow    12 kms !
Earth from Interplanetary Fast     14 kms !!

Once supersonic retropropulsion is used, the differences become much smaller. The hardest reentry is back to earth. If it can do that, Mars speed is no problem. Negative lift is needed on Mars. but that is no more than some attitude control. Also much more fuel. If the vehicle has that fuel = delta v Mars landing is no problem. On earth landing needs a lot less fuel due to the dense atmosphere.

So there are the two major differences. More attitude control for Mars EDL, more capacity of the heatshield for earth reentry.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #43 on: 06/16/2015 04:48 AM »
The hardest reentry is back to earth. If it can do that, Mars speed is no problem.

But it CAN'T that's the rub, the vehicle can't be the large low density tank people are imagining, it would be crushed.  The Apollo heat shield alone was 15% of the mass, the structure was 27% and this was to for a compact and easy to protect shape.  So we can't just wave our arms and say MCT will be able to do this.

How much retro-propulsion do you think we can do upon return to Earth?  Any propellent to do this with is added to our DeltaV from Mars surface which is already 6-7 kms for a direct Earth return.  The propellent fraction is already near the limits of credibility.

Offline Krevsin

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Re: MCT Speculation and Discussion Thread 4
« Reply #44 on: 06/16/2015 05:50 AM »

At these kinds of packaging densities you would need 1200-5000 m^3 to use that mass effectively for launching satellites.  But their is no way the vehicle can have such a huge cargo hold, it would make the overall vehicle too large and require too much structural mass to make it survive re-entry.
This is an entirely wrong assumption. An Ariane V, per your words, has ~390 cubic meters of volume and can manage to bring two satellites into GTO and costs about $200 million per launch, at a price of $100 million per satellite. A falcon 9 has ~275 cubic meters and can get a single satellite into GTO in fully expendable mode at a price of $90 million per launch/satellite.

Therefore, the only thing a MCT needs to do in order to be competitive as a satellite delivery platform is lower the cost of getting satellites into GTO, not increase the amount of satellites into GTO and since the goal of the MCT is to be rapidly reusable, a "gas & go" type of system, the only costs involved with the launch would be processing, operation and the cost of the methalox rather than building an entire vehicle, the MCT is cheaper and if the internal volume of its cargo bay is only equal to that of an Ariane V fairing, it can still get two satellites into GTO so the only way a MCT is not competitive is if the cost of its launch approaches the $200 million mark.

And given the MCT's much touted price tag of $50 million per 100 people ($500 000 per passenger) I just don't see that as a likely event.
Commercial communication satellites are LOW DENSITY, just look at the size of current payload fairing and you can see that their is no way you could put 100 mT of satellites into the kind of volumes were looking at for a MCT cargo-hold.

Falcon 9 payload fairing has a volume of ~275 m^3 and it launches only 5 mT to GTO, Ariane 5 has ~390 m^3 and launches a maximum of 12 mT to GTO.  Shuttle had ~300 m^3 payload bay and could carry 24 mT to LEO. 

At these kinds of packaging densities you would need 1200-5000 m^3 to use that mass effectively for launching satellites.  But their is no way the vehicle can have such a huge cargo hold, it would make the overall vehicle too large and require too much structural mass to make it survive re-entry.

Volume is VERY important, MANY space launch systems face volume limitations, Dragon capsule for example is volume rather then mass limited for most cargoes that need to be launched to ISS.
You would never pack 100 metric tonnes' worth of satellites in a MCT.

Mostly because an MCT without refuelling can do much less than that in tonnes to GTO. (I remember that a number of 10-15 tonnes was mentioned somewhere in the previous topic, which amounts to 2 conventional satellites or 3-4 SEP ones, so about an Ariane 5 worth of payload to GTO). The reason why you'd want to do this without refuelling is to lower operating costs and complexity of the mission.

So you pack it with as many satellites as you can given the volume and mass constraints and that still makes the volume of the MCT a non-issue because it's still cheaper than the alternatives and has enough of a capability for at least 2 satellites.

The only time when the volume of a MCT becomes an issue is if you consider a commercial depot stationed in LEO, to which the MCT could deliver its full payload in mass and from which tugs would take over. But even then it'd still be cheaper

Offline The Amazing Catstronaut

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Re: MCT Speculation and Discussion Thread 4
« Reply #45 on: 06/16/2015 06:51 AM »

But it CAN'T that's the rub, the vehicle can't be the large low density tank people are imagining, it would be crushed.  The Apollo heat shield alone was 15% of the mass, the structure was 27% and this was to for a compact and easy to protect shape.  So we can't just wave our arms and say MCT will be able to do this.

How much retro-propulsion do you think we can do upon return to Earth?  Any propellent to do this with is added to our DeltaV from Mars surface which is already 6-7 kms for a direct Earth return.  The propellent fraction is already near the limits of credibility.

What about retro burning before the MTC hits the troposphere - how much of a burn would that require to be effective? Is there any way lunar gravity could be helpful for negating some of that velocity?
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Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #46 on: 06/16/2015 07:13 AM »
How much retro-propulsion do you think we can do upon return to Earth?  Any propellent to do this with is added to our DeltaV from Mars surface which is already 6-7 kms for a direct Earth return.  The propellent fraction is already near the limits of credibility.

None, of course. It will all be done by the heat shield. None, that is, except the few seconds landing burn, maybe 200m/s delta-v. And PicaX is not only vastly better than Avcoat, it is also vastly lighter.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #47 on: 06/16/2015 11:15 AM »
MCT is a radical design. Acknowledge that and move on.
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Offline AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #48 on: 06/16/2015 12:06 PM »
Using a BFR and MCT to launch satellites is comparable to using the Queen Mary as an ore barge.
We should also acknowledge that and move on.

It will also refuel at least twice, so Apollo-like 'missions' to Mars are off the table... for SpaceX anyway.
« Last Edit: 06/16/2015 12:11 PM by AncientU »
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #49 on: 06/16/2015 12:37 PM »
Using a BFR and MCT to launch satellites is comparable to using the Queen Mary as an ore barge.
We should also acknowledge that and move on.

It will also refuel at least twice, so Apollo-like 'missions' to Mars are off the table... for SpaceX anyway.
I'm sure you would be better off using the Queen Mary to haul ore than throwing away part of an ore ship each time. You can always build a freighter version, just like the cargo 747 or whatever.

But yeah, MCT will be refueled at least twice. (well, once for sure... Has to be refueled on Mars... But a lightweighted version could possibly take a slow trip to Mars on a single shot.)
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Offline R7

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Re: MCT Speculation and Discussion Thread 4
« Reply #50 on: 06/16/2015 12:40 PM »
This is why it is not valid to design spacecraft by only looking at Delta-V and tank sizes and imagining that a giant 2nd stage can do the job of direct Earth return from Mars surface just because it can hold the propellents to launch to Earth.  It would literally be crushed like an empty beer can against ones forehead when it hits the Earth's atmosphere.

Try crushing a beer can containing several bars of pressure against your forehead.
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Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #51 on: 06/16/2015 12:49 PM »
Try crushing a beer can containing several bars of pressure against your forehead.

Or better - don't.  :)

Offline spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #52 on: 06/16/2015 01:50 PM »
If the MCT is completely refueled on Mars for launch back to earth, all of the fuel will not be needed to get the TEI burn.  There will be quite a bit of fuel left.  So the MCT could fire and slow down before earth reentry, or it could slow down using aerocapture for a few orbits to slow down.  Also, there will probably be only a crew return, not 100 people. 

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #53 on: 06/16/2015 02:19 PM »
No. Retropulsion before reentry on Earth isn't going to happen with chemical propulsion. It's always better to improve the heatshield.
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Offline JamesH

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Re: MCT Speculation and Discussion Thread 4
« Reply #54 on: 06/16/2015 02:44 PM »
No. Retropulsion before reentry on Earth isn't going to happen with chemical propulsion. It's always better to improve the heatshield.

Why?

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #55 on: 06/16/2015 02:55 PM »
Mass penalty is way too high. Increasing the required delta-v for the Mars stage from 7-7.5km/s to ~10km/s would double the required mass, and that's assuming no increase in tankage mass.

Just no.

PICA-X is crazy awesome stuff. Just use a little more of it.
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Offline RonM

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Re: MCT Speculation and Discussion Thread 4
« Reply #56 on: 06/16/2015 02:59 PM »
Take advantage of Earth's thick atmosphere. A PICA-X heatshield can handle the high speed reentry and will be lighter, less complicated, and cheaper than any retropulsion system.

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #57 on: 06/16/2015 03:07 PM »
Mass penalty is way too high. Increasing the required delta-v for the Mars stage from 7-7.5km/s to ~10km/s would double the required mass, and that's assuming no increase in tankage mass.

Just no.

PICA-X is crazy awesome stuff. Just use a little more of it.

Might it be possible to refuel the MCT in Mars orbit?  Not the first ones, but once the transit system is set up?  There could be a few ships permanently at Mars, operating as SSTO's hauling up fuel, and a used MCT would make a good orbital tanker, as it already has the cooling systems required to prevent evaporation.  If the MCT (most of the times) goes back empty of crew, with no radiation shielding, that should allow for pretty high deltaV?

Depends a lot on how complicated orbital refueling turns out to be, I expect.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #58 on: 06/16/2015 03:09 PM »
The first stage is a different story. The delta-v penalty is far less for first stage retropulsion and RTLS.
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Online Doesitfloat

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Re: MCT Speculation and Discussion Thread 4
« Reply #59 on: 06/16/2015 03:10 PM »
Yeah but,..
Any chance they would use both.  We have seen the results of firing an engine in a braking burn. (F-9 booster recovery attempts)  Also they showed computer modeling of hypersonic reentry with engine firing.  ISTM the results show the engine pushes the superheated plasma from reentry, away from the spacecraft.

So say they use an appropriately sized engine to push the plasma away from the spacecraft, but the majority of the braking is done by the atmosphere.  Would lead to less wear on the pica-X heat shield and is reusable by refueling the engine.

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