Author Topic: MCT Speculation and Discussion Thread 4  (Read 611042 times)

Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #2720 on: 09/01/2016 10:29 AM »
Isn't the simplest solution to foreign object damage an engine that is robust enough to survive the odd pebble? And maybe some redundancy in case one engine breaks?
I have a really hard time to imagine how something could enter an active combustion chamber against the flow of the combusted fuel.

Not all engines are active on landing. But probably all are required for launch when fully fuelled. At least when engine out capability on launch is to be maintained.

Offline Oersted

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Re: MCT Speculation and Discussion Thread 4
« Reply #2721 on: 09/01/2016 12:04 PM »
I think the answer to this issue is landing on bedrock or somewhere else deemed reasonably safe. Extra engines will just be dead weight which you can't afford under the constraints of the mission profile. Spacex have shown with Dragon 2 what I think will be the probable approximate configuration for the MCT. We'll all be much wiser in a few days...


Offline jpo234

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Re: MCT Speculation and Discussion Thread 4
« Reply #2722 on: 09/01/2016 12:22 PM »
I think the answer to this issue is landing on bedrock or somewhere else deemed reasonably safe.

Is "landing on bedrock" compatible with ISRU, e.g. would you have access to water?
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Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #2723 on: 09/01/2016 12:45 PM »
I think the answer to this issue is landing on bedrock or somewhere else deemed reasonably safe.

Is "landing on bedrock" compatible with ISRU, e.g. would you have access to water?

All 40 potential landing sites, considered during the NASA workshop, had both water, some of them huge amounts of glacial water and hard flat surfaces for landing.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #2724 on: 09/02/2016 08:07 AM »
Isn't the simplest solution to foreign object damage an engine that is robust enough to survive the odd pebble? And maybe some redundancy in case one engine breaks?

The odd pebble?



9 Merlins, roughly the same force generated as three Raptors. People are anticipating up to five Raptors on the lander.

So picture something with 2/3rd more power than what is depicted here, sitting on landing legs, a metre or two above hard flat surface. Even bedrock would spall. And it's not just damage to the engines, you also can't damage that heat-shield either, because you'll need it to get the ship back to Earth.

In NASA's proposals (DRA 5.0), the Earth return vehicle is vastly smaller, and launched from a "launch platform" made from its own landing system. Similar to the two-stage ascent/descent modules on the Apollo LM.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #2725 on: 09/02/2016 08:19 AM »
Aside: No engine retesting on Mars before relaunch back to Earth. Three months in deep space, slammed into Mars-entry, supersonic retropropulsion, landing on - at best - a rock surface. Sitting in a dusty, cold, semi-vacuum during refuelling for however long that takes, and where the atmosphere freezes out at the temperatures you are using for the fuel. Plus there might be repair/refurb performed by the crew, "Sensors say valve four was sticky during landing".

But you can't test your engines before a launch into a direct Earth return trajectory, because there's no infrastructure that can withstand the test (or hold down the vehicle.)

Or would you perform a short vertical (sub-orbital) hop to test the engines before final fuel and re-launch?

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #2726 on: 09/02/2016 08:21 AM »
Perhaps this is better. Low-powered Merlin 1a testing nozzle ablative.

« Last Edit: 09/02/2016 08:23 AM by Paul451 »

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2727 on: 09/03/2016 12:17 AM »
Isn't the simplest solution to foreign object damage an engine that is robust enough to survive the odd pebble? And maybe some redundancy in case one engine breaks?

The odd pebble?....
So picture something with 2/3rd more power than what is depicted here, sitting on landing legs, a metre or two above hard flat surface. Even bedrock would spall. ...
They did this above concrete, which spalls. Short turnaround time.
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Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2728 on: 09/03/2016 12:27 AM »
But you can't test your engines before a launch into a direct Earth return trajectory, because there's no infrastructure that can withstand the test (or hold down the vehicle.)

Actually you could test the engines if they were sufficiently canted, and the ship was fully fuelled. For a T/W of 1 at launch, cos^-1(1/2.8) = 69.1°, so set the cant at 70° or more, and you could perform a short full thrust static fire.
Admittedly, I have some trepidation regarding static fires at the moment.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2729 on: 09/03/2016 12:30 AM »
But you can't test your engines before a launch into a direct Earth return trajectory, because there's no infrastructure that can withstand the test (or hold down the vehicle.)

Actually you could test the engines if they were sufficiently canted, and the ship was fully fuelled. For a T/W of 1 at launch, cos^-1(1/2.8) = 69.1°, so set the cant at 70° or more, and you could perform a short full thrust static fire.
Admittedly, I have some trepidation regarding static fires at the moment.
Static fires are fine (if maybe expensive), provided you don't put your payload on top.
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #2730 on: 09/03/2016 01:06 AM »
Making the vehicle strong enough for a static fire in two different directions is a somewhat questionable design choice, even if we were to assume that the gimbals for canting posed no weight issues or technical unknowns.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2731 on: 09/03/2016 01:22 AM »
Making the vehicle strong enough for a static fire in two different directions is a somewhat questionable design choice, even if we were to assume that the gimbals for canting posed no weight issues or technical unknowns.
I wasn't suggesting that, by the way. I don't yet believe in the idea of extreme canting for MCT.
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Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2732 on: 09/03/2016 01:28 AM »
Making the vehicle strong enough for a static fire in two different directions is a somewhat questionable design choice, even if we were to assume that the gimbals for canting posed no weight issues or technical unknowns.

Perhaps it would help to consider the idea of 15 1/3 scale Raptors, arranged in a circle. Their 'dance floor' would also be circular and made strong enough to support full inline thrust. The engineering required to make a such a structure also support a radial load is a solved problem. In fact, a circular structure is ideal for bearing radial loads, e.g. the wheel.

Offline Patchouli

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Re: MCT Speculation and Discussion Thread 4
« Reply #2733 on: 09/03/2016 04:09 AM »
I've tried to fit landing some methalox in, but even for a small portion of the vehicle (an 'escape pod'), the math doesn't work for return to orbit (much less return to Earth), it swamps other payload.  If you could wring 4.5km/s out of an escape pod, you could return to MLO and a waiting lifeboat there for return to Earth.  But that's too much.  Landing mostly dry just works a lot better.  Every kilogram of propellant landed reduces the ISRU gear landed by 1kg, which reduces the propellant generated by many kilograms per synod.

Now...  that's not to say landing *completely* dry is the preferred option.  Liquid propellants are useful for a number of things other than full-on Mars ascent.  Solar storm shielding, EDL mass redistribution maneuvers, and high-thrust RCS landing maneuvers are all made considerably easier if you pack a sizable quantity of, say, monomethyl hydrazine on board.

Why not put the first habs in place before sending crew so you can just abort to the Martian surface vs MLO?

Once there are assets on Mars it be a safe haven as well.
« Last Edit: 09/03/2016 04:12 AM by Patchouli »

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #2734 on: 09/03/2016 05:09 PM »
Who here still believes that some new pad at Cape Canaveral or some future pad at Boca Chica is still viable for BFR launches?  Given the recent pad anomaly, I believe that more stringent safety zones will force the BFR to be launched off shore, assuming any US based launch site, which I do because of ITAR.

There are shallow seas offshore from both SX launch sites.  At first a platform could be anchored to the sea floor and serviced by barges and hydrofoils while launch rates remained low.  A very expensive causeway could later be constructed to support higher flight rates.

I do not feel that on shore launch remains an option assuming the BFR is really ~2x Saturn V size or more.

The noise dB problem remains as RTLS sonic booms propagate tens of miles with little attenuation.
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Online RonM

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Re: MCT Speculation and Discussion Thread 4
« Reply #2735 on: 09/03/2016 05:57 PM »
Who here still believes that some new pad at Cape Canaveral or some future pad at Boca Chica is still viable for BFR launches?  Given the recent pad anomaly, I believe that more stringent safety zones will force the BFR to be launched off shore, assuming any US based launch site, which I do because of ITAR.

There are shallow seas offshore from both SX launch sites.  At first a platform could be anchored to the sea floor and serviced by barges and hydrofoils while launch rates remained low.  A very expensive causeway could later be constructed to support higher flight rates.

I do not feel that on shore launch remains an option assuming the BFR is really ~2x Saturn V size or more.

The noise dB problem remains as RTLS sonic booms propagate tens of miles with little attenuation.

I agree an offshore platform is the best option for the BFR we've been discussing. But with the schedule Elon mentioned there really isn't enough time to build a pad or platform before initial BFR testing. Maybe the BFR isn't going to be as big as we think. Perhaps the first generation BFS will be 50 tonnes cargo to Mars instead of 100 tonnes, requiring a BFR about the size of a Saturn V. That's still impressive. Pad 39A can handle that. We'll find out in a few weeks.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2736 on: 09/03/2016 07:00 PM »
Who here still believes that some new pad at Cape Canaveral or some future pad at Boca Chica is still viable for BFR launches?  Given the recent pad anomaly, I believe that more stringent safety zones will force the BFR to be launched off shore, assuming any US based launch site, which I do because of ITAR....
This has no bearing on BFR.

Explosions on pads are not new. If it wasn't feasible to have BFR some place after this accident, it wouldn't have been feasible before.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2737 on: 09/03/2016 10:49 PM »
I tend to agree, the possibility of pad detonation had to already be present and evaluated in sizing the BFR before the recent incident.  Also consider that NASA had plans for rockets considerably larger then Saturn V, such as NOVA which they must have considered possible to launch from land at the cape.

Also we know BFR is to have approximately twice the Saturn V thrust but that doesn't mean it carries twice the propellant load, the considerably greater efficiency of Raptor engines may make for a rocket that is smaller then expected.

Offline zubenelgenubi

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Re: MCT Speculation and Discussion Thread 4
« Reply #2738 on: 09/03/2016 11:53 PM »
A side note:
IIRC, the Advanced Saturn and/or NOVA pads would have been constructed on land originally purchased for the center to the north of the current boundary--including what is now Canaveral National Seashore.
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Offline Zed_Noir

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Re: MCT Speculation and Discussion Thread 4
« Reply #2739 on: 09/04/2016 06:24 AM »
...
I do not feel that on shore launch remains an option assuming the BFR is really ~2x Saturn V size or more.
...

I agree an offshore platform is the best option for the BFR we've been discussing. But with the schedule Elon mentioned there really isn't enough time to build a pad or platform before initial BFR testing. Maybe the BFR isn't going to be as big as we think. Perhaps the first generation BFS will be 50 tonnes cargo to Mars instead of 100 tonnes, requiring a BFR about the size of a Saturn V. That's still impressive. Pad 39A can handle that. We'll find out in a few weeks.

Maybe a shipyard somewhere is already slated to work on floating BFR launch platform of some sort already designed.

Could be a converted semi-submersible oil production platform.

A more radical approach is to convert a VLCC (aka supertanker) as a launch platform and LV hangar. Pour some concrete as semi-ablative coating over the ship's top surface. Installed a flame trench section on the ship with the usual water suppression system. Build a reinforced hangar to stored the complete BFR/BFS stack. Voila a fast mobile autonomous sea going launch facility. There is a lot of candidate surplus VLCC available currently. Before anyone ask, the LV launch crew is taken off the ship in helicopter to support ship prior to launch. The Royal Navy convert the RFA Argus (A135) aviation training/hospital ship from a container ship by in part adding 1800 tons of concrete.

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