Poll

When will full-scale hot-fire testing of Raptor begin?

Component tests - 2017
3 (0.6%)
Component tests - 2018
21 (4.2%)
Integrated tests -  2017
19 (3.8%)
Integrated tests -  2018
237 (47%)
Integrated tests -  2019
181 (35.9%)
Raptor is not physically scaled up
33 (6.5%)
Never
10 (2%)

Total Members Voted: 504


Author Topic: SpaceX Raptor engine (Super Heavy/Starship Propulsion) - General Thread 1  (Read 869985 times)

Offline Kaputnik

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #520 on: 10/16/2017 11:32 am »
How will they get the required tank pressure if they only need a short burst? Seeing as they need to build up some energy in the heat exchanger.
(Unless he was serious about the Harry Potter thing)...
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

Offline Peter.Colin

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #521 on: 10/16/2017 11:58 am »
How will they get the required tank pressure if they only need a short burst? Seeing as they need to build up some energy in the heat exchanger.
(Unless he was serious about the Harry Potter thing)...

You don’t need high tank presure just high chamber pressure.
For this they could use an air driven hydraulic pump, like this one.
Instant respons high presure, up to 7000 bar.
It’s similar to a turbo pump but much lower flow, and better presure control.

http://www.haskel.com/products/pneumatic-pumps/liquid-pumps/

« Last Edit: 10/16/2017 12:45 pm by Peter.Colin »

Offline livingjw

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #522 on: 10/16/2017 12:23 pm »
How will they get the required tank pressure if they only need a short burst? Seeing as they need to build up some energy in the heat exchanger.
(Unless he was serious about the Harry Potter thing)...

There will be separate high pressure gaseous LOX and CH4 tanks for RCS. Liquid propellants can easily be electrically pumped in and vaporized by heating. Heating could be electrical, chemical or, if engines are running, tapped off  the engines.

The heat exchangers on the Raptor are primarily for autogenesis main tank pressurization.

John
« Last Edit: 10/16/2017 03:57 pm by livingjw »

Offline livingjw

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #523 on: 10/16/2017 04:00 pm »
Are RCS motors fed with gaseous propellants or liquid? I seem to remember someone stating that they would be gaseous, but now I am not so sure.

John

Online envy887

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #524 on: 10/16/2017 04:07 pm »
Are RCS motors fed with gaseous propellants or liquid? I seem to remember someone stating that they would be gaseous, but now I am not so sure.

John

Musk said at 2016 IAC that they would be gaseous. That hasn't changed, as far as I know.

Online Oersted

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #525 on: 10/16/2017 04:11 pm »
From the Reddit AMA:

QUESTION 1

Why was Raptor thrust reduced from ~300 tons-force to ~170 tons-force?

One would think that for (full-flow staged combustion...) rocket engines bigger is usually better: better surface-to-volume ratio, less friction, less heat flow to handle at boundaries, etc., which, combined with the target wet mass of the rocket defines a distinct 'optimum size' sweet spot where the sum of engines reaches the best thrust-to-weight ratio.

Yet Raptor's s/l thrust was reduced from last year's ~300 tons-force to ~170 tons-force, which change appears to be too large of a reduction to be solely dictated by optimum single engine TWR considerations.
What were the main factors that led to this change?

Elon Musk initial reply:

We chickened out

Elon Musk follow-up reply:

The engine thrust dropped roughly in proportion to the vehicle mass reduction from the first IAC talk. In order to be able to land the BF Ship with an engine failure at the worst possible moment, you have to have multiple engines. The difficulty of deep throttling an engine increases in a non-linear way, so 2:1 is fairly easy, but a deep 5:1 is very hard. Granularity is also a big factor. If you just have two engines that do everything, the engine complexity is much higher and, if one fails, you've lost half your power. Btw, we modified the BFS design since IAC to add a third medium area ratio Raptor engine partly for that reason (lose only 1/3 thrust in engine out) and allow landings with higher payload mass for the Earth to Earth transport function.

Redditor comment:

You can't land on moon using 3MN engine

Elon Musk reply:

Yes, you can. - Bob, the Builder

Offline vaporcobra

From the Reddit AMA:
Redditor comment:

You can't land on moon using 3MN engine

Elon Musk reply:

Yes, you can. - Bob, the Builder

It's funny because current Raptor is < 2MN. Elon could have just been being silly (he was self-admittedly drunk :D) but also lends credence to the idea that SpaceX will likely eventually move to a > 3MN Raptor.

Offline CuddlyRocket

In order to be able to land the BF Ship with an engine failure at the worst possible moment, you have to have multiple engines. The difficulty of deep throttling an engine increases in a non-linear way, so 2:1 is fairly easy, but a deep 5:1 is very hard. Granularity is also a big factor. If you just have two engines that do everything, the engine complexity is much higher and, if one fails, you've lost half your power. Btw, we modified the BFS design since IAC to add a third medium area ratio Raptor engine partly for that reason (lose only 1/3 thrust in engine out) and allow landings with higher payload mass for the Earth to Earth transport function.

I'm interested in the phrase "a third medium area ratio Raptor engine". Everyone's taken this to mean a third SL engine; but, if so, why didn't he just say so? (Are SL engines referred to as medium area ratio engines?) But, I wonder if he actually meant a third type of engine, with an area ratio between that of the SL and Vac engines. One that will work at sea-level without the usual adverse consequences of attempting to run a Vac engine at those atmospheric pressure, but one that is more efficient than the SL engines when used in regimes of lower atmospheric pressure (though not as efficient as a Vac engine).

Are they proposing to land on the Moon and Mars using the SL engines? If so, such an engine would be more efficient. How high will the BSF be when it separates from the booster and could such an engine be useful at such an altitude? I'm not a rocket engineer (does it show! :) ), but could there be benefits from having such an intermediate engine?

The obvious argument against is having a third type of engine, with the design and manufacturing complexity etc. I suppose this depends on the level of commonality with the SL and/or Vac engines, and whether any benefits are worth the cost.

Offline DreamyPickle

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #528 on: 10/17/2017 10:24 am »
I'm interested in the phrase "a third medium area ratio Raptor engine". Everyone's taken this to mean a third SL engine; but, if so, why didn't he just say so? (Are SL engines referred to as medium area ratio engines?) But, I wonder if he actually meant a third type of engine, with an area ratio between that of the SL and Vac engines. One that will work at sea-level without the usual adverse consequences of attempting to run a Vac engine at those atmospheric pressure, but one that is more efficient than the SL engines when used in regimes of lower atmospheric pressure (though not as efficient as a Vac engine).

There is some speculation in this thread that the 2016 ITS already had 3 types of engine.

Offline livingjw

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #529 on: 10/17/2017 12:09 pm »
In order to be able to land the BF Ship with an engine failure at the worst possible moment, you have to have multiple engines. The difficulty of deep throttling an engine increases in a non-linear way, so 2:1 is fairly easy, but a deep 5:1 is very hard. Granularity is also a big factor. If you just have two engines that do everything, the engine complexity is much higher and, if one fails, you've lost half your power. Btw, we modified the BFS design since IAC to add a third medium area ratio Raptor engine partly for that reason (lose only 1/3 thrust in engine out) and allow landings with higher payload mass for the Earth to Earth transport function.

I'm interested in the phrase "a third medium area ratio Raptor engine". Everyone's taken this to mean a third SL engine; but, if so, why didn't he just say so? (Are SL engines referred to as medium area ratio engines?) But, I wonder if he actually meant a third type of engine, with an area ratio between that of the SL and Vac engines. One that will work at sea-level without the usual adverse consequences of attempting to run a Vac engine at those atmospheric pressure, but one that is more efficient than the SL engines when used in regimes of lower atmospheric pressure (though not as efficient as a Vac engine).

Are they proposing to land on the Moon and Mars using the SL engines? If so, such an engine would be more efficient. How high will the BSF be when it separates from the booster and could such an engine be useful at such an altitude? I'm not a rocket engineer (does it show! :) ), but could there be benefits from having such an intermediate engine?

The obvious argument against is having a third type of engine, with the design and manufacturing complexity etc. I suppose this depends on the level of commonality with the SL and/or Vac engines, and whether any benefits are worth the cost.

I thought the phrasing was odd as well. If they put a 70:1 ER engine in the middle and two landing engines on either side, this would increase their allowable landing weight and increase BFS thrust to weight during ascent. This would be an improvement if BFS's T/W was a little low to begin with.

John

Offline Bynaus

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #530 on: 10/17/2017 12:43 pm »
I was confused at first, but I think what this means is:

"large area ratio engine" = vacuum-engine
"medium area ratio engine" = sea-level-engine
"small area ratio engine" = Raptor-based RCS thruster (?)

I think overall it just means that he wants have these engine descriptions capture an unchanging property of the engine (the area ratio) as opposed to the conditions they are used (which isn't as clear cut).

I don't know if the "small" version is really an RCS thruster, but that seems to make most sense.
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Offline nacnud

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #531 on: 10/17/2017 12:51 pm »
Would there be any difference in the best area ratio for an air start raptor that then needs to land versus a ground start raptor that then needs to land?

Small, ground start and landing
Medium, air start (well space really) and landing
Large, vacuum optimized

Offline Nomic

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #532 on: 10/17/2017 02:55 pm »
I'm interested in the phrase "a third medium area ratio Raptor engine".

Booster might have a lower ER Raptor then the landing Raptors on BFS? ISP doesn't matter as much for the booster, can squeeze more engines on to it with a smaller nozzle and save a bit of weight.

So BFR with low ER raptor, BFS has mid size and vac?

Online envy887

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #533 on: 10/17/2017 07:45 pm »
I'm interested in the phrase "a third medium area ratio Raptor engine".

Booster might have a lower ER Raptor then the landing Raptors on BFS? ISP doesn't matter as much for the booster, can squeeze more engines on to it with a smaller nozzle and save a bit of weight.

So BFR with low ER raptor, BFS has mid size and vac?

That is my interpretation. The booster needs a smaller ER nozzle to pack more engines into a small space.

Offline intrepidpursuit

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #534 on: 10/19/2017 08:47 pm »
I'm interested in the phrase "a third medium area ratio Raptor engine".

Booster might have a lower ER Raptor then the landing Raptors on BFS? ISP doesn't matter as much for the booster, can squeeze more engines on to it with a smaller nozzle and save a bit of weight.

So BFR with low ER raptor, BFS has mid size and vac?

As far as I can tell, the situation where you need the most efficiency out of the center engines on the BFS is on takeoff. That will never happen at earth sea level. It may be the biggest ratio they can get away with at SL so it is better optimized for mars and moon operations.

Offline rockets4life97

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #535 on: 10/20/2017 03:15 am »
Blue Origin recently reported a successful test of the B-4. Does this put the B-4 ahead or behind raptor in terms of development? or is it hard to compare given SpaceX's decision to test a subscale engine first?

Offline Space Ghost 1962

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #536 on: 10/20/2017 04:43 am »
Blue Origin recently reported a successful test of the B-4. Does this put the B-4 ahead or behind raptor in terms of development? or is it hard to compare given SpaceX's decision to test a subscale engine first?

Both Raptor and BE-4 engines are in the early stages of test. To advance a test program for large engines like these is a long term program, not the kind of thing you can handicap like a horse race.

They also have different goals, chamber pressures, and scale of size. The interesting commonality is the combustion of largely similar propellants at similar mass flows.

SX has about a year lead on operating a more complex and scaleable engine against BO getting one to operate (an enormous achievement nonetheless). Both of these are "firsts" in different ways - SX in hydrocarbon FFSC globally, and BO in the first non-Russian, non-Ukrainian ORSC engine.

BE-4 is intended for use in multiple vehicles, so from this POV the tests progress building confidence in a single design instance at high fidelity. Raptor as currently implemented is a compact, extremely high chamber pressure engine apparently not intended for use, but to allow many derivatives that will be used,  to be rapidly developed for an exotic two stage vehicle intended to land on other moons/planets. So the first follows an immediate path of critical development/review, while the other's path requires even more reliability/application over a more elaborate development path.

So they are necessarily hard to compare. Even if ULA selects BE-4 by end of year, the nature of the Vulcan engine role isn't the same as with NG nor that of the Raptors that will actually be flown.

And even with the first flown BE-4's ... that will occur far enough into the future, that understanding where Raptor in an actual vehicle will be in comparison, isn't possible.

So the best one can do is compare test programs now.

In short - Raptor has a lead on time/reliability. They're both at about the same thrust currently, though this will change as duration and power level increases with next steps. Both have gotten by a major achievement in start-up/shutdown.

Next big steps for each - Raptor needs to move on to a flight scale/quality engine (it was wise to do a 1MN one first, makes this next step easier),  BE-4 needs to expand its operating range to its design limits while stably functioning and thus proving that its model of operation matches its actual function. Both are tall orders.

Offline hkultala

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #537 on: 10/20/2017 05:20 am »

They're both at about the same thrust currently, though this will change as duration and power level increases with next steps.

Are you seriously claiming that BE-4 is only running at about 40% thrust currently?

Offline mnelson

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #538 on: 10/20/2017 05:49 am »

They're both at about the same thrust currently, though this will change as duration and power level increases with next steps.

Are you seriously claiming that BE-4 is only running at about 40% thrust currently?

Eric Berger's article at ARSTechnica says the test was at 50% thrust: https://arstechnica.com/science/2017/10/blue-origin-has-successfully-tested-its-powerful-be-4-rocket-engine/

Offline Rabidpanda

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #539 on: 10/20/2017 06:06 am »

They're both at about the same thrust currently, though this will change as duration and power level increases with next steps.

Are you seriously claiming that BE-4 is only running at about 40% thrust currently?

Eric Berger's article at ARSTechnica says the test was at 50% thrust: https://arstechnica.com/science/2017/10/blue-origin-has-successfully-tested-its-powerful-be-4-rocket-engine/

Still... there is a big difference between a fullscale engine being tested at a lower thrust level and a subscale development engine.

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