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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)...
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
From the Reddit AMA:Redditor comment:You can't land on moon using 3MN engineElon Musk reply:Yes, you can. - Bob, the Builder
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).
Quote from: Oersted on 10/16/2017 04:11 pmIn 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'm interested in the phrase "a third medium area ratio Raptor engine".
Quote from: CuddlyRocket on 10/17/2017 09:01 amI'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?
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?
They're both at about the same thrust currently, though this will change as duration and power level increases with next steps.
Quote from: Space Ghost 1962 on 10/20/2017 04:43 amThey'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?
Quote from: hkultala on 10/20/2017 05:20 amQuote from: Space Ghost 1962 on 10/20/2017 04:43 amThey'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/