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| Starship SN10 High-Altitude Flight Test |
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| raptor1956:
--- Quote from: wraper on March 09, 2021, 12:10:19 pm --- --- Quote from: raptor1956 on March 09, 2021, 11:55:12 am --- --- Quote from: rdl on March 06, 2021, 03:18:19 am ---They're using three engines, so most of the time during these tests the thrust is not along the centerline. Pretty impressive how well they deal with that. I'm amazed they landed one. --- End quote --- Normally, when you use three engines, they are arranged in a row so that you can have balanced thrust with 3 engines, 2 engines or 1 engine -- not sure why that wasn't used. In an operational rocket requiring more than one engine you would prefer three in a row, five arranged with one in the center and four arranged 90 degrees around the center, or seven with one in the center and the others spaced at 60 degrees. Again, not sure what SpaceX is trying to prove with the arrangement they're using. --- End quote --- Maybe think a little bit and realize you cannot make a round rocket with such arrangement. Then think a bit again and realize you will have thrust vectoring issues in certain directions. Not to say starship will have 6 engines in total (3 of which vacuum engines with large bells). Then think for a third time, and realize you cannot switch between 2 engines and 1 or 3 in a way that thrust remains in center. You either need to fire additional engine during switchover or shut down them all. Or deal with even more off center thrust compared with current arrangement. Just because you have no understanding, does not mean somebody tries to prove anything. --- Quote ---Normally, when you use three engines, they are arranged in a row so that you can have balanced thrust with 3 engines, 2 engines or 1 engine -- not sure why that wasn't used --- End quote --- Normally where? Delta IV Heavy which uses 3 separate boosters and does not even have an option to shut them down separately? --- Quote ---five arranged with one in the center and four arranged 90 degrees around the center, or seven with one in the center and the others spaced at 60 degrees. --- End quote --- These are inefficient in regards to space usage, especially 5 engine variant. A lot of space wasted for nothing. --- End quote --- Jesus, where to begin... Yes, you can make a round rocket using three engines in a row -- it's been done so it follows its possible. It's not uncommon to alter the thrust by shutting down one or more engines -- the Falcon 9 tends to shutdown one or more near the end of the first stage burn to limit acceleration to about 4g's. They then use three engines for the entry burn and one for the landing. If the max thrust of a rocket stage is equal to it's weight then it will accelerate at one g initially (in space outside of a gravity field) BUT, since propellent often equals 90% or more of the entire mass the acceleration at the end of the burn could be 10g's and that's not good with people onboard and it's not wise to design and build a rocket stressed to handle 10g's as the extra mass needed would not be helpful -- so, being able to reduce thrust is important! Operating a rocket engine at partial thrust comes at the penalty of lowered ISP so it's better, where possible, to shutdown one or more engines so you can run the remaining engines closer to rated thrust. The Apollo second stage used 5 J2 engines arranged with one in the center and four around it at 90 degrees. The Falcon 9 uses 9 Merlin engines arranged with one in the center and eight around it at 45 degrees. Brian |
| raptor1956:
--- Quote from: rdl on March 09, 2021, 01:14:44 pm --- --- Quote ---five arranged with one in the center and four arranged 90 degrees around the center, or seven with one in the center and the others spaced at 60 degrees. --- End quote --- The Saturn V used the first arrangement. The Falcon 9 uses the second, though with eight engines in the outer ring. The first version of Falcon 9, at least one of them, had the nine engines in 3x3 pattern which seems really weird. The Shuttle Orbiter's three main engines were in a triangle. It seems the designers use whatever they think is best. --- End quote --- The Shuttle was a weird beast that differs from traditional rockets based on the cylinder and the trust of the three main engines was not directed directly aft do to the off center mass of external fuel tank. The angle of the offset was pretty significant so the net thrust was less than the thrust produced do to cosine error. Brian |
| wraper:
--- Quote from: raptor1956 on March 10, 2021, 01:17:04 am ---It's not uncommon to alter the thrust by shutting down one or more engines -- the Falcon 9 tends to shutdown one or more near the end of the first stage burn to limit acceleration to about 4g's. --- End quote --- It does not. It uses part of its engines only during descent. During ascent it always uses all of its engines. And I don't know any other rocket which can use part of its engines. |
| Brumby:
Starship Raptor engines are run in 3 stages: All 3 for lift off, 2 part way up and 1 near the top of these 10km trials - for good engineering reasons, I might add. All the old rules were necessary from having zero experience in order to get things up in space. Now that we know a lot more and have much more advanced technology, those old rules can be revisited. Gimballed engines providing thrust vectoring being one brilliant example. One engine off to the side of a central axis can control a craft for propulsive landing. We've seen it done. |
| raptor1956:
--- Quote from: wraper on March 10, 2021, 01:41:20 am --- --- Quote from: raptor1956 on March 10, 2021, 01:17:04 am ---It's not uncommon to alter the thrust by shutting down one or more engines -- the Falcon 9 tends to shutdown one or more near the end of the first stage burn to limit acceleration to about 4g's. --- End quote --- It does not. It uses part of its engines only during descent. During ascent it always uses all of its engines. And I don't know any other rocket which can use part of its engines. --- End quote --- Here ya go... https://spaceflightnow.com/2015/01/05/falcon-9-spacex-5-launch-timeline/ "Moments after two of the Falcon 9’s first stage engines shut down, the remaining seven Merlin engines cut off at an altitude of 80 kilometers, or about 50 miles, and a velocity of Mach 10." But even in your reply you admit that they use less than all 9 engines for entry and landing -- not sure what your particular problem is! Brian |
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