So successful 3 engine landing. Presumably they were running somewhere close to full throttle. I've been surprised that the airframe of an an almost empty stage could take the G loading, but obviously it can - at least once. It's diminishing return but I wonder if it could handle a 'we who are about to die' 5 engine burn. Re-plumbing for 5 vs 3 engine TEA/TEB would allow changing both the boost back and landing burns. Did we ever establish for sure that only 1 engine is used for the entry burn 'plasma shield'?
So successful 3 engine landing. Presumably they were running somewhere close to full throttle. I've been surprised that the airframe of an an almost empty stage could take the G loading, but obviously it can - at least once. It's diminishing return but I wonder if it could handle a 'we who are about to die' 5 engine burn. Re-plumbing for 5 vs 3 engine TEA/TEB would allow changing both the boost back and landing burns. Did we ever establish for sure that only 1 engine is used for the entry burn 'plasma shield'? IIRC Elon commented recently that small percentage changes add up in the rocket business.
Simplistic model in table form Engines T/W Accel t h g-loss ratio fuel (m/s^2) (sec) (m) (m/s) 1 1.75 7.5 20 1500 200 2.33 100% 2 3.5 25.0 6 450 60 1.40 60% 3 5.3 42.5 3.5 265 35 1.24 53% 4 7.0 60.0 2.5 188 25 1.17 50% 5 8.8 77.5 1.9 145 19 1.13 48% 6 10.5 95.0 1.6 118 16 1.11 47% 7 12.3 112.5 1.3 100 13 1.09 47% 8 14.0 130.0 1.2 87 12 1.08 46% 9 15.8 147.5 1.0 76 10 1.07 46%
But, if it's already pulling ~10Gs with 3 engines then 5 would push that up to ~16 which is likely beyond the structural limit.
Quote from: Comga on 03/14/2016 02:39 pmSimplistic model in table form Engines T/W Accel t h g-loss ratio fuel (m/s^2) (sec) (m) (m/s) 1 1.75 7.5 20 1500 200 2.33 100% 2 3.5 25.0 6 450 60 1.40 60% 3 5.3 42.5 3.5 265 35 1.24 53% 4 7.0 60.0 2.5 188 25 1.17 50% 5 8.8 77.5 1.9 145 19 1.13 48% 6 10.5 95.0 1.6 118 16 1.11 47% 7 12.3 112.5 1.3 100 13 1.09 47% 8 14.0 130.0 1.2 87 12 1.08 46% 9 15.8 147.5 1.0 76 10 1.07 46% This is up thread. *IF* it could be made to work 5 vs 3 would save a bit more fuel and little bits add up with rocket. But, if it's already pulling ~10Gs with 3 engines then 5 would push that up to ~16 which is likely beyond the structural limit. Although, the published G limit may be more about the payload than the rocket. I guess if they ever try 5 we can surmise that it's higher than we thought. As I understand it, if the the stage was made 'stationary' relative to the surface the terminal velocity it would achieve would be a relatively benign event. The figure ' about 15 seconds' was mentioned in the web cast for the entry burn. I'm sure that produces significant slowing of the stage but comes nowhere close to 'stationary' so the rest of the kinetic energy has to be bled off as drag and the resulting heat. The 'plasma shield' I was referring to is the engine(s) exhaust diverting much of that heating around the stage and keeping it from cooking and ablating the exterior surface. That being the case there might be an advantage of using a single engine since you could run it longer to create an increased protection duration. If that's not true, then there would be no benefit to waiting to initiate the burn once you were out of proximity to S2 and could effectively combine the boost back and entry burns.
Quote from: Okie_Steve on 05/06/2016 11:21 pmQuote from: Comga on 03/14/2016 02:39 pmSimplistic model in table form Engines T/W Accel t h g-loss ratio fuel (m/s^2) (sec) (m) (m/s) 1 1.75 7.5 20 1500 200 2.33 100% 2 3.5 25.0 6 450 60 1.40 60% 3 5.3 42.5 3.5 265 35 1.24 53% 4 7.0 60.0 2.5 188 25 1.17 50% 5 8.8 77.5 1.9 145 19 1.13 48% 6 10.5 95.0 1.6 118 16 1.11 47% 7 12.3 112.5 1.3 100 13 1.09 47% 8 14.0 130.0 1.2 87 12 1.08 46% 9 15.8 147.5 1.0 76 10 1.07 46% This is up thread. *IF* it could be made to work 5 vs 3 would save a bit more fuel and little bits add up with rocket. But, if it's already pulling ~10Gs with 3 engines then 5 would push that up to ~16 which is likely beyond the structural limit. Although, the published G limit may be more about the payload than the rocket. I guess if they ever try 5 we can surmise that it's higher than we thought. As I understand it, if the the stage was made 'stationary' relative to the surface the terminal velocity it would achieve would be a relatively benign event. The figure ' about 15 seconds' was mentioned in the web cast for the entry burn. I'm sure that produces significant slowing of the stage but comes nowhere close to 'stationary' so the rest of the kinetic energy has to be bled off as drag and the resulting heat. The 'plasma shield' I was referring to is the engine(s) exhaust diverting much of that heating around the stage and keeping it from cooking and ablating the exterior surface. That being the case there might be an advantage of using a single engine since you could run it longer to create an increased protection duration. If that's not true, then there would be no benefit to waiting to initiate the burn once you were out of proximity to S2 and could effectively combine the boost back and entry burns.Given the diminishing returns of 5 engines vs 3 for landing, I think it's more likely SpaceX tries a 5 (or more) engine reentry burn instead. There's a lot more fuel mass onboard (15-20 tons?) when it starts, so a 5 engine reentry burn should give more of a benefit to T/W without pulling too many Gs. The reentry burn for JCSAT-14 was about 15 seconds with 3 engines, so would a 5 engine burn cut that to under 10 seconds? What kind of fuel savings would that give?
The reentry burn for JCSAT-14 was about 15 seconds with 3 engines...
Merlin takes about 3 sec to start so whatever fuel savings gained in reduced gravity losses using 5 engines might get lost in 6 sec of wasting fuel of extra 2 engines start up
Huh? In what way do you think fuel is "wasted" during engine start-up?
Quote from: ChrisWilson68 on 05/07/2016 08:38 amHuh? In what way do you think fuel is "wasted" during engine start-up?The fuel/gases that come out during spin-up are nowhere near the exit velocity of a fully running engine, so they only give little thrust for their mass.
a) What kind of upper stages might one design for a Falcon-3R, or a Falcon-5R?(Is a Falcon-3R with parachute light enough to be caught be a helicopter?)