Quote from: IainMcClatchie on 02/15/2018 01:37 amI am suspicious of the idea that backpressure in the engines causes relighting difficulty. The stagnation pressure during the landing relight is 17 kPa (boosters) and 13 kPa (core), according to Zach's simulation here. That's less than 0.2 bar, which is going to be less than the pressure drop across whatever nozzle injects the TEA-TEB for relight. My guess is the TEA-TEB tank is probably at least ten times that pressure, and if the nozzle is choked flow, no minor pressure fluctuation downstream like that is going to affect the mass flow.The entry burn relight is at even lower stagnation pressure: under 300 Pa, so that's not a problem either.I think it's interesting that the peak stagnation pressure during the entry burn is so low, about 10 kPa. These values seemed way too low to me, based strictly on experience of sticking my hand out the window of a car and feeling the forces.But rockets are light for their size. If a landing rocket masses 30 tonnes, and is falling at terminal velocity, then the force must be 300 kN.
I am suspicious of the idea that backpressure in the engines causes relighting difficulty. The stagnation pressure during the landing relight is 17 kPa (boosters) and 13 kPa (core), according to Zach's simulation here. That's less than 0.2 bar, which is going to be less than the pressure drop across whatever nozzle injects the TEA-TEB for relight. My guess is the TEA-TEB tank is probably at least ten times that pressure, and if the nozzle is choked flow, no minor pressure fluctuation downstream like that is going to affect the mass flow.The entry burn relight is at even lower stagnation pressure: under 300 Pa, so that's not a problem either.I think it's interesting that the peak stagnation pressure during the entry burn is so low, about 10 kPa.
Quote from: TheRadicalModerate on 02/13/2018 09:48 pmI wonder if, as part of the throttle-down of the center core immediately after launch, they actually did a shutdown of some of the engines, no shutdowns
I wonder if, as part of the throttle-down of the center core immediately after launch, they actually did a shutdown of some of the engines,
Quote from: Jim on 02/14/2018 10:36 amQuote from: TheRadicalModerate on 02/13/2018 09:48 pmI wonder if, as part of the throttle-down of the center core immediately after launch, they actually did a shutdown of some of the engines, no shutdowns>So how do you know for sure it didn't happen?
Only 3 engines can restart because they're the only ones tied to the rockets internal TEA-TEB system. As the FH center core proved you don't want to exhaust the TEA-TEB. Better to throttle.
Quote from: Roy_H on 02/22/2018 03:13 amQuote from: Jim on 02/14/2018 10:36 amQuote from: TheRadicalModerate on 02/13/2018 09:48 pmI wonder if, as part of the throttle-down of the center core immediately after launch, they actually did a shutdown of some of the engines, no shutdowns>So how do you know for sure it didn't happen?Only 3 engines can restart because they're the only ones tied to the rockets internal TEA-TEB system. As the FH center core proved you don't want to exhaust the TEA-TEB. Better to throttle.
Quote from: docmordrid on 02/22/2018 03:39 amQuote from: Roy_H on 02/22/2018 03:13 amQuote from: Jim on 02/14/2018 10:36 amQuote from: TheRadicalModerate on 02/13/2018 09:48 pmI wonder if, as part of the throttle-down of the center core immediately after launch, they actually did a shutdown of some of the engines, no shutdowns>So how do you know for sure it didn't happen?Only 3 engines can restart because they're the only ones tied to the rockets internal TEA-TEB system. As the FH center core proved you don't want to exhaust the TEA-TEB. Better to throttle.How are the engines that are not tied to the rockets internal TEA-TEB system initially startet?/Svend
Quote from: docmordrid on 02/22/2018 03:39 amOnly 3 engines can restart because they're the only ones tied to the rockets internal TEA-TEB system. As the FH center core proved you don't want to exhaust the TEA-TEB. Better to throttle.Merlin 1D can probably throttle to 40% or so of maximum.Shutting down 3/9 engines gets you to 26%.So, for it to be worth it, it would have to be valulable to operate in that 14% range. This seems not hugely likely on the face of it.
Quote from: speedevil on 02/22/2018 03:46 amQuote from: docmordrid on 02/22/2018 03:39 amOnly 3 engines can restart because they're the only ones tied to the rockets internal TEA-TEB system. As the FH center core proved you don't want to exhaust the TEA-TEB. Better to throttle.Merlin 1D can probably throttle to 40% or so of maximum.Shutting down 3/9 engines gets you to 26%.So, for it to be worth it, it would have to be valulable to operate in that 14% range. This seems not hugely likely on the face of it.You guys are missing the point here. Rocket engines are less efficient when throttled.
Now I don't know how much but at 40% throttle maybe it uses 60% fuel rate. Anybody know actual figures?
Quote from: Roy_H on 02/22/2018 06:44 pmQuote from: speedevil on 02/22/2018 03:46 amQuote from: docmordrid on 02/22/2018 03:39 amOnly 3 engines can restart because they're the only ones tied to the rockets internal TEA-TEB system. As the FH center core proved you don't want to exhaust the TEA-TEB. Better to throttle.Merlin 1D can probably throttle to 40% or so of maximum.Shutting down 3/9 engines gets you to 26%.So, for it to be worth it, it would have to be valulable to operate in that 14% range. This seems not hugely likely on the face of it.You guys are missing the point here. Rocket engines are less efficient when throttled.yes, but...Quote Now I don't know how much but at 40% throttle maybe it uses 60% fuel rate. Anybody know actual figures?No, isp does not drop from 300 seconds to 200 seconds because of throttling to 40%.it drops maybe something like from 300 seconds to 290-294 seconds, like 2-3% instead of 33%.
Quote from: smndk on 02/22/2018 08:32 amHow are the engines that are not tied to the rockets internal TEA-TEB system initially startet?On the ground TEA-TEB is supplied externally.
How are the engines that are not tied to the rockets internal TEA-TEB system initially startet?
Quote from: docmordrid on 02/22/2018 08:42 amQuote from: smndk on 02/22/2018 08:32 amHow are the engines that are not tied to the rockets internal TEA-TEB system initially startet?On the ground TEA-TEB is supplied externally. I am amazed that the post-ignition quick disconnect for a hypergolic fluid is lighter weight than just having a larger tank on board.
Quote from: IainMcClatchie on 02/23/2018 05:02 amQuote from: docmordrid on 02/22/2018 08:42 amQuote from: smndk on 02/22/2018 08:32 amHow are the engines that are not tied to the rockets internal TEA-TEB system initially startet?On the ground TEA-TEB is supplied externally. I am amazed that the post-ignition quick disconnect for a hypergolic fluid is lighter weight than just having a larger tank on board.Why? A larger tank (or tanks) would have to hold 9 more starts than the tank (or tanks, because we don't know if there is a central tank pair or one pair per engine) that only has to support restarts.
Quote from: IainMcClatchie on 02/23/2018 05:02 amI am amazed that the post-ignition quick disconnect for a hypergolic fluid is lighter weight than just having a larger tank on board.Why? A larger tank (or tanks) would have to hold 9 more starts than the tank (or tanks, because we don't know if there is a central tank pair or one pair per engine) that only has to support restarts.
I am amazed that the post-ignition quick disconnect for a hypergolic fluid is lighter weight than just having a larger tank on board.
Quote from: Lar on 02/23/2018 05:11 amQuote from: IainMcClatchie on 02/23/2018 05:02 amI am amazed that the post-ignition quick disconnect for a hypergolic fluid is lighter weight than just having a larger tank on board.Why? A larger tank (or tanks) would have to hold 9 more starts than the tank (or tanks, because we don't know if there is a central tank pair or one pair per engine) that only has to support restarts.Because scaling up a tank is very lightweight. For small quantity things like this, the fittings and plumbing can chew up most of the mass. I wouldn't be surprised to hear that there is enough TEA-TEB in the plumbing for a full restart.Also note that the ground support equipment, and the rocket plumbing must have sufficient pipe diameter to deliver the shot of TEA-TEB in much less than 1 second. That plumbing goes from two sources (ground and flight tanks) to 9 destinations.
Depends on altitude. I'll plot isp curves for Merlin as a function of throttle and altitude in RPA tomorrow. If I remember OK, here's the plot. Some assumptions here but should be pretty close. At SL throttling to 40% costs about 15% in isp. But this very rapidly drops with altitude and at optimal expansion (only 2.4 km up!) it only costs about 1%.
Quote from: envy887 on 02/23/2018 04:31 amDepends on altitude. I'll plot isp curves for Merlin as a function of throttle and altitude in RPA tomorrow. If I remember OK, here's the plot. Some assumptions here but should be pretty close. At SL throttling to 40% costs about 15% in isp. But this very rapidly drops with altitude and at optimal expansion (only 2.4 km up!) it only costs about 1%.Thank you for clearing this up. Obviously I was wrong as full thrust from all 3 cores is required at lift-off and throttling only occurs when g-forces exceed desired limit. I don't know where that is, but I would expect it to be about the 2.4km altitude so at only 1% loss it becomes a non-issue. I think there would still be some gain by shutting down center core engines to save fuel until after side boosters have done their job, but now I think all 9 engines would have to be shut down and re-started to make any significant difference.
I really love it when people are smarter than professionals like Tom Mueller.
Quote from: IainMcClatchie on 02/23/2018 05:02 amI am amazed that the post-ignition quick disconnect for a hypergolic fluid is lighter weight than just having a larger tank on board.Quote from: Herb Schaltegger on 02/24/2018 12:29 amI really love it when people are smarter than professionals like Tom Mueller.Let me say it differently then.Can someone point out the non obvious thing that makes having the post-ignition quick disconnect preferrable to a bigger tank?For instance, deleting the quick disconnect means that, should they start the engines and then shut down on the pad, they probably can't try an engine restart without reconnecting, which requires draining the fuel tanks and so on. Maybe that's the reason.Your statement contained no information about rockets.
Quote from: IainMcClatchie on 02/24/2018 11:35 pmQuote from: IainMcClatchie on 02/23/2018 05:02 amI am amazed that the post-ignition quick disconnect for a hypergolic fluid is lighter weight than just having a larger tank on board.Quote from: Herb Schaltegger on 02/24/2018 12:29 amI really love it when people are smarter than professionals like Tom Mueller.Let me say it differently then.Can someone point out the non obvious thing that makes having the post-ignition quick disconnect preferrable to a bigger tank?For instance, deleting the quick disconnect means that, should they start the engines and then shut down on the pad, they probably can't try an engine restart without reconnecting, which requires draining the fuel tanks and so on. Maybe that's the reason.Your statement contained no information about rockets.The tail service masts don't release until the hold-downs do. And fluid QD's are very light, very easy to service/replace and when they leak, it's very obvious, neither of which are true for internal fluid tanks and lines. (DISCLAIMER: I have spec'd/designed fluid QD for crewed spaceflight applications in life-critical operations; I'm not talking out of my ass).