@Jim, as you seem to have the most experience in this. Are most payloads built to cope with those kind of forces? I just imagined 6 g's was the maximum normal (off course you could get higher if your launch vehicle and payload can cope with it) as the user manuals of multiple launchers stated it as their max loads. I have no idea what most payloads are calculated on though, maybe you can clarify it a bit? thanks
Did you take into account the loss of efficiency resulting from throttling the engines below their optimum (design) thrust level? Actually, throttled engines seem to perform quite well with less than 10% loss in Isp over a wide range of thrust. Above 80% thrust, Isp increases to 100% of the nominal value, but between ~30% to ~80% throttle, ~10% reduction in Isp seems to be typical. Thrust can be increased to compensate by throttling up, but fuel consumption would increase. Also, throttleable engines seem to be noteably more massive than the fixed thrust counterpart. Of course, 1% of engine weight is only 5 added kg so maybe the added mass wouldn't be much in absolute terms.Do a Google search on "rocket engine throttling," or something like that, there are a lot of references.
Quote from: MP99 on 07/03/2012 05:44 pmQuote from: Lobo on 07/03/2012 04:44 pmNot sure if this question has already been answered, as I've not read all the pages on this thread, but I was wondering the reasons for doing crossfeed, rather than simply throttling down the central core during ascent to preserve it's fuel, and then after booster separationthere is still fuel left in the core. I guess this would lead to higher staging, as the boosters aren't drainging their propellant into the central core, but wouldn't it accomplish roughly the same thing?or not?And if not, why not?Well, you do start off with the same prop in the three first stage tanks, but if you throttle down the core, you reduce the thrust when the vehicle is heaviest. That increases gravity losses (but does assume the vehicles' structure can take all the thrust).Also, if speculation is correct that the outriggers feed twelve engines each (their own, plus 3x core engines), then the outriggers will drain 4x faster than the core. At outrigger burnout, the core should have at least 75% of it's prop load remaining.If you were to do the same via throttling the core, you'd need to throttle down to 25% (and lose a lot of T/W). M1Ds seem to throttle only down to 70%.cheers, MartinWhich leads me to my 2nd question, would it be cheaper/easier to develop M1d with that deep of throttling? Or the extra complexity of crossfeed?I don't know, just curious.
Quote from: Lobo on 07/03/2012 04:44 pmNot sure if this question has already been answered, as I've not read all the pages on this thread, but I was wondering the reasons for doing crossfeed, rather than simply throttling down the central core during ascent to preserve it's fuel, and then after booster separationthere is still fuel left in the core. I guess this would lead to higher staging, as the boosters aren't drainging their propellant into the central core, but wouldn't it accomplish roughly the same thing?or not?And if not, why not?Well, you do start off with the same prop in the three first stage tanks, but if you throttle down the core, you reduce the thrust when the vehicle is heaviest. That increases gravity losses (but does assume the vehicles' structure can take all the thrust).Also, if speculation is correct that the outriggers feed twelve engines each (their own, plus 3x core engines), then the outriggers will drain 4x faster than the core. At outrigger burnout, the core should have at least 75% of it's prop load remaining.If you were to do the same via throttling the core, you'd need to throttle down to 25% (and lose a lot of T/W). M1Ds seem to throttle only down to 70%.cheers, Martin
Not sure if this question has already been answered, as I've not read all the pages on this thread, but I was wondering the reasons for doing crossfeed, rather than simply throttling down the central core during ascent to preserve it's fuel, and then after booster separationthere is still fuel left in the core. I guess this would lead to higher staging, as the boosters aren't drainging their propellant into the central core, but wouldn't it accomplish roughly the same thing?or not?And if not, why not?
Quote from: Lobo on 07/03/2012 04:44 pmNot sure if this question has already been answered, as I've not read all the pages on this thread, but I was wondering the reasons for doing crossfeed, rather than simply throttling down the central core during ascent to preserve it's fuel, and then after booster separationthere is still fuel left in the core. I guess this would lead to higher staging, as the boosters aren't drainging their propellant into the central core, but wouldn't it accomplish roughly the same thing?or not?And if not, why not?Comparison using my simulator. The only change is shut down cross feed and activate throttle down. Even the initial pitch is the same.EDIT: Updated sheet to make MECO with ~300 m/s residual prop.Edit again: Updated with data for cross feed between 1 and 8 engines.
Quote from: modemeagle on 07/03/2012 05:14 pmQuote from: Lobo on 07/03/2012 04:44 pmNot sure if this question has already been answered, as I've not read all the pages on this thread, but I was wondering the reasons for doing crossfeed, rather than simply throttling down the central core during ascent to preserve it's fuel, and then after booster separationthere is still fuel left in the core. I guess this would lead to higher staging, as the boosters aren't drainging their propellant into the central core, but wouldn't it accomplish roughly the same thing?or not?And if not, why not?Comparison using my simulator. The only change is shut down cross feed and activate throttle down. Even the initial pitch is the same.EDIT: Updated sheet to make MECO with ~300 m/s residual prop.Edit again: Updated with data for cross feed between 1 and 8 engines.Nice simulator. How do you program the circularization burn so nicely?And wrt gravity loss. I always wondered wether that needs to take centrifugal force into account since the flightpathangle is not defined wrt an inertial reference frame. My apologies if i'm taking this too off topic.
