I wonder if the Aerojet guy that Blackstar ate lunch next to was working on the NASA project to build a common upper stage discussed in this thread: http://forum.nasaspaceflight.com/index.php?topic=35144 . Or maybe this Falcon Heavy upper stage project evolved out of that NASA-funded project?
The engine would be DOD certified if not the stage, one plus going for.
There is a lot I do not understand on the Falcon Heavy SpaceX page. It says the core engines throttle down shortly after liftoff (presumably to limit max Q), and throttle up after booster separation. This implies they stay throttled down between those two events. It seems to me that once past max Q, I'd want those engines right back up to 100% again to reduce gravity losses. Maybe they just don't mention those two more throttle moves.
The core is throttled down until separation to leave as much propellant as possible in the core at separation. This improves performance.
Quote from: Lars-J on 02/17/2015 04:13 amThe core is throttled down until separation to leave as much propellant as possible in the core at separation. This improves performance.Right, that's the part I don't understand.I don't have access to a simulator anymore, but when I used to run simulations, it was always better to burn as much propellant as soon as possible, subject to the limitations of the airframe, and the staging effect. MaxQ is the first limitation, most rockets have to throttle down a bit to avoid getting too fast in the lower atmosphere. I know that the Space Shuttle, at least, throttled all the way back up after maxQ. Typically aero loads have an early maximum just after mach 1, less than 1/3 of the way through the first stage burn, and then trend down from there, as air density drops off faster than velocity^2 increases. The second limitation is maximum acceleration. I know the Saturn V and the Falcon 9 both have to throttle down their first stages shortly before engine cutoff to avoid too much acceleration. But that limitation comes in fairly late in the first stage burn, in the last 20-30 seconds. Finally, staging helps by eliminating the weight of empty tankage and engines that have had to be turned off.The longer you wait around to burn off your propellant, the more gravity loss you suffer, especially early in the flight. This is a strong effect. There must be some really strong reason to throttle down the core stage on an F9 or D4H launch, to overcome the gravity loss effect. What is it?I thought maybe the stress loads on the vehicle would be smaller with less thrust, but upon further thought I think stress loads on the interconnect between the boosters and core are minimized by minimizing the difference between core thrust and booster thrust.
Schillings presumably uses cross-feed;
Hope this is the right thread to post in.I think I have a scheme which gets the Falcon Heavy most of the benefit of crossfeed with only a small portion of the risk.At liftoff, five of the core engines slurp the core tanks, and two of the core engines run off each of the booster tanks. The five core engines throttle down as necessary to limit max Q and max acceleration. So far, standard crossfeed.If the center five engines can get down to 50% throttle, the boosters run out of propellant (certainly in the reuse case) before we hit maximum acceleration. So, the booster engines run at 100% for their whole trip. When the booster engine cutoff is signaled around +162 seconds, all 22 engines fed by booster tanks cut off.The four dead engines on the core stage are never restarted. There is no crossfeed valve. This is the extent of the idea. The five remaining engines ramp back up to 100%, and stay at 100% until core engine cutoff at around 310 seconds into flight.The new hardware needed are new propellant plenums for the core and booster stages, and unions between the adjacent fuel and oxidizer manifolds that can be isolated, drained, and disconnected in flight.This scheme is not as good as full-blown crossfeed. It dumps the empty booster weight almost as early as possible. Perfect crossfeed (100% full core at booster engine cutoff) would dump them 15 seconds earlier, gaining perhaps 60-70 m/s more delta-V. Perfect crossfeed would also have all nine core engines running after booster separation, which would make the core engine cutoff about 50 seconds earlier, which would reduce gravity losses. I'm not sure how much delta-V that is worth, perhaps 100 m/s, perhaps more.This scheme is better than no crossfeed. Without crossfeed, the booster engine cutoff happens around 200 seconds (assuming reuse). The extra momentum carried away by the separated boosters costs 160-180 m/s delta-V compared to my scheme. Also, either the boosters will have to be throttled, or core engines will have to be shut down to avoid overacceleration before booster separation. The first gets the boosters going even faster when they separate, and the second requires either a midflight restart (with the payload still attached) or delaying core engine cutoff, either of which has penalties.There is a lot I do not understand on the Falcon Heavy SpaceX page. It says the core engines throttle down shortly after liftoff (presumably to limit max Q), and throttle up after booster separation. This implies they stay throttled down between those two events. It seems to me that once past max Q, I'd want those engines right back up to 100% again to reduce gravity losses. Maybe they just don't mention those two more throttle moves.The diagram also shows the boosters about 15% larger than the core stage, the upper stage identical in propellant mass to the F9, and the total liftoff mass 54,702 kg shy of 3x the Falcon 9. First, why would the FH need a faster liftoff acceleration than the F9? By loading more propellant, you could get more payload. Second, I'd expect the booster propellant load to be approximately the same as the F9's first and second stage propellant loads combined, but the increase is only 2/3 of that. There is plenty of room to stretch those tanks further.
From the Europa science thread Blackstar posted some juicy Falcon Heavy info:I wonder what sort of fuel that upper stage would use. Aerojet Rocketdyne has a suitable hydrogen engine (RL-10), various hypergolic engines (e.g. Shuttle OMS) and solids experience (e.g. Orion FTS jettison motor) so there are a lot of plausible options.If that guy's project is official it sounds like Aerojet Rocketdyne is trying to build a future for itself that doesn't rely on ULA. I was going to write that this was surprising back-stabbing of its close business partner ULA but then I remembered that ULA has already cheated on that marriage with their funding of XCOR's RL-10 competitor.about cross-feed and was told by one of the people working on the rocket that they are not developing it. It's a potential upgrade if somebody pays for it, but they're not doing the development. So you shouldn't use it in your calculations.
If hypergolic's are suitable would/could SpaceX use a kick stage with super dracos?
Hope this is the right thread to post in.I think I have a scheme which gets the Falcon Heavy most of the benefit of crossfeed with only a small portion of the risk.At liftoff, five of the core engines slurp the core tanks, and two of the core engines run off each of the booster tanks. The five core engines throttle down as necessary to limit max Q and max acceleration. So far, standard crossfeed.If the center five engines can get down to 50% throttle, the boosters run out of propellant (certainly in the reuse case) before we hit maximum acceleration. So, the booster engines run at 100% for their whole trip. When the booster engine cutoff is signaled around +162 seconds, all 22 engines fed by booster tanks cut off.The four dead engines on the core stage are never restarted. There is no crossfeed valve. This is the extent of the idea. The five remaining engines ramp back up to 100%, and stay at 100% until core engine cutoff at around 310 seconds into flight.The new hardware needed are new propellant plenums for the core and booster stages, and unions between the adjacent fuel and oxidizer manifolds that can be isolated, drained, and disconnected in flight.
It looks like you can bite the bullet and just mount those two engines on the boosters instead of the core. Then at booster separation you do not need any disconnects and you are not carrying dead weight on the core.
The Soyuz rocket uses cross feed and has since the 1950's. They just send a signal and they disconnect.