Author Topic: SpaceX Falcon Heavy Discussion (Thread 3)  (Read 202793 times)

Offline Herb Schaltegger

Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #20 on: 02/16/2015 09:03 PM »
Bear in mind, too, that in Aerospace-eese, "working on" something can mean anything from one guy running some Excel spreadsheet numbers and ginning up some PowerPoints, to a whole team of designers and mission planners working up a full RFP submittal, to a trucking the finished product out to meet a delivery date.

So I'm sure Blackstar's source told him right, just didn't tell him much at all. ;)
Ad astra per aspirin ...

Offline Jim

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #21 on: 02/16/2015 09:07 PM »
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?

Bingo, i would put money on that. It is for a kick stage and not a replacement for the existing second stage or an additional liquid stage.
« Last Edit: 02/16/2015 09:10 PM by Jim »

Offline punder

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #22 on: 02/16/2015 09:08 PM »
An RL-10 upper stage would go completely against SpaceX's demonstrated philosophy of engine and stage commonality.  It would blow launch costs out of the water and create a lot of technical and contractual entanglements that they seem very eager to avoid.

Doesn't mean they won't do it if they have to; just low-probability.  YMMV.

Offline TrevorMonty

Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #23 on: 02/16/2015 11:08 PM »
The engine would be DOD certified if not the stage, one plus going for.

Offline Jim

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #24 on: 02/17/2015 12:01 AM »
The engine would be DOD certified if not the stage, one plus going for.

That is meaningless.  Components are not certified, only vehicles.

Offline deltaV

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #25 on: 02/17/2015 01:58 AM »
Here are some sketchy performance comparisons between no-cross-feed Falcon Heavy (FH) and Delta IV Heavy (DIVH) and Atlas V 551 (AV) for interplanetary trajectories. I'm getting numbers for ULA's launchers from the chart at http://www.nap.edu/openbook.php?record_id=12554&page=109. I'm getting numbers for FH from my rocket equation spreadsheet (a revised version of the one posted at http://forum.nasaspaceflight.com/index.php?topic=33897), which involves lots of numbers pulled out of thin air but is calibrated to match SpaceX's reported 45 tonnes to LEO.

Comparing DIVH to FH it seems FH does better below around C3 = 60 km^2/s^2 (= above 3.5 tonnes payload) and DIVH does better above. Comparing AV to FH it seems FH does better below around C3 = 98 km^2/s^2 (= above 0.8 tonnes payload) and AV does better above. If you give FH a Star 48B kick stage (0.13 tonnes dry, 2.14 tonnes wet, 292.1 s ISP) it outperforms both DIVH and AV to all orbits. Consequently for any possible combination of kick stages for DIVH you can beat it with FH if you add an additional Star 48B to the kick stage collection. Note that FH plus kick stage is probably cheaper than DIVH without a kick stage so giving FH an extra kick stage is not necessarily unfair.

If you give all three launchers a Star 48 then the performance comparison comes down to whether or not the payload plus kick state combined exceed the critical values above, i.e. DIVH does better than FH when payload is less than about 3.5 - 2.14 = 1.36 tonnes, which both launchers can send to around C3 = 131 km^2 / s^2.

Conclusion: even without cross-feed FH does better than DIVH and AV for almost all missions, including LEO, GTO, GEO, Moon and Mars. For the highest delta-vee missions the winner depends on the kick stages used. With no kick stages DIVH beats FH for high C3 missions. With optimal kick stages for each vehicle FH can always throw more, but for the same performance FH may require more complicated kick stage(s) than DIVH.

Concretely:
-Solar Probe Plus is 0.685 tonnes and C3 = 154 km^2/s^2 (http://forum.nasaspaceflight.com/index.php?topic=26605.msg1188790#msg1188790), which is within the capabilities of Star 48B plus either FH or DIVH. The DIVH has slightly better performance here but FH can meet requirements too.
-NASA is considering a Europa mission involving SLS sending about 5 tonnes to C3 = 86 km^2/s^2. As I described elsewhere (http://forum.nasaspaceflight.com/index.php?topic=27871.msg1331978#msg1331978) that's out of reach of FH, DIVH and AV, even with hypergolic or solid kick stages. In theory it's within reach of FH plus a hydrogen kick stage such as Centaur or DCSS, but that's unlikely to be worth the trouble compared to the alternatives of using SLS or a lower energy mission plan with a Venus flyby.

P.S. To sanity-check the above I asked Schillings (http://www.silverbirdastronautics.com/LVperform.html) for FH performance to C3=98 km^2/s^2 and got 2.46 tonnes, substantially more than the 0.8 tonnes I had found. Schillings presumably uses cross-feed; if I turn cross feed on my model increases performance to 1.8 tonnes, a lot closer. My point is this difference between 1.8 tonnes and 2.46 tonnes gives a hint at the magnitude of the error in my figures.
« Last Edit: 02/17/2015 02:01 AM by deltaV »

Offline IainMcClatchie

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #26 on: 02/17/2015 04:03 AM »
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.

Online Lars-J

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #27 on: 02/17/2015 04:13 AM »
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. This is what all 100% liquid heavies do - including Delta IV Heavy and Angara A5.

In the case of FH, it also allows the boosters to stage earlier, so they have an easier time to get back to the launch site.
« Last Edit: 02/17/2015 04:15 AM by Lars-J »

Offline IainMcClatchie

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #28 on: 02/17/2015 06:17 AM »
The 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.

Online RonM

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #29 on: 02/17/2015 06:43 AM »
The 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.

If you go full throttle on the core and the two boosters you are hauling the mass of all three until you run out of propellant. If you throttle back on the core it still has propellant after you dump the mass of the boosters.

You can think of the boosters as the first stage and the core as the second stage. Of course, you have to run the core and the boosters at full throttle to get the thing off the ground.

Offline MP99

Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #30 on: 02/17/2015 06:45 AM »


Schillings presumably uses cross-feed;

Only if you configure the prop load for stages 0 and 1 for "booster and a bit of core prop" and "rest of the core prop", respectively.

Configure differently, and you'll have the non-crossfeed config.

Cheers, Martin

Offline Ben the Space Brit

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #31 on: 02/17/2015 08:49 AM »
Purely FWIW, I think that RL-10C's thrust is too low for use on Falcon Heavy, even if you use cross-feed on the core and boosters. Staging is just too low and slow (because of RTLS performance limitations). Given that limitation and given that I think that Messrs Musk and Bezos would prefer to gouge out their own eyes than work together, the only hydrolox engine likely to be used on Falcon Heavy, IMHO at least (for a certain percentage of 'likely' anyway) is MB-60.
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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #32 on: 02/17/2015 10:41 AM »
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.

All of my simulations of FH show the maximum payload to orbit was only cross feeding 4 of the 9 engines.  This was simulating the lower performance Merlin 1C before Merlin 1D was created.  The numbers may change now, but I was showing 4 was the best match.  I published a bunch of my simulation runs on earlier Falcon Heavy threads including boost back of the three cores to launch site.

Offline jak Kennedy

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #33 on: 02/17/2015 11:32 AM »
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?

Offline Ben the Space Brit

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #34 on: 02/17/2015 11:41 AM »
If hypergolic's are suitable would/could SpaceX use a kick stage with super dracos?

As is, the Super Draco is unsuited for upper atmosphere/vacuum firing. The engine would need an expansion nozzle and maybe some other tweaks. Whilst it might be suitable, it is more likely that, should a payload need a kick stage, SpaceX would require the customer to provide them. Orbital-ATK and AJR make lots of suitable products for that purpose.
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Offline gospacex

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #35 on: 02/17/2015 02:58 PM »
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.

Online abaddon

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #36 on: 02/17/2015 03:04 PM »
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.

Er... what?  Where exactly are you planning on putting these extra engines?  Even if you could the boosters would then be radically different from the core.  This is a non-starter.

Offline spacenut

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #37 on: 02/17/2015 04:13 PM »
Disconnect can be with quick disconnect type couplings.  The Soyuz rocket uses cross feed and has since the 1950's.  They just send a signal and they disconnect.  Very few failures if any.  I'm surprised Delta IV heavy hasn't used cross feed to get another 5-10 tons or so to orbit.  I think the original Falcon Heavy 53 tons to LEO was with cross feed.  I think it is 45 or so without.  Someone can chime in here with exacts.  Cross feed doesn't have to go straight to the engines, it can go to the fuel and oxidizer tanks on the boosters to the core.  That way there is only 4 connections to disconnect.   

Online Lars-J

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #38 on: 02/17/2015 04:14 PM »
The Soyuz rocket uses cross feed and has since the 1950's.  They just send a signal and they disconnect.

Huh? Since when?

Offline spacenut

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #39 on: 02/17/2015 04:23 PM »
I read somewhere that the 4 strap ons burn with the core and transfer fuel to the core at the same time.  Then they drop off and the core continues.  Maybe they don't since they are smaller.  However I read that years ago. 

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