Author Topic: SpaceX Falcon Heavy Discussion (Thread 3)  (Read 332246 times)

Online edkyle99

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #140 on: 02/26/2015 09:03 pm »
Boosters carrying more propellant than core makes no sense unless crossfeed is used, which is apparently not going to be the case.  There is no performance advantage.  If anything, there is a performance penalty. 

I am not an expert, so could you elaborate more on why this would not provide any performance advantage? Center core will be throttled down = it feels like the more propelant you have in boosters, the higher delta V you can give to the center core.
The comparison I made is for a given GLOW, assumes no crossfeed, and assumes full recovery of boosters and core.  The choice is between all three cores being identical and having the boosters carry, say, 10% more propellant than the core.  In my modeling, I find that the identical cores example provides slightly more LEO payload than the bigger boosters example, partly because there is less propellant left in the core at staging for the latter.  The goal isn't just to boost the core to some velocity, it is also to stage it with as much propellant still on board as possible.  The longer-burning boosters would cause the core to burn more propellant before staging.

 - Ed Kyle
« Last Edit: 02/26/2015 09:04 pm by edkyle99 »

Offline MarekCyzio

Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #141 on: 02/26/2015 09:13 pm »
The longer-burning boosters would cause the core to burn more propellant before staging.

Can't the core be throttled even more? Or some core engines may even be shut down to save fuel? But I think I see your point - adding more fuel to boosters makes them carry their own weight for a longer period and the goal is to get rid of the boosters as soon as possible.

Offline Robotbeat

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #142 on: 02/26/2015 09:16 pm »
I would give you 20:1 odds and still take the bet. The boosters are stretched, we have it from multiple sources.
Who?

 - Ed Kyle
SpaceX multiple times and Jim.
Maybe all three are stretched from v1.1. That would make sense.  Boosters carrying more propellant than core makes no sense unless crossfeed is used, which is apparently not going to be the case.  There is no performance advantage.  If anything, there is a performance penalty. 

Meanwhile, we have these images to ponder.

 - Ed Kyle
The interstage, man.

Also, there IS a performance advantage. Merlin 1D can throttle widely (and can relight). That provides much the performance advantage that cross-feed does.

EDIT:Okay, I would still take the bet that they're stretched, but you've made a good enough case that I wouldn't offer you 20:1 odds.
« Last Edit: 02/26/2015 09:20 pm by Robotbeat »
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Online edkyle99

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #143 on: 02/26/2015 09:29 pm »
The longer-burning boosters would cause the core to burn more propellant before staging.
Can't the core be throttled even more? Or some core engines may even be shut down to save fuel? But I think I see your point - adding more fuel to boosters makes them carry their own weight for a longer period and the goal is to get rid of the boosters as soon as possible.
The longer-burning boosters would cause the core to burn more propellant before staging.

Can't the core be throttled even more? Or some core engines may even be shut down to save fuel? But I think I see your point - adding more fuel to boosters makes them carry their own weight for a longer period and the goal is to get rid of the boosters as soon as possible.
The core can be throttled to the minimum for both cases, but will still have less propellant left for the longer-burning booster example.  The only advantage of a longer booster would be if it transferred propellant to the core while still attached, but crossfeed seems unlikely to appear for awhile if ever.

 - Ed Kyle
« Last Edit: 02/26/2015 09:30 pm by edkyle99 »

Offline cscott

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #144 on: 02/26/2015 09:47 pm »
I would give you 20:1 odds and still take the bet. The boosters are stretched, we have it from multiple sources.

I'll take the bet.  20:1 odds are pretty good!

I think unstretched boosters makes sense given SpaceX's incremental development philosophy, plus the fact that FH is already rather late.

FHv1: unstretched boosters, no crossfeed, identical tanks, interstage & grid fins for all cores, "just" stage interconnects, core throttling, and everything else which is "essential" for FH.  Get 'er flying ASAP!
FHv1.1: stretched boosters, new interstage for boosters, grid fin relocation, etc.  Probably all in development, just not on the critical path for first flight anymore.  Low-hanging fruit.
FHv1.2: crossfeed.  Developed only if/when a customer has need, may be bypassed by current events.
...
FHv2: S2 reusability? (A man can dream!)

That seems like a nice incremental development path, assuming that all three of FHv1, FHv1.1, FHv1.2 offer monotonic performance improvements over F9R.

Online Semmel

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #145 on: 02/27/2015 11:19 am »
Do we have any knowledge what payload the first FH will have and where it would be going? I am looking for actual information here, not speculation.

I couldnt really find info on that question. From Shit Elon Sais from a talk in 2011 which is quite outdated. (Thx QuantumG !!!)  link
Quote
The first mission is really a demonstration flight. It's there to prove that Falcon Heavy will work. That it will deliver the payload that we say it can, and we don't have a primary customer for it, but we are likely to have several smaller secondary satellites on-board that will do a variety of things, and if we get lucky, maybe there will be a big satellite at the last minute that wants to buy the flight at a reduced price.

Offline macpacheco

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #146 on: 02/28/2015 12:52 am »
I would give you 20:1 odds and still take the bet. The boosters are stretched, we have it from multiple sources.

I'll take the bet.  20:1 odds are pretty good!

I think unstretched boosters makes sense given SpaceX's incremental development philosophy, plus the fact that FH is already rather late.

FHv1: unstretched boosters, no crossfeed, identical tanks, interstage & grid fins for all cores, "just" stage interconnects, core throttling, and everything else which is "essential" for FH.  Get 'er flying ASAP!
FHv1.1: stretched boosters, new interstage for boosters, grid fin relocation, etc.  Probably all in development, just not on the critical path for first flight anymore.  Low-hanging fruit.
FHv1.2: crossfeed.  Developed only if/when a customer has need, may be bypassed by current events.
...
FHv2: S2 reusability? (A man can dream!)

That seems like a nice incremental development path, assuming that all three of FHv1, FHv1.1, FHv1.2 offer monotonic performance improvements over F9R.
The snag is USAF/NASA certification, unless they could upgrade with a single launch to confirm everything is ok, SpaceX has a big incentive for doing all much as possible on V1.0.
Besides, what does SpaceX has to learn from FH, compared to everything they got through already ? Yes, there are variables, but it seems from F9R to FH v1.0 its less of a leap than from F9 to F9R. So go to what you consider FHv1.1 directly instead.
FH might be 50% DoD missions long run. That tends to prevent SpaceX from doing major incrementalism.
Finally, most missions conceived (and all booked) for FH don't need anything beyond FHv1.1. Leave something for BFR.

The one way to enable incrementalism is recovery of all 3 stages, which would enable upgrading lots of F9R launches to FH at minimal cost, while giving them lots of flight data to certificate from. However I would think USAF wouldn't be too fond of 3 certifications.

Also, with Dragon2 and FH mostly done frees up resources to focus on BFR designs. I would hope the simpler upscaled F9R using Raptor engines could see major design work start 2016.
« Last Edit: 02/28/2015 02:12 am by macpacheco »
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Offline Space Ghost 1962

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #147 on: 02/28/2015 01:39 am »
Keep in mind payloads and who you sell capacity to.

FH commercial payloads (GSO) that launch on Ariane V only are a prime candidate.

AV 551 and DIVH govt payloads are other prime candidates. Keep also in mind likely orbital insertion requirements.

For now that's all you'd need to certify under a single configuration called FH.

There are only two much bigger payload sources - theoretically NASA and, of course, someone else who has ambitions for Mars. For these two, likely a "special case" arrangement might be in order. Also still called FH.

You don't really need more, do you?

Offline cuddihy

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #148 on: 02/28/2015 04:14 am »
Ok, I'm aware this sound insane from a risk standpoint. But what if you not only throttled down the core engines but actually shut down the center engine as well? They've had a lot of experience now re-lighting it, just not while the other ones are running. So you throttle the core mains to say 80% and shut down the center engine while the boosters keep burning at 115%. Now you'd have almost half the fuel left in the core when you stage. That's significant.  If the stage can survive this thrust differential at Max Q, and the additional gravity losses...hmm, needs more work...

Offline Robotbeat

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #149 on: 02/28/2015 04:15 am »
They relight 3 engines, not just the center one.
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Offline cuddihy

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #150 on: 02/28/2015 04:34 am »
Yes, but shutting those down 30s into flight, that would be missing another 220,000 lbf of first stage thrust you need to get to orbit. There are limits...you'd get close to overall vehicle thrust < weight. Bad for gravity losses.

Offline Nilof

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #151 on: 02/28/2015 10:55 am »
Question: how much will the increased thrust of the Merlin 1D+ affect the benefits of crossfeed? If the side boosters have more thrust, you can throttle down the center core more without being eaten up alive by gravity drag, and the benefits of crossfeed should be smaller.
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline Robotbeat

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #152 on: 02/28/2015 01:14 pm »
Question: how much will the increased thrust of the Merlin 1D+ affect the benefits of crossfeed? If the side boosters have more thrust, you can throttle down the center core more without being eaten up alive by gravity drag, and the benefits of crossfeed should be smaller.
Precisely.
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Online edkyle99

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #153 on: 02/28/2015 02:05 pm »
Ok, I'm aware this sound insane from a risk standpoint. But what if you not only throttled down the core engines but actually shut down the center engine as well? They've had a lot of experience now re-lighting it, just not while the other ones are running. So you throttle the core mains to say 80% and shut down the center engine while the boosters keep burning at 115%. Now you'd have almost half the fuel left in the core when you stage. That's significant.  If the stage can survive this thrust differential at Max Q, and the additional gravity losses...hmm, needs more work...
If engines aren't needed, why have them at all?  Thrust costs money.

It seems more likely that the rocket is designed to use, and need, the thrust.

 - Ed Kyle

Offline cscott

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #154 on: 02/28/2015 02:46 pm »
Re: certification --- it may well be that FHv1.1 is the version they get certified.   And the gap between v1 and v1.1 needn't be long. This is exactly how F9 development worked. F9v1.0 flew for only a few flights, F9v1.1 is the variant that got certified.

Offline Robotbeat

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #155 on: 02/28/2015 02:55 pm »
Ok, I'm aware this sound insane from a risk standpoint. But what if you not only throttled down the core engines but actually shut down the center engine as well? They've had a lot of experience now re-lighting it, just not while the other ones are running. So you throttle the core mains to say 80% and shut down the center engine while the boosters keep burning at 115%. Now you'd have almost half the fuel left in the core when you stage. That's significant.  If the stage can survive this thrust differential at Max Q, and the additional gravity losses...hmm, needs more work...
If engines aren't needed, why have them at all?  Thrust costs money.

It seems more likely that the rocket is designed to use, and need, the thrust.

 - Ed Kyle
Huh? Where'd you get that from? You'd still want full thrust right at lift-off but throttle down the core (perhaps shut down an engine or two) soon after then throttle the core to full right after the boosters stage off.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline TomH

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #156 on: 02/28/2015 04:36 pm »
Ok, I'm aware this sound insane from a risk standpoint. But what if you not only throttled down the core engines but actually shut down the center engine as well? They've had a lot of experience now re-lighting it, just not while the other ones are running. So you throttle the core mains to say 80% and shut down the center engine while the boosters keep burning at 115%. Now you'd have almost half the fuel left in the core when you stage. That's significant.  If the stage can survive this thrust differential at Max Q, and the additional gravity losses...hmm, needs more work...
If engines aren't needed, why have them at all?  Thrust costs money.

It seems more likely that the rocket is designed to use, and need, the thrust.

 - Ed Kyle

The reader is supposed to draw an inference that said engine(s) run when needed-at liftoff and after booster separation. In essence they're throttling down-all the way down to 0% before throttling back up after boosters are gone.

Offline sanman

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #157 on: 02/28/2015 08:15 pm »
I'm curious - does Falcon Heavy automatically fall under the Reusable Rockets category?

Is there any mission type where none of it gets retrieved or re-used?

Offline Robotbeat

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #158 on: 02/28/2015 08:18 pm »
I'm curious - does Falcon Heavy automatically fall under the Reusable Rockets category?

Is there any mission type where none of it gets retrieved or re-used?
Yeah, anything which needs its full performance. None really exist, yet, except maybe some high energy missions like New Horizons.
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Offline sanman

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Re: SpaceX Falcon Heavy Discussion (Thread 3)
« Reply #159 on: 02/28/2015 08:27 pm »
Yeah, anything which needs its full performance. None really exist, yet, except maybe some high energy missions like New Horizons.

But not even the side-boosters get recovered in that case?

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