Author Topic: SpaceX Falcon Heavy Discussion (Thread 6)  (Read 622025 times)

Offline Jim

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #660 on: 04/26/2018 06:10 pm »
STAR-48 is too small

To back up this statement:
Star 48B masses about 2000kg, and supplies 591,000 kg(force)seconds. So if we add this to a 6000 kg probe, the starting mass will be 8000 kg and the ending about 6000 kg.  With an ISP of 292, this gives a delta-V of 292*9.8*ln(8/6) = 823 m/s.

But the extra mass takes performance from the second stage, which now ends at 13t, instead of 11t.  The loss is 348*9.8*ln(13/11) = 570 m/s.

So the net gain is only 823-570 = 253 m/s.  That's not enough to erase the shortfall at all launch opportunities, though it would help for some, since the FH is pretty close already.

What about Star 48GXV tested for the Parker Solar Probe mission as the upper stage on a Atlas V 551 vehicle but was cancelled in favor of DIVH
 cited Wiki via Orbital ATK

PSP has a Star 48 kick motor

Offline envy887

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #661 on: 04/26/2018 06:11 pm »
STAR-48 is too small

To back up this statement:
Star 48B masses about 2000kg, and supplies 591,000 kg(force)seconds. So if we add this to a 6000 kg probe, the starting mass will be 8000 kg and the ending about 6000 kg.  With an ISP of 292, this gives a delta-V of 292*9.8*ln(8/6) = 823 m/s.

But the extra mass takes performance from the second stage, which now ends at 13t, instead of 11t.  The loss is 348*9.8*ln(13/11) = 570 m/s.

So the net gain is only 823-570 = 253 m/s.  That's not enough to erase the shortfall at all launch opportunities, though it would help for some, since the FH is pretty close already.

That's a little simplistic as it assumes the 2nd stage starts at the same velocity every time and ignores the dry mass of the upper stage.

Here's a slightly higher fidelity model that counts the FH as a 3-stage vehicle. Still some rough assumptions, but it shows the optimal kick stage to be about 8-10 tonnes. However, the 2 tonne STAR-48 adds a significant boost, almost half the additional dv of what a 13.6 tonne Castor 30 does.
« Last Edit: 04/26/2018 06:14 pm by envy887 »

Offline Jim

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #662 on: 04/26/2018 06:14 pm »
Also, 6000kg spacecraft is too large to be supported by a STAR-48. 

Also, Falcon facilities are not sited for large motors.

Offline envy887

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #663 on: 04/26/2018 06:21 pm »
Also, 6000kg spacecraft is too large to be supported by a STAR-48. 

Also, Falcon facilities are not sited for large motors.

Good to know. What kick stage do you think JPL was modeling on FH then?

Offline Proponent

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #664 on: 04/26/2018 06:31 pm »
Hans' NEAF 2018 talk....

Quote
“[Crossfeed] may be introduced a bit later on”

What's NEAF?  Would you have a link for the talk?

Falcon Heavy having been de-emphasized by SpaceX, I'm surprised there is still talk of cross-feed, which I would think is a pretty big project.

Offline rockets4life97

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #665 on: 04/26/2018 06:36 pm »
Falcon Heavy having been de-emphasized by SpaceX, I'm surprised there is still talk of cross-feed, which I would think is a pretty big project.

I'd put it in the same category as human rating FH. If BFR is delayed then maybe. But likely not.

Offline Proponent

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #666 on: 04/26/2018 07:02 pm »
If I recall about wasn't there some concern from the German team about putting Helios-A on the second flight of the Titan IIIE?  I thought the first Titan IIIE launch failed or had an issue.

Yes, the first one failed, though not, as far as I can tell, because of any fundamental flaw in the integration of Titan III with Centaur.  But the upshot is that Titan IIIE had flown before it was used for a very high-value payload.

Offline russianhalo117

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #667 on: 04/26/2018 07:12 pm »
Hans' NEAF 2018 talk....

Quote
“[Crossfeed] may be introduced a bit later on”

What's NEAF?  Would you have a link for the talk?

Falcon Heavy having been de-emphasized by SpaceX, I'm surprised there is still talk of cross-feed, which I would think is a pretty big project.
Here:
http://www.rocklandastronomy.com/neaf.html
https://www.youtube.com/channel/UCZ03kS2hnMqta3Goq8u_2Pg

Offline EnigmaSCADA

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #668 on: 05/29/2018 11:41 pm »
I'm curious about staging strategies but simply don't know enough to fully figure various options out. Irrespective of the economics, business strategy, or likelihood of implementation, what sort of impact on payload and reachable orbits would the following setups result in?

1) shortening the center core and elongating the 2nd stage by the same amount for more prop in the US.

2) launch using side cores, igniting center core at booster sep. Is this even possible? I presume this would make the core expendable also?

3) crossfeed vs the current throttling of the core. Does this result in the same performance as scenario 2?

Lastly, can anyone point me to a good resource that compares various parallel vs serial staging strategies? I would have thought this would be easy information to come across but maybe I'm just struggling with right search terms.

Offline RonM

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #669 on: 05/30/2018 12:20 am »
From a cost point of view, changing the length of the stages and crossfeed don't make sense. SpaceX optimizes on cost, not performance.

Don't know if igniting the core engines later would improve performance. I think only three of the engines can be fired up in flight, so doing all nine would require modifications (cost more).

Online philw1776

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #670 on: 05/30/2018 12:40 am »
2) won't work because rocket will never leave the launch pad.

3) is different from 2. 
Crossfeed would leave the center core with more propellant than today as side cores burnt out.  Center core would then take the vehicle higher and faster.  It would not be recoverable.
« Last Edit: 05/30/2018 12:43 am by philw1776 »
FULL SEND!!!!

Online ZachS09

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #671 on: 05/30/2018 12:47 am »
If SpaceX used the crossfeed method, the only way to recover the side cores would be to land them on two drone ships ("Of Course I Still Love You" and "A Shortfall of Gravitas") while expending the center core.

Although the company throws away some money by expending the center core, at least they're bringing back the side cores to either use them again for another Falcon Heavy flight or turn them into Falcon 9 first stages.
« Last Edit: 05/30/2018 12:48 am by ZachS09 »
Liftoff for St. Jude's! Go Dragon, Go Falcon, Godspeed Inspiration4!

Online wannamoonbase

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #672 on: 05/30/2018 01:03 am »
If SpaceX used the crossfeed method, the only way to recover the side cores would be to land them on two drone ships ("Of Course I Still Love You" and "A Shortfall of Gravitas") while expending the center core.

Although the company throws away some money by expending the center core, at least they're bringing back the side cores to either use them again for another Falcon Heavy flight or turn them into Falcon 9 first stages.

I don't see that the center core must be expended.  Get a third ASDS and use 3 engine braking burns for the extra velocity.

Surely there are numbers too for the side boosters doing RTLS, they would burn out much sooner than non-crossfeed and therefore closer to land.

Seems that SpaceX decided to abandon crossfeed with the Block 5 power and lack of any commercial payload that can't be served by the Block 5 FH and of course starting the BFR development. 
We very much need orbiter missions to Neptune and Uranus.  The cruise will be long, so we best get started.

Offline EnigmaSCADA

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #673 on: 05/30/2018 01:53 am »
Would the optimal goal (in terms of spacex's desire to recover the first stage) be to bring the largest US possible up to the very limit of altitude/velocity that still allows recovery of the first stage? Sorry, I'm sure this is elementary to some of you. Is there a well known velocity limit, beyond which, the first stage (in its current form) is no longer capable of landing and being reused? I assume distance from the launch site isn't the limiting factor, right? Just means you park the barge further out?

Offline Lars-J

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #674 on: 05/30/2018 04:38 am »
If SpaceX used the crossfeed method, the only way to recover the side cores would be to land them on two drone ships ("Of Course I Still Love You" and "A Shortfall of Gravitas") while expending the center core.

No. With crossfeed, the side cores would actually stage *sooner*. (their propellant is used up faster since the central core would be siphoning off propellant too) So side core recovery would be EASIER. The central core would go much farther, though.

Offline speedevil

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #675 on: 05/30/2018 09:01 am »
2) won't work because rocket will never leave the launch pad.
With the latest engine thrust numbers, there might be T:W of 1.02 or so, even counting the heavier core and the attachment stuff.
This is technically leaving the launch pad, though at so low thrust, it might be unable to do so safely.

It would also not be an improvement, of course, which is a separate issue, as gravity losses murder you.

Offline rsdavis9

Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #676 on: 05/30/2018 11:56 am »
Something like the current profile but shutdown the center core part way up and then restart center after staging. Alternatively deep throttling could do much of this without stopping and restarting.

EDIT: we do know that the throttling of the first FH was VERY conservative. They only ran the engines at 90% thrust? How deep can a M1D throttle? 30% ?
Probably a LOT of room for improvement.
« Last Edit: 05/30/2018 12:01 pm by rsdavis9 »
With ELV best efficiency was the paradigm. The new paradigm is reusable, good enough, and commonality of design.
Same engines. Design once. Same vehicle. Design once. Reusable. Build once.

Offline rpapo

Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #677 on: 05/30/2018 12:05 pm »
Something like the current profile but shutdown the center core part way up and then restart center after staging. Alternatively deep throttling could do much of this without stopping and restarting.

EDIT: we do know that the throttling of the first FH was VERY conservative. They only ran the engines at 90% thrust? How deep can a M1D throttle? 30% ?
Probably a LOT of room for improvement.
(1) Without some changes to the Octoweb, only three of the engines can be started (or restarted) during flight.
(2) It is not clear (to me, at least) whether the Merlins can be throttled down from 100% to 60% (40% down), or from 100% to 40% (60% down).
Following the space program since before Apollo 8.

Offline envy887

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #678 on: 05/30/2018 01:02 pm »
Something like the current profile but shutdown the center core part way up and then restart center after staging. Alternatively deep throttling could do much of this without stopping and restarting.

EDIT: we do know that the throttling of the first FH was VERY conservative. They only ran the engines at 90% thrust? How deep can a M1D throttle? 30% ?
Probably a LOT of room for improvement.

The sides ran at 90%. The center much lower than that, you can see the difference in the length of the exhaust plume clearly right off the pad.

Something like the current profile but shutdown the center core part way up and then restart center after staging. Alternatively deep throttling could do much of this without stopping and restarting.

EDIT: we do know that the throttling of the first FH was VERY conservative. They only ran the engines at 90% thrust? How deep can a M1D throttle? 30% ?
Probably a LOT of room for improvement.
(1) Without some changes to the Octoweb, only three of the engines can be started (or restarted) during flight.
(2) It is not clear (to me, at least) whether the Merlins can be throttled down from 100% to 60% (40% down), or from 100% to 40% (60% down).

Only 3 engines can restart. But the center one at least can throttle to 40% thrust (60% down from full), and it seems likely to me that all 9 can do that.

Offline envy887

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Re: SpaceX Falcon Heavy Discussion (Thread 6)
« Reply #679 on: 05/30/2018 01:05 pm »
I'm curious about staging strategies but simply don't know enough to fully figure various options out. Irrespective of the economics, business strategy, or likelihood of implementation, what sort of impact on payload and reachable orbits would the following setups result in?

1) shortening the center core and elongating the 2nd stage by the same amount for more prop in the US.

2) launch using side cores, igniting center core at booster sep. Is this even possible? I presume this would make the core expendable also?

3) crossfeed vs the current throttling of the core. Does this result in the same performance as scenario 2?

Lastly, can anyone point me to a good resource that compares various parallel vs serial staging strategies? I would have thought this would be easy information to come across but maybe I'm just struggling with right search terms.

1) doesn't help much unless also combined with crossfeed

2) gravity losses probably cause this to be worse than the current launch profile with the center core throttling down early.

3) Crossfeed is MUCH better. it empties and ditches the side boosters sooner while having the most thrust available and a full core stage at staging. The only way to improve on crossfeed would be to stretch the upper stage.

 

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