Author Topic: SpaceX: General Falcon and Dragon discussion (Thread 3)  (Read 568097 times)

Online meekGee

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #20 on: 02/22/2011 06:49 pm »
My guess is that they won't throttle the core down early - they'll kill more engines on the center core, and earlier.

You want all three cores at full blast on liftoff, but since all cores are getting lighter and you're only killing center core engines, you can do it faster.

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Offline ugordan

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #21 on: 02/22/2011 06:49 pm »
* Warning: speculation ahead *

I agree that this would either suggest crossfeed (not likely any time soon IMHO) or a reduced number of engines on the core. Eyeballing the strapons, I'd say they'd carry 25% more propellant.

Let's assume for a moment the core would have 5 engines instead of 9.

I've done a WAG T/W calculation : assuming a core weighing 280 tons + 2nd stage weighing 40 tons + payload weighing 30 tons and 2 strapons weighing 350 tons that totals about 1050 tons GLOW.

9+9+5 Merlin 1d engines should produce about 1230 tons liftoff thrust. So it could get off the ground, albeit slowly.

Now assuming the engines still won't be throttleable and assuming no strapon engines do G-limiting early cutoff (probably not a valid assumption), that propellant load difference would give the core 1.44 times a longer burn time than the strapons.

As for the actual payload this thing would have? I haven't a slightest idea.

Of course, having 9 engines on the core would still work but you'd want to cut some off real early and in a sense you'd be throwing engines away after a minute or so of work.

« Last Edit: 02/22/2011 06:55 pm by ugordan »

Offline Jim

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #22 on: 02/22/2011 06:51 pm »

But!  if the rocket can handle the thrust imbalance, which it should, then you can have three engines fail *anywhere* in the 27 cluster, and  probably have even more engines fail if the distribution is favorable.


How does the 3 body vehicle handle the thrust imbalance?  It can't use the same method as the single core vehicle.

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #23 on: 02/22/2011 07:07 pm »
Of course, having 9 engines on the core would still work but you'd want to cut some off real early and in a sense you'd be throwing engines away after a minute or so of work.

I think that's the preferred approach.  You're indeed using some of the engines for only a short period, but these are small 1/9 engines.  It's not prohibitive, and it factors into the whole reliability issue.  It's a small price to pay, and you haven't modified the center core.

It's no different than having a single engine per core, with the center core engine throttled down by increments of 11%.   Whether they can (or want) to re-light some engines after side core separation is an optimization thing - maybe another reason why the side cores are stretched, so they don't have to relight.
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Offline ugordan

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #24 on: 02/22/2011 07:09 pm »
I think that's the preferred approach.  You're indeed using some of the engines for only a short period, but these are small 1/9 engines.  It's not prohibitive, and it factors into the whole reliability issue.

The core in a parallel-staged vehicle is supposed to burn longer so the energy at burnout would be significantly higher than on a single stick F9. Hence you would probably have to kiss any stage recovery goodbye.

Offline Jim

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #25 on: 02/22/2011 07:10 pm »

Flying at an angle is a bitch of course, but unlike the Shuttle for example where the biggest thrusters are very far apart, the aspect ratio inside the single cluster and between the cluster is not that much different, so the flight angle will not be much different.


You still can't arm wave it away and say the vehicle can take it.  The shuttle is designed for those loads. 

Offline kevin-rf

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #26 on: 02/22/2011 07:11 pm »
My guess is that they won't throttle the core down early - they'll kill more engines on the center core, and earlier.

You want all three cores at full blast on liftoff, but since all cores are getting lighter and you're only killing center core engines, you can do it faster.



Didn't an N1 fail that way, just kept on shutting down engines to balance it until it could no longer lift itself over the lithosphere?
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Online meekGee

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #27 on: 02/22/2011 07:14 pm »
The core in a parallel-staged vehicle is supposed to burn longer so the energy at burnout would be significantly higher than on a single stick F9. Hence you would probably have to kiss any stage recovery goodbye.
That's correct, but without cross-feed, the stretched side-core configuration is going there no matter what...   It's their ship  :)

All I'm saying is that with extended side-cores, I'd keep the center core as is, and start killing engines early.  Maybe with a three-barrel, they are content recovering only 2/3.
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Online meekGee

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #28 on: 02/22/2011 07:18 pm »
You still can't arm wave it away and say the vehicle can take it. 

Go back to the list I posted above, you can easily ascertain the thrust imbalance and the torque arm in each of the scenarios - the angle is chiefly determined by that, since you still need the thrust vector to pass through the CG.

k?
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Offline Jim

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #29 on: 02/22/2011 07:20 pm »
You still can't arm wave it away and say the vehicle can take it. 

Go back to the list I posted above, you can easily ascertain the thrust imbalance and the torque arm in each of the scenarios - the angle is chiefly determined by that, since you still need the thrust vector to pass through the CG.

k?


That was my point that the same method for a single core is not applicable to the 3 core.

What says the vehicle can take the aero side loads?

These cores are to be the same as the F9 single core.
« Last Edit: 02/22/2011 07:22 pm by Jim »

Offline ugordan

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #30 on: 02/22/2011 07:24 pm »
Didn't an N1 fail that way, just kept on shutting down engines to balance it until it could no longer lift itself over the lithosphere?

Keep in mind N1 performed steering via differential throttling so loss of an engine on one side could play havoc on control authority. Losing an engine in a gimballing setup would be less of an issue in this respect, but still nontrivial.
« Last Edit: 02/22/2011 07:24 pm by ugordan »

Online meekGee

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #31 on: 02/22/2011 07:27 pm »
No, I'm saying the angle of attack is similar, and the vehicle is actually stronger in that plane. Otherwise, I'm presenting the best calcs I can make from existing data.

I still have to hear a single argument why it will be worse - other than "nobody proved it to my satisfaction". 

We've got this dynamics going where I follow up on the data as best I can, and all you guys are saying is "you're arm waiving" - but you're not presenting anything to the contrary...   

Of course I can't absolutely prove stuff, since I don't have the SpaceX data...   But you're not arguing to the contrary, you're just complaining...
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Offline Nate_Trost

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #32 on: 02/22/2011 07:28 pm »
There is, of course, the open question of how the engines would be controlled on the F9H. Side booster engines locked down, independent avionics on each core and only center core gimbaling engines? That would itself seem to present some compensation limitations.

Offline Jim

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #33 on: 02/22/2011 07:33 pm »
No, I'm saying the angle of attack is similar, and the vehicle is actually stronger in that plane. Otherwise, I'm presenting the best calcs I can make from existing data.

I still have to hear a single argument why it will be worse - other than "nobody proved it to my satisfaction". 

We've got this dynamics going where I follow up on the data as best I can, and all you guys are saying is "you're arm waiving" - but you're not presenting anything to the contrary...   

Of course I can't absolutely prove stuff, since I don't have the SpaceX data...   But you're not arguing to the contrary, you're just complaining...

a.  How is the vehicle "stronger" in that plane?
b.  The fairing is not stronger

It is worse because the vehicle is flying at an angle of attack it is not designed for.  The vehicle is using the same cores as the existing F9.

And you have yet to address the propellant utilization.
« Last Edit: 02/22/2011 07:34 pm by Jim »

Offline moose103

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #34 on: 02/22/2011 07:39 pm »
But it makes more sense to stretch the core instead of the boosters.

Wow.  Is that ever a presumptive statement.  You were able to reverse engineer that entire design just by looking at a 2D sketch.

I especially enjoyed the use of the word "stretch" when there are no "unstretched" boosters to stretch.


That appears to make little sense... Unless they can throttle down the core significantly. (which they cannot do on the current Merlin 1c, AFAIK).

SpaceX employees have said in the past they are working on a newer Merlin 1 engine design.  There is no contradiction here.

Online meekGee

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #35 on: 02/22/2011 07:40 pm »
a.  How is the vehicle "stronger" in that plane?
b.  The fairing is not stronger

It is worse because the vehicle is flying at an angle of attack it is not designed for.  The vehicle is using the same cores as the existing F9.

And you have yet to address the propellant utilization.

Since the angle is similar, the leading fairing will see the same flow as the single F9 flying with one engine out, but less the compressive loads from the second stage/payload. 

The center core will see the compressive loads, but not the main off-axis aerodynamic load.

the trailing core will have a fine day, thank you very much.

[and yes, this is only first order estimates, I don't have the model to run CFD on....]

I addresses propellant issues before - this is an optimization issue - one of the affects of losing an engine is less efficient use of propellant, and you need to allocate margin for it.


And again - where is the counter-argument?  This is still the same pattern of discussion.  You're just shooting things down, but not bringing anything to the table to argue the other way.
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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #37 on: 02/22/2011 07:51 pm »
Get a load of what they appear to be contemplating, from Tim Hughes' AIAA presentation:

The image in the PDF is called 'Falcon 9 Heavy extended'. Stretched tanks don't exactly strike me as being just 2 more first stages strapped to the side.

Where did you get the term "extended"?  I don't see it on the presentation.  Is there a chance that this image might merely be a mistake?

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Offline gladiator1332

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #38 on: 02/22/2011 07:58 pm »
Get a load of what they appear to be contemplating, from Tim Hughes' AIAA presentation:

The image in the PDF is called 'Falcon 9 Heavy extended'. Stretched tanks don't exactly strike me as being just 2 more first stages strapped to the side.

Where did you get the term "extended"?  I don't see it on the presentation.  Is there a chance that this image might merely be a mistake?

 - Ed Kyle

When you hold your curser over the image in adobe, you see an image title of Falcon 9 Heavy Extended.
I wouldn't read too much into it until we hear something from SpaceX. It's a powerpoint presentation. Companies often use placeholder images in those things. Then again, I am somewhat confused why they didn't just use the standard Falcon 9 Heavy image that has floated around since the design first came out.
« Last Edit: 02/22/2011 07:58 pm by gladiator1332 »

Offline ugordan

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Re: SpaceX: General Falcon and Dragon discussion (Thread 3)
« Reply #39 on: 02/22/2011 08:06 pm »
Yes, we're probably reading too much into a single image, but I don't think it's a mistake. Many of their recent presentations for the wider audience repeat the same old sections, going back a year or so. Max Vozoff's presentation from 2010 AIAA conference shows a "classic" F9H which is pretty much an identical drawing to this one, except the strapons aren't stretched.

Since I'm already reading too much into a single drawing, I'll just point out that the LOX tank appears to be stretched much more than the RP-1 tank - MR optimized for thrust?

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