Author Topic: General Falcon and Dragon discussion (Thread 12)  (Read 280924 times)

Online FutureSpaceTourist

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #40 on: 03/02/2015 07:34 AM »
Any insights on how these F9 changes may affect USAF certification, which I assume has been based on F9 v1.1? I don't see SpaceX operating two F9 variants?

Offline cambrianera

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #41 on: 03/02/2015 09:26 AM »
A tweet from Musk about F9 performance upgrades coming soon: (we already knew about the densification and M1D thrust upgrade, but the upper stage stretch(?) is new)

https://twitter.com/elonmusk/status/572257004938403840

Quote
Elon Musk ‏@elonmusk
Upgrades in the works to allow landing for geo missions: thrust +15%, deep cryo oxygen, upper stage tank vol +10%

Well, it was a low hanging fruit.
http://forum.nasaspaceflight.com/index.php?topic=36045.msg1285819#msg1285819

a stretch is all you need to improve your GTO margins.
A two meter stretch would add 20000 kg of propellant to the second stage for a mere 300 kg of extra tankage (only 2m sidewalls added).
With 20% more propellant you get at least 1000 kg of extra payload.
This would have no influence on first stage T/W, and small influence on second stage T/W; most of this extra mass is masked by GTO payload reduction compared to LEO.


And +10% is more like 1 m stretch.
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Offline Dave G

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #42 on: 03/02/2015 09:43 AM »
It all makes sense.  First stage has already been stretched as far as it can.  Any longer and it wouldn't be road transportable. 

Offline kevinof

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #43 on: 03/02/2015 09:46 AM »
The tweet from Musk didn't say anything about a stretch - wonder if they can do this by increasing the tank size while keeping the stage dimensions the same?



A tweet from Musk about F9 performance upgrades coming soon: (we already knew about the densification and M1D thrust upgrade, but the upper stage stretch(?) is new)

https://twitter.com/elonmusk/status/572257004938403840

Quote
Elon Musk ‏@elonmusk
Upgrades in the works to allow landing for geo missions: thrust +15%, deep cryo oxygen, upper stage tank vol +10%

Well, it was a low hanging fruit.
http://forum.nasaspaceflight.com/index.php?topic=36045.msg1285819#msg1285819

a stretch is all you need to improve your GTO margins.
A two meter stretch would add 20000 kg of propellant to the second stage for a mere 300 kg of extra tankage (only 2m sidewalls added).
With 20% more propellant you get at least 1000 kg of extra payload.
This would have no influence on first stage T/W, and small influence on second stage T/W; most of this extra mass is masked by GTO payload reduction compared to LEO.


And +10% is more like 1 m stretch.

Online ugordan

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #44 on: 03/02/2015 11:14 AM »
Any insights on how these F9 changes may affect USAF certification, which I assume has been based on F9 v1.1? I don't see SpaceX operating two F9 variants?

I'll throw in a question about NASA's own certification and how this will play into it? Can anyone in the know comment? Not interested in speculation for the sake of speculation.

Offline cuddihy

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #45 on: 03/02/2015 11:43 AM »
It actually could be no stretch if the U/S really has been flying at less than 90% fuel load, which has been speculated before. Perhaps they designed it with subcooling in mind and the lower stage couldn't provide enough for the extra prop  to be loaded until subcooling increased the lower stage total impulse enough to make it work.

Offline kevinof

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #46 on: 03/02/2015 11:55 AM »
I think this is more realistic than stretching the 2nd stage. The latter would require re-tooling, aerodynamic analysis and more. Can't see this happening, at least in the short term.

If you're right and they are only now topping up the tanks then that possibly gives them the extra 10%. Add in better performance of the 1d+ which should increase power by 15-20% and you have the extra grunt to push the added fuel uphill.

then again, what do I know?

It actually could be no stretch if the U/S really has been flying at less than 90% fuel load, which has been speculated before. Perhaps they designed it with subcooling in mind and the lower stage couldn't provide enough for the extra prop  to be loaded until subcooling increased the lower stage total impulse enough to make it work.

Offline su27k

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #47 on: 03/02/2015 12:16 PM »
It actually could be no stretch if the U/S really has been flying at less than 90% fuel load, which has been speculated before. Perhaps they designed it with subcooling in mind and the lower stage couldn't provide enough for the extra prop  to be loaded until subcooling increased the lower stage total impulse enough to make it work.

Or the Upper Stages have been flying had shorter tanks?

Offline Kabloona

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #48 on: 03/02/2015 12:42 PM »
It actually could be no stretch if the U/S really has been flying at less than 90% fuel load, which has been speculated before. Perhaps they designed it with subcooling in mind and the lower stage couldn't provide enough for the extra prop  to be loaded until subcooling increased the lower stage total impulse enough to make it work.

Read it again. He said tank volume +10%.
« Last Edit: 03/02/2015 12:43 PM by Kabloona »

Offline Norm38

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #49 on: 03/02/2015 01:00 PM »
A two meter stretch would add 20000 kg of propellant to the second stage for a mere 300 kg of extra tankage (only 2m sidewalls added).

And +10% is more like 1 m stretch.

We know that SpaceX likes to make incremental changes, and they treat each expendable like a flying experiment.  So I wouldn't be at all surprised if they first stretched 1m, evaluated performance, checked for vibrations etc, then later stretched another meter if it bought them more performance.  But first they may want to get hard numbers on the first stage upgrade performance.

They shouldn't have to retool, as first and second stages are built on the same tooling today.  Length is a free variable.

Offline cambrianera

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #50 on: 03/02/2015 01:01 PM »
I think this is more realistic than stretching the 2nd stage. The latter would require re-tooling, aerodynamic analysis and more. Can't see this happening, at least in the short term.
All speculations obviously, but re-tooling seems not needed for a simple stretch; ditto for aerodynamic analysis given 1m stretch on 60 m rocket.
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Offline llanitedave

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #51 on: 03/02/2015 02:15 PM »
Any insights on how these F9 changes may affect USAF certification, which I assume has been based on F9 v1.1? I don't see SpaceX operating two F9 variants?

I'll throw in a question about NASA's own certification and how this will play into it? Can anyone in the know comment? Not interested in speculation for the sake of speculation.

Just speculation, but EM only mentioned it in the context of GEO missions, so the LEO configuration may be unaffected, and hence certification is unchanged.


But that would stick them with two different second stage models to have to manufacture in parallel, which doesn't seem to be their M.O.
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Offline Kabloona

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #52 on: 03/02/2015 02:21 PM »
Also, once you do the S2 stretch, you get more margin for S1 engine-out performance, and more margin for  boostback burn to the Cape, so there's added incentive to fly all future missions with the stretch.

Offline GWH

Re: General Falcon and Dragon discussion (Thread 12)
« Reply #53 on: 03/02/2015 02:29 PM »
With densified LOX they are only changing one variable, which leaves (3) scenarios for the rocket:
1.) All tank volumes stay the same, vary the combustion mixture so that the engines are now running on proportionally higher O2.
2. ) Marginally lengthen kerosene tanks while shortening LOX tanks to maintain same fuel/oxidizer ratios
3.) Shorten the overall first stage proportionally relative to the lower volume required by the denser LOX. Increase the length of the first stage by the same amount. 

Without looking at exact numbers of tank length vs. oxidizer density it seems entirely plausible that SpaceX can increase the total tank volume of the second stage by 10% without increasing the total rocket length through the use of densified LOX.

Offline Kabloona

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #54 on: 03/02/2015 02:59 PM »
it seems entirely plausible that SpaceX can increase the total tank volume of the second stage by 10% without increasing the total rocket length through the use of densified LOX.

Densified LOX does not change the volume of a tank. It changes the mass of LOX in a given volume.

Also, Elon mentioned the 10% increase in S2 tank volume right after mentioning the densified LOX. These are clearly not the same thing.

Offline GWH

Re: General Falcon and Dragon discussion (Thread 12)
« Reply #55 on: 03/02/2015 03:02 PM »
Yes that's exactly what my post implies. You need less volume of lox to maintain same ratio.

Offline Kabloona

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #56 on: 03/02/2015 03:22 PM »
Yes that's exactly what my post implies. You need less volume of lox to maintain same ratio.

I don't see how it's possible to increase total tank volume 10% without stretching the stage length (assuming no change in diameter, which seems a safe assumption).

The following paper suggests a 9% increase in LOX mass is possible by supercooling. So maybe they keep the LOX tank volume as is, and increase the RP-1 tank volume 10% to maintain the ratio.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050203875.pdf

In any case, hard to see how you do that without stretching the stage slightly. As others have pointed out, changing S2 tank length slightly is not a big deal from a manufacturing POV, and probably not a big deal structurally or aerodynamically.
« Last Edit: 03/02/2015 03:34 PM by Kabloona »

Offline GWH

Re: General Falcon and Dragon discussion (Thread 12)
« Reply #57 on: 03/02/2015 03:47 PM »
The following paper suggests a 9% increase in LOX mass is possible by supercooling. So maybe they keep the LOX tank volume as is, and increase the RP-1 tank volume 10% to maintain the ratio.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050203875.pdf

In any case, hard to see how you do that without stretching the stage slightly. As others have pointed out, changing S2 tank length slightly is not a big deal from a manufacturing POV, and probably not a big deal structurally or aerodynamically.

Or, leave the first stage total mass of propellant/oxidizer the exact same.  Now that the 1st stage LOX is denser you don't need as long of a tank (volume) to maintain the same mass balance.  Take that additional length and add it to the second stage.  Gain your 10% total volume increase in the second stage without changing the overall length of rocket.  10% less volume on the 1st stage LOX is probably pretty close to 10% overall tanks volume of the second stage. 
 Launch pad and infrastructure and erector changes can also be less since again the overall length of the rocket isn't changing. 
Proportionally they will see much larger gains in performance by increasing the tank volume of the second stage 10% rather than the 1st stage by a couple %.

Offline cambrianera

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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #58 on: 03/02/2015 04:00 PM »

Well, It's only 1 meter.  You'd have to lengthen the tank by only about 19 inches on each end to get 1 meter.  Perhaps within the current shell, they can reposition some equipment and wiring at either end to squeeze in a slightly bigger tank.
Hmm.. the tank is the shell.
If you stretch the tank, you stretch the stage.
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Re: General Falcon and Dragon discussion (Thread 12)
« Reply #59 on: 03/02/2015 09:47 PM »
Can someone run the numbers on what additional performance these upgrades would allow in expendable mode versus the current spec? More thrust, fuel densification and now a second stage stretch it would seem to be a nice increase. Should be a point or so increase in ISP as well from the increased chamber pressure. I know it is mostly to allow landing attempts on more launches but if it allows them to grab more expendable launches too I doubt they will mind.

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