Author Topic: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD  (Read 602509 times)

Offline nisse

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1080 on: 12/09/2013 03:13 pm »
Wow, that's a great video - and yes, that was the reaction control thrusters of the first stage firing. This is the best view of it that we have seen so far.

I'm not sure where it was filmed, but that was some pretty impressive tracking to follow the second stage until it went below the horizon. If this was truly filmed all the way from Orlando, then WOW again! More rockets should launch at this time to have the same great lighting conditions.  ;D
I understand they use cold gas thrusters? Does that give the most energy compared to the weight of the total package? ISP is not that great for cold gas but don't know about total efficiency. What rcs options exist for that are relevant for a first stage reentry control? How much weight does the rcs package add? Thought they were going to maximize performance on this one.

Offline Kabloona

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1081 on: 12/09/2013 03:26 pm »
Wow, that's a great video - and yes, that was the reaction control thrusters of the first stage firing. This is the best view of it that we have seen so far.

I'm not sure where it was filmed, but that was some pretty impressive tracking to follow the second stage until it went below the horizon. If this was truly filmed all the way from Orlando, then WOW again! More rockets should launch at this time to have the same great lighting conditions.  ;D
I understand they use cold gas thrusters? Does that give the most energy compared to the weight of the total package? ISP is not that great for cold gas but don't know about total efficiency. What rcs options exist for that are relevant for a first stage reentry control? How much weight does the rcs package add? Thought they were going to maximize performance on this one.

Cold gas systems are not optimized for performance, since cold gas Isp is low. They are optimized for cost, safety (no hazardous props), and simplicity. There's not much point in trying to optimize for performance, since the ACS is a small percentage of the total stage mass, and the added cost/complexity/hazards of, say, a monoprop hydrazine ACS is not an advantageous tradeoff.

Offline Lars_J

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1082 on: 12/09/2013 06:43 pm »
Wow, that's a great video - and yes, that was the reaction control thrusters of the first stage firing. This is the best view of it that we have seen so far.

I'm not sure where it was filmed, but that was some pretty impressive tracking to follow the second stage until it went below the horizon. If this was truly filmed all the way from Orlando, then WOW again! More rockets should launch at this time to have the same great lighting conditions.  ;D
I understand they use cold gas thrusters? Does that give the most energy compared to the weight of the total package? ISP is not that great for cold gas but don't know about total efficiency. What rcs options exist for that are relevant for a first stage reentry control? How much weight does the rcs package add? Thought they were going to maximize performance on this one.

Optimizing for performance != optimizing for cost.

Yes, the RCS package for 1st stage was technically not needed for this mission - but flying the same 1st stage configuration is easier and more preferred than flying a different one for this flight... and if the RCS was there, they might as well run some tests with it.
« Last Edit: 12/09/2013 06:44 pm by Lars_J »

Offline Ben the Space Brit

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1083 on: 12/09/2013 08:27 pm »
@ Lars_J,

I suppose there must be a point where carrying not-required weight is so detrimental that it outweighs the simplification of production. For example, I doubt that the landing struts would be on all flights after SpX-CRS-3. The performance penalty would be too great.
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Offline beancounter

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1084 on: 12/10/2013 01:10 am »
@ Lars_J,

I suppose there must be a point where carrying not-required weight is so detrimental that it outweighs the simplification of production. For example, I doubt that the landing struts would be on all flights after SpX-CRS-3. The performance penalty would be too great.
Well I'd guess that that depends on whether or not they plan to return the first stage.  Yes you'd pay a mass penalty but so long as your payload and legs don't exceed the minimum performance requirements of the msssion then staying with a standard manufactured structure is probably worth it. 

Probably the answer lies in how you've set up the leg design.  Is it easy simply to leave them off or have you designed them to be an integral part of the first stage.  Anyone?
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Offline guckyfan

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1085 on: 12/10/2013 01:38 am »

Probably the answer lies in how you've set up the leg design.  Is it easy simply to leave them off or have you designed them to be an integral part of the first stage.  Anyone?

The stage has already flown without legs. So if not for performance, they will leave the legs off because they are not cheap. They are the only major component that distinguishes reusable from non reusable.

Yes I know there are the avionics, the relight capability and the cold gas thrusters. All of these will probably be present on the expendable, because they are integrated.


Offline AJA

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1086 on: 12/10/2013 08:14 am »

...
There is no propellant for a controlled deorbit in GTO missions.  The stages have limited life and usually are dead before reaching apogee for a deorbit burn.  Also, it might be a bad point to do the entry at that time.


CCAM is not done with the main engine, it is done with thrusters.  The depletion or inerting "burn" is not even a burn, it is just a dump of the propellants.


But in this case, the perigee was raised quite a bit. By a 102 km! (295 km published super sync GTO perigee vs NORAD tracked 397 km). That's definitely a burn isn't it? Unless they decided to inject in a higher orbit than published in the first place...


The "burn"/dump doesn't have to have happened at apogee. But in that case, the argument of perigee would've changed significantly too, wouldn't it? i.e. imagine an orbit with perigee at point P, and apogee at point A, and an intermediate orbital point X. Burn prograde at X, making X the new perigee. (Obviously you'll also have a new apogee, opposite X).

Offline Jim

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1087 on: 12/10/2013 12:19 pm »
That's definitely a burn isn't it? Unless they decided to inject in a higher orbit than published in the first place...


Venting can do that.  But will leave it to Spacex to say whether it was burn or vent

Offline fatjohn1408

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1088 on: 12/10/2013 02:27 pm »
http://www.americaspace.com/?p=46034

Any idea how accurate the statements are about second stage burn times?
Namely first burn of 320 seconds and second burn of 71 seconds?

Offline beancounter

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1089 on: 12/11/2013 12:47 am »

Probably the answer lies in how you've set up the leg design.  Is it easy simply to leave them off or have you designed them to be an integral part of the first stage.  Anyone?

The stage has already flown without legs. So if not for performance, they will leave the legs off because they are not cheap. They are the only major component that distinguishes reusable from non reusable.

Yes I know there are the avionics, the relight capability and the cold gas thrusters. All of these will probably be present on the expendable, because they are integrated.
Ok thanks, that's sounds reasonable.
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Offline fatjohn1408

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1090 on: 12/11/2013 02:37 pm »
http://www.americaspace.com/?p=46034

Any idea how accurate the statements are about second stage burn times?
Namely first burn of 320 seconds and second burn of 71 seconds?

The wired article that musk tweeted quotes a burn of just 1 minute, but this could have been presumably taken from the mission kit which stated a burn of over one minute.
http://www.wired.com/autopia/2013/11/spacex-thanksgiving-launch/?cid=co15077314

Offline fatjohn1408

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1091 on: 12/19/2013 09:31 am »
http://www.americaspace.com/?p=46034

Any idea how accurate the statements are about second stage burn times?
Namely first burn of 320 seconds and second burn of 71 seconds?

The wired article that musk tweeted quotes a burn of just 1 minute, but this could have been presumably taken from the mission kit which stated a burn of over one minute.
http://www.wired.com/autopia/2013/11/spacex-thanksgiving-launch/?cid=co15077314

Wow, hadn't even checked it, was mainly focused on second burn of 71 seconds.
But the first burn was not 320 it was at least 338 (before they killed the feed)
Even if they do the transfer at constant 6G somehow miraculously, they would need an additional 49s.
since there press kit states plus 1 minute and most articles state 71 seconds I'm willing to venture that the second stage burned for more than 400s.

=> NO WAY THAT THE MERLIN VAC 1D PRODUCES 801KN OF THRUST! ISP AND THUS MASS FLOW AND UPPER STAGE PROPELLANT AND BURN TIME ARE NOT CONSISTENT AT ALL.

just if anyone is interested

Offline ChrisWilson68

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1092 on: 12/19/2013 09:51 am »
http://www.americaspace.com/?p=46034

Any idea how accurate the statements are about second stage burn times?
Namely first burn of 320 seconds and second burn of 71 seconds?

The wired article that musk tweeted quotes a burn of just 1 minute, but this could have been presumably taken from the mission kit which stated a burn of over one minute.
http://www.wired.com/autopia/2013/11/spacex-thanksgiving-launch/?cid=co15077314

Wow, hadn't even checked it, was mainly focused on second burn of 71 seconds.
But the first burn was not 320 it was at least 338 (before they killed the feed)
Even if they do the transfer at constant 6G somehow miraculously, they would need an additional 49s.
since there press kit states plus 1 minute and most articles state 71 seconds I'm willing to venture that the second stage burned for more than 400s.

=> NO WAY THAT THE MERLIN VAC 1D PRODUCES 801KN OF THRUST! ISP AND THUS MASS FLOW AND UPPER STAGE PROPELLANT AND BURN TIME ARE NOT CONSISTENT AT ALL.

just if anyone is interested

You realize that the Merlin 1Dvac can throttle down to 70% thrust, maybe less, right?  So even if the average thrust of the second stage was less than 801kN on the SES-8 flight, that doesn't mean the engine isn't capable of higher thrust.

Since we don't know how much the engine was throttled down compared to what it's capable of, I don't see how you could possibly conclude it isn't capable of 801kN based on what happened on SES-8.

Offline malu5531

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1093 on: 12/19/2013 11:26 am »

=> NO WAY THAT THE MERLIN VAC 1D PRODUCES 801KN OF THRUST! ISP AND THUS MASS FLOW AND UPPER STAGE PROPELLANT AND BURN TIME ARE NOT CONSISTENT AT ALL.

just if anyone is interested

My model of M1DVac predict 147 isp/801 kN at full thrust. I believe SpaceX is underselling the M1DVac Isp on their website, or the inconsistency is due to different numbers assume different throttle + rounded for nicer looking numbers.

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1094 on: 12/19/2013 12:10 pm »

=> NO WAY THAT THE MERLIN VAC 1D PRODUCES 801KN OF THRUST! ISP AND THUS MASS FLOW AND UPPER STAGE PROPELLANT AND BURN TIME ARE NOT CONSISTENT AT ALL.

just if anyone is interested

My model of M1DVac predict 147 isp/801 kN at full thrust. I believe SpaceX is underselling the M1DVac Isp on their website, or the inconsistency is due to different numbers assume different throttle + rounded for nicer looking numbers.
Apologies if this was either already brought up or has no relation but hasn't it been established by Elon that the M1D is currently operating at -15% of capable thrust? An increase of which they will optimize for as they continue flight operations.  Would this not also transfer over to M1DVacs current and future operating parameters?
« Last Edit: 12/19/2013 12:12 pm by rcoppola »
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Offline fatjohn1408

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1095 on: 12/19/2013 12:28 pm »
You realize that the Merlin 1Dvac can throttle down to 70% thrust, maybe less, right?  So even if the average thrust of the second stage was less than 801kN on the SES-8 flight, that doesn't mean the engine isn't capable of higher thrust.

Since we don't know how much the engine was throttled down compared to what it's capable of, I don't see how you could possibly conclude it isn't capable of 801kN based on what happened on SES-8.

Yes off course I realize that, but there is no reason to throttle if you do not hit 6g is there? Perhaps a structural reason but then the premise remains the same that the upper stage cannot produce 801kN.
Otherwise why throttle? It increases gravity losses and decreases Isp.


=> NO WAY THAT THE MERLIN VAC 1D PRODUCES 801KN OF THRUST! ISP AND THUS MASS FLOW AND UPPER STAGE PROPELLANT AND BURN TIME ARE NOT CONSISTENT AT ALL.

just if anyone is interested

My model of M1DVac predict 147 isp/801 kN at full thrust. I believe SpaceX is underselling the M1DVac Isp on their website, or the inconsistency is due to different numbers assume different throttle + rounded for nicer looking numbers.

First off all that is one heck of a spreadsheet.
Pardon me for not going through it all before replying but when you have the following given:
A first burn of more than 337s (probably around 345)
A second burn of more than 60 seconds in which you know 2860 m/s delta v is given => hence you know the mass ratio between the beginning and the end of the burn given a certain Isp and you know the fuel rate going on in this burn hence the average throttle settings.
How could you not conclude that even with a high Isp of 347 seconds that it does not burn straight through more than 88.8t of propellant? according to me its more around 94t. Taking the 340s of the website its 97t
Now I don't think it can fit that much propellant so the thrust must be lower.

Only other conclusion would be that they - just for fun - decreased the throttle setting a bit in the first burn and went full power in the second burn. Which does not make any sense.

I think it is easier to assume that they cant go to 801.






« Last Edit: 12/19/2013 12:48 pm by fatjohn1408 »

Offline Jason1701

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1096 on: 12/19/2013 03:34 pm »
801 kN sounds about right for MVacD's maximum thrust. It is a lot more than M1D's.

Offline Lars_J

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1097 on: 12/19/2013 04:17 pm »
FYI: The M1DVac can apparently throttle to 50%.

Offline malu5531

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1098 on: 12/19/2013 04:59 pm »
First off all that is one heck of a spreadsheet.
Pardon me for not going through it all before replying but when you have the following given:
A first burn of more than 337s (probably around 345)
A second burn of more than 60 seconds in which you know 2860 m/s delta v is given => hence you know the mass ratio between the beginning and the end of the burn given a certain Isp and you know the fuel rate going on in this burn hence the average throttle settings.

Using 71 second burn time and 2860 m/s delta-v, I figure the second burn was @ 55% throttle on average. (132 kg/s instead of 236 kg/s).

In my model, this would mean there was 79500 kg of fuel for the first burn, or about 338 seconds @ 100% thrust. However, I don't believe they ran on full thrust for the entire first burn. I calculate there was about 500m/s delta-v margin left after stage 2 second MECO.

I also calculate that s2 thrust @ 55% would mean acceleration of 2.74g at the start of the second burn and 6.37g at the end. (Doesn't SpaceX cap the acceleration at 5g though?)

Calculations (green box)

Only other conclusion would be that they - just for fun - decreased the throttle setting a bit in the first burn and went full power in the second burn. Which does not make any sense.

Why do you think they went full power in the second burn?
« Last Edit: 12/19/2013 05:23 pm by malu5531 »

Offline mlindner

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1099 on: 12/19/2013 05:04 pm »
Making any conclusion off of peanut gallery spreadsheet calculations using random figures from different locations on the internet is a fool's errand. (Even if that is one awesome spreadsheet.)
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