Author Topic: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD  (Read 442580 times)

Offline malu5531

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1100 on: 12/19/2013 05:19 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.)

Perhaps, but I think it's better to make a somewhat educated guesstimate than having no clue at all and making all sorts of poorly supported claims such as "no way the engine can make 801 kN thrust", actually the data point from fatjohn1408 was surprisingly in line with the previous calculations.

After iterating this model and getting closer to reality and matching more pieces of evidence, it offers a way to learn and understand; i.e., it's fun. :)
« Last Edit: 12/19/2013 05:22 PM by malu5531 »

Offline LouScheffer

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1101 on: 12/19/2013 07:30 PM »
First off all that is one heck of a spreadsheet.
 
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?

In fact, it would be very surprising if they went full power in the second burn.   If running the engine at 55% generates 6.37 g, then running at 100% would generate 11.58 gs.  With a light payload like SES-8, they are probably throttled down as far as they dare.
« Last Edit: 12/19/2013 07:31 PM by LouScheffer »

Offline Nomadd

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1102 on: 12/19/2013 09:37 PM »
 I'm getting a little lost at the various claims here, but SpaceX is claiming 375 seconds burn time at 801 kN on their site.
http://www.spacex.com/falcon9
I don't see an updated ISP to go along with that.
 The 50% percent throttle number might refer to that 801 and not the 720KN or so they run at now. That would make sense if you figured 50% of 801 or 55% of 720. Either one comes to around 400 kN minimum.
« Last Edit: 12/19/2013 09:56 PM by Nomadd »

Offline Chris Bergin

Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1103 on: 12/19/2013 09:40 PM »
That slide on the Russian forum was actually from a NASA presentation on NASA.gov. Someone messed up in inserting that slide. I was actually the one who made SpaceX aware of it, and they got NASA to remove it. Obviously, anyone who had already downloaded it still has it (as that Russian page suggests), but we're not going to be involved in spreading it further.

Online ChrisWilson68

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1104 on: 12/20/2013 12:12 AM »
I'm getting a little lost at the various claims here, but SpaceX is claiming 375 seconds burn time at 801 kN on their site.
http://www.spacex.com/falcon9

That's the generic Falcon 9 page, not a page for the mission profile of SES-8 or any other mission.  The 375 second burn time is presumably for some model mission profile.  There's nothing that says they can't use a different profile for different missions.

Also, it doesn't exactly say even that 375 seconds of burn time is all at 801kN.  It just lists both a "THRUST" and "BURN TIME" under the second stage section.

Online ChrisWilson68

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1105 on: 12/20/2013 12:16 AM »
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.

We don't know what reasons they might have to throttle.  Off the top of my head two come to mind: (1) to reduce the load on the payload and spacecraft, and (2) because it might be easier on the engine and make it less likely to fail.  Apparently, they had excess margin on SES-8.  So why not use some of that margin to reduce the g-loads or to take it easier on the engine?

And SpaceX might have other reasons we don't even know of to throttle down.  The point is that our lack of knowledge doesn't justify a conclusion that the engine just can't do 801kN.

Online ChrisWilson68

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1106 on: 12/20/2013 12:20 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.
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?

My understanding was that he was saying he thinks a future rev of the Merlin 1 engine might be able to get 15% more thrust than the numbers currently claimed by SpaceX.  So it would mean the Merlin 1Evac or Merlin 1Fvac could get 15% more than that 801kN thrust, not that they are currently getting 15% less than that.

Offline malu5531

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1107 on: 12/20/2013 12:27 AM »

My understanding was that he was saying he thinks a future rev of the Merlin 1 engine might be able to get 15% more thrust than the numbers currently claimed by SpaceX.  So it would mean the Merlin 1Evac or Merlin 1Fvac could get 15% more than that 801kN thrust, not that they are currently getting 15% less than that.

That's my understanding as well. "Merlin 1E Vac" should do ~200klbf @ 147s isp.

Offline fatjohn1408

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1108 on: 12/20/2013 10:17 AM »
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?

I think they went full power until they hit 6G, which is the limit they acknowledge in the old F9 block2 user manual.
After that I think they throttle to keep the g-forces at 6g.
Is it difficult to change throttle settings continually?
You assume one switch in throttle to 55%? Don't forget throtlleing costs Isp, 20+s worth of it if you throttle to 55%.

Now I could agree with the one time switch to 55% of throttle level. And yes that would mean that my somewhat hysterical conclusion was a bit too trigger happy. But why wouldn't they do the first burn at full settings? You lose a lot due to gravity losses that way.
« Last Edit: 12/20/2013 10:20 AM by fatjohn1408 »

Online ChrisWilson68

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1109 on: 12/21/2013 09:13 AM »
why wouldn't they do the first burn at full settings? You lose a lot due to gravity losses that way.

This has already been answered: they had enough margin that they could accept some gravity loss in exchange for a lower throttle setting, and throttling down means less stress on both payload and stage from acceleration, and potentially less wear-and-tear on the engine, meaning potentially less chance of engine failure.  And SpaceX might have other reasons we haven't even thought of.

The real question is, if they have extra performance margin to burn at less than full throttle, why not do it?

Offline malu5531

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SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1110 on: 12/21/2013 10:55 AM »
I think they went full power until they hit 6G, which is the limit they acknowledge in the old F9 block2 user manual.
After that I think they throttle to keep the g-forces at 6g.
Is it difficult to change throttle settings continually?
You assume one switch in throttle to 55%? Don't forget throtlleing costs Isp, 20+s worth of it if you throttle to 55%.

Now I could agree with the one time switch to 55% of throttle level. And yes that would mean that my somewhat hysterical conclusion was a bit too trigger happy. But why wouldn't they do the first burn at full settings? You lose a lot due to gravity losses that way.

I do not assume 55% throttle, it was an example of average thrust and I have no guesstimate for the thrust profile.

My 55% was a counter example to show a possible average throttle which allows for a 71s burn, adding 2480 delta-v while leaving fuel for a 338s 100% stage 2 first burn. I.e, it's possible to find consistency in the numbers and you were way too trigger happy. ;)

As for low throttle; you gain precision at lower acceleration and there is very little gravity loss during S2 burn. They might even have kept later parts of first burn below a lower acceleration limit.
« Last Edit: 12/21/2013 10:56 AM by malu5531 »

Offline fatjohn1408

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1111 on: 01/05/2014 03:49 PM »
I think they went full power until they hit 6G, which is the limit they acknowledge in the old F9 block2 user manual.
After that I think they throttle to keep the g-forces at 6g.
Is it difficult to change throttle settings continually?
You assume one switch in throttle to 55%? Don't forget throtlleing costs Isp, 20+s worth of it if you throttle to 55%.

Now I could agree with the one time switch to 55% of throttle level. And yes that would mean that my somewhat hysterical conclusion was a bit too trigger happy. But why wouldn't they do the first burn at full settings? You lose a lot due to gravity losses that way.

I do not assume 55% throttle, it was an example of average thrust and I have no guesstimate for the thrust profile.

My 55% was a counter example to show a possible average throttle which allows for a 71s burn, adding 2480 delta-v while leaving fuel for a 338s 100% stage 2 first burn. I.e, it's possible to find consistency in the numbers and you were way too trigger happy. ;)

As for low throttle; you gain precision at lower acceleration and there is very little gravity loss during S2 burn. They might even have kept later parts of first burn below a lower acceleration limit.

Actually funny thing that I noticed.
Given a constant thrust, a certain duration of a burn and a certain value of maximum acceleration, the Delta-V of a stage will only be dependent on Isp and not on the size of the stage or the dry mass or any other characteristic.

This is basically so because the specific impulse defines the slope of the acceleration curve.
If you assume 6G, 71 seconds and 340 Isp, your Delta-V will always be around 2,746 m/s. This is inversely related to specific impulse.

If you assume no dog leg was performed toward the parking orbit and the parking orbit was thus 185x185x28.5 and the burn over the equator thus needed to do an 8 degree inclination change and go to 185x79135x20.5 then you'd need 3102 m/s.

This would be impossible to do with a constant thrust burn. In fact even assuming a dog leg manoeuvre during the first burns that brings the parking orbit to 185x185x20.5 means that there is still a need of 2830 m/s. Which is a decent close match, but a slight stretch.

Anyone have information about the dog leg manoeuvre in the first phase of flight and what the resulting parking orbit could have been?

I'm just doing this exercise to understand more what happens during such a flight, what parking orbit, what thrust profile and if there are some conclusions to be drawn regarding Isp, thrust or dry mass.

In the end its physics and also SpaceX's rockets are ruled by those laws. :)

Offline fatjohn1408

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Re: SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD
« Reply #1112 on: 02/12/2014 09:53 AM »
CelesTrack is showing something labeled "Object A" with a launch date of 12/3/2013 from Florida.  It has international designator 2013-071A and NORAD catalog number 39460.

The TLE for this object is:

1 39460U 13071A   13337.40768818 -.00000413  00000-0  00000+0 0    37
2 39460  20.5531 242.7832 8534855 179.4250 185.5374  0.87290738    01

So it looks like 397 km by 79341 km at 20.55 degrees.

Neat. Decay is estimated to be on 25 Mar 2014.. after 174 orbits. The lifetime is 112 days.

Using the same (wrong) guesses as before for drag.

Still going very strong though: http://www.n2yo.com/satellite/?s=39461
I don't think it's going to decay soon...
What program did you use again and what orbit did it predict it would be at this moment in time?

Also I totally do not get why the perigee went down but the apogee stayed roughly the same.
Shouldn't drag at perigee diminish the apogee of the stage until the orbit naturally circularizes and only then re-entry occurs?

Source: orbital mechanics text books and kerbal space program.

Additionally I looked up some A5 ECA's and you have to go back to 2008 (more than 7 years ago) to see significant decay:
http://www.n2yo.com/satellite/?s=29646 (hardly any decay)
http://www.n2yo.com/satellite/?s=29498 (significant decay)

Note that SES-8 upper stage has 3 times the orbital period than ariane upper stages and also has a higher perigee, perhaps its ballistic coefficient is a fair bit lower but I do not think it will make up for it.

SES-8 upper stage is here to stay, for a very very long time.
« Last Edit: 02/12/2014 10:16 AM by fatjohn1408 »

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