Author Topic: SpaceX: Advanced Launch Abort System (LAS)  (Read 271431 times)

Offline spectre9

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #500 on: 10/31/2012 10:42 pm »
How does a shortened Falcon 9 get the same data?

I'm guessing it doesn't.

Ascent abort will be done with the real deal with all the computers and gizmos that will be on a real flight.

Offline meekGee

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #501 on: 10/31/2012 11:42 pm »
I'm a he, but, oh wow, I'm speechless  :)

---

Max-q is a combination of altitude, velocity, and acceleration.  You can hit all of them if you can control the amount of fuel loaded and the thrust profile.   (Reference back to Little Joe II)   

As an aside, max-q is the worst case loading on a *normally* operating rocket and the worst case escape scenario should not be the same, since we assume the rocket stopped accelerating.

EDIT - I went reading, I think I have my definition wrong, but the point still holds...   [going to read some more...]

EDIT2 - yeah, I had some other explanation in mind.  no matter.  Good use of the the strike-through feature.
« Last Edit: 10/31/2012 11:55 pm by meekGee »
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Offline Jim

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #502 on: 11/01/2012 12:23 am »

As an aside, max-q is the worst case loading on a *normally* operating rocket and the worst case escape scenario should not be the same, since we assume the rocket stopped accelerating.


No, it is the worst case.  max-q is a period of time and not instantaneous.

Offline meekGee

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #503 on: 11/01/2012 12:51 am »
Yes, the aerodynamic load peaks as a function of speed and air density. 

The rocket structure also experiences an acceleration load, but if the engines throttle to keep it constant, than the peak of aerodynamic load is also the peak of total load, and we're all in agreement.

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Offline hkultala

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #504 on: 11/01/2012 07:50 am »
Yes, the aerodynamic load peaks as a function of speed and air density. 

The rocket structure also experiences an acceleration load, but if the engines throttle to keep it constant, than the peak of aerodynamic load is also the peak of total load, and we're all in agreement.


You are making a big false assumption with your if.

The engines throttle to keep acceleration under some structural limit. That limit is always much higher than the acceleration at max-q.

The rocket starts going vertically to quickly rise to lower atmospheric density, max-q happens relatively early phase of flight where the air still has some density left.

Limiting max acceleration to the acceleration at max-q would mean couple of things:
1) there would be much more gravity losses
2) if the throttling is not done by shutting down whole engines, throttling down engine thrust would also decrease isp (all rocket engines have best isp at full thrust)

So making max acceleration happen at max-q would mean the rocket would waste a lot of potential capasity.


Offline Jim

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #505 on: 11/01/2012 12:34 pm »
Yes, the aerodynamic load peaks as a function of speed and air density. 

The rocket structure also experiences an acceleration load, but if the engines throttle to keep it constant, than the peak of aerodynamic load is also the peak of total load, and we're all in agreement.

Aero loads are not the same as structural loads.  Peak acceleration occurs near engine shutdown and not max-q.

Offline meekGee

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #506 on: 11/01/2012 01:37 pm »
I agree.  peak acceleration occurs as the rocket becomes lighter and so easier to push, and also emptier,  and so easier for the tanks to handle.

Max-q, as I corrected earlier, is only about the aerodynamic pressure, and occurs relatively early.

Does anyone know, btw, at what angle to the horizontal the F9 flies (let's be specific - in the last mission) when hitting max-Q?
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Offline Jim

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #507 on: 11/01/2012 02:56 pm »
and so easier for the tanks to handle.


Can't say that either.  It is a function of pressure, acceleration and propellant remaining, and therefore peak structural loading of the tanks may occur before peak acceleration, whereas for non tank structures, it is a peak acceleration.
« Last Edit: 11/01/2012 02:58 pm by Jim »

Offline simonbp

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #508 on: 11/01/2012 03:50 pm »

Offline meekGee

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #509 on: 11/01/2012 05:23 pm »
and so easier for the tanks to handle.


Can't say that either.  It is a function of pressure, acceleration and propellant remaining, and therefore peak structural loading of the tanks may occur before peak acceleration, whereas for non tank structures, it is a peak acceleration.

Didn't say the peaks coincided, just that towards the end of the burn, two processes are at play - increasing acceleration due to lower mass, and depletion of propellant and thus reduction of column height in the tanks.  The overall effect of loads is a combination of those.

Anyway, back to LAS.
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Offline MP99

Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #510 on: 11/01/2012 06:36 pm »
Heh. No Internet fighting!

Obligitory. http://xkcd.com/386

Ironic, when there was only one word!   ;)  http://www.merriam-webster.com/dictionary/obligatory

Had to point it out in the circumstances! And it's not even bedtime, yet.

cheers, Martin

Offline meekGee

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #511 on: 11/01/2012 07:53 pm »
Heh. No Internet fighting!

Obligitory. http://xkcd.com/386

Ironic, when there was only one word!   ;)  http://www.merriam-webster.com/dictionary/obligatory

Had to point it out in the circumstances! And it's not even bedtime, yet.

cheers, Martin

Seriously funny....
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Offline corrodedNut

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #512 on: 11/16/2012 09:57 pm »
Details? From today's Royal Aeronautical Society lecture:

"Duncan Law-Green @Rocketeer_UK
SuperDracos 6-8gs, will boost through sound barrier in 4s"

Offline docmordrid

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #513 on: 11/16/2012 11:11 pm »
Details? From today's Royal Aeronautical Society lecture:

"Duncan Law-Green @Rocketeer_UK
SuperDracos 6-8gs, will boost through sound barrier in 4s"

Whooah....and I thought 0-100 in 5-6 seconds was a rush :-p
DM

Offline AndyX

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #514 on: 07/15/2013 01:40 am »
Details? From today's Royal Aeronautical Society lecture:

"Duncan Law-Green @Rocketeer_UK
SuperDracos 6-8gs, will boost through sound barrier in 4s"


What do the Tower LAS do in comparison to that?

Offline Lars_J

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #515 on: 07/15/2013 02:48 am »
Details? From today's Royal Aeronautical Society lecture:

"Duncan Law-Green @Rocketeer_UK
SuperDracos 6-8gs, will boost through sound barrier in 4s"


What do the Tower LAS do in comparison to that?

What tower LAS do you mean? It all depends on how fast it needs to be able to get away from the LV. F9 should have a relatively benign worst case scenario compared to something sitting on top of solid(s). One could make a tower LAS with SD thrusters if one wanted.

Offline beancounter

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #516 on: 07/15/2013 03:40 am »
Details? From today's Royal Aeronautical Society lecture:

"Duncan Law-Green @Rocketeer_UK
SuperDracos 6-8gs, will boost through sound barrier in 4s"


What do the Tower LAS do in comparison to that?

What tower LAS do you mean? It all depends on how fast it needs to be able to get away from the LV. F9 should have a relatively benign worst case scenario compared to something sitting on top of solid(s). One could make a tower LAS with SD thrusters if one wanted.

Ok more specifically, does anyone know what g forces is the MPCV tower LAS expected to pull and is it more or less that the SDs?  I'm assuming it would be greater as solids can't be turned off and so the capsule has to escape an accelerating lv.  Yes?  No?  Anyone?
Beancounter from DownUnder

Offline Elmar Moelzer

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #517 on: 07/15/2013 04:02 am »

Ok more specifically, does anyone know what g forces is the MPCV tower LAS expected to pull and is it more or less that the SDs?  I'm assuming it would be greater as solids can't be turned off and so the capsule has to escape an accelerating lv.  Yes?  No?  Anyone?
I might be remembering this wrong, but AFAIK, yes they need to pull more gs because of the solids. IIRC that was one often critizised point about the Ares1.

Offline Garrett

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Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #518 on: 07/15/2013 10:43 am »
Details? From today's Royal Aeronautical Society lecture:

"Duncan Law-Green @Rocketeer_UK
SuperDracos 6-8gs, will boost through sound barrier in 4s"

Whooah....and I thought 0-100 in 5-6 seconds was a rush :-p

Amazing acceleration, but for exact times to the sound barrier, here's what I work out:
avg acceleration     
time to sound barrier
4g
8.7 s
5g6.9 s
6g5.8 s
7g5.0 s
8g4.3 s
8.7g4.0 s
9g3.9 s

Assumptions:
 - sound barrier is 340 m/s (i.e. sea level sound barrier)
 - 1g = 9.8 m/s2
 - initial velocity is zero
« Last Edit: 07/15/2013 10:44 am by Garrett »
- "Nothing shocks me. I'm a scientist." - Indiana Jones

Offline MP99

Re: SpaceX: Advanced Launch Abort System (LAS)
« Reply #519 on: 07/15/2013 11:27 am »
Details? From today's Royal Aeronautical Society lecture:

"Duncan Law-Green @Rocketeer_UK
SuperDracos 6-8gs, will boost through sound barrier in 4s"

Whooah....and I thought 0-100 in 5-6 seconds was a rush :-p

Amazing acceleration, but for exact times to the sound barrier, here's what I work out:
avg acceleration     
time to sound barrier
4g
8.7 s
5g6.9 s
6g5.8 s
7g5.0 s
8g4.3 s
8.7g4.0 s
9g3.9 s

Assumptions:
 - sound barrier is 340 m/s (i.e. sea level sound barrier)
 - 1g = 9.8 m/s2
 - initial velocity is zero

Which direction are you firing relative to ground? Suspect you need to do a vector addition of 1g downwards for gravity. If you start out horizontally, the fall adds slightly to your net speed.

But, for instance, 0.999g whilst sitting on the ground - never. Not enough thrust to lift off the ground.

cheers, Martin

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