Author Topic: Falcon 9 R landing decelleration / Minimum Merlin throttle levels.  (Read 2240 times)

speedevil

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Falcon 9 R landing decelleration / Minimum Merlin throttle levels.
« on: 01/06/2015 05:02 PM »
I've done some searching, and failed to find anything other than the general assumptions based on 'Merlin 1D can throttle to 70%' 'Stage weight about 25 tons'.

However.
The ORBCOMM video raises questions for me.

At 0:38 - 25:42 - the engine ignites.
At 26:06 - the engine is last seen.

At 70% of specified launch thrust, the thrust would be 455kN or so.
Or 18m/s^2 decelleration, with a 25000kg stage.
8m/s once you subtract gravity.

But - the 28 seconds of engine firing above implies 210m/s or so of delta-v, and the stage is falling well under 100m/s as I understood the aerodynamics.

Dropping the specified launch thrust and going to the original engine design thrust and mulplying by .7 helps a little, but not much. (160m/s).

If Merlin can throttle to a little under 50% - the numbers work out.
Or have I dropped a zero somewhere?

cscott

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Re: Falcon 9 R landing decelleration / Minimum Merlin throttle levels.
« Reply #1 on: 01/06/2015 05:08 PM »
"Throttle 70%"  == total thrust 30% of maximum.

speedevil

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Re: Falcon 9 R landing decelleration / Minimum Merlin throttle levels.
« Reply #2 on: 01/06/2015 05:10 PM »
That would, indeed do it.

But - http://aviationweek.com/awin/spacex-unveils-plans-be-world-s-top-rocket-maker -as an example.
"The 1D design incorporates many lessons learned from the earlier Merlins and is of a simpler design with an increased fatigue life. “We've added the ability to throttle between 70% and 100%. "

That seems to not leave the possibility that it means that.
« Last Edit: 01/06/2015 05:12 PM by speedevil »

cambrianera

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Re: Falcon 9 R landing decelleration / Minimum Merlin throttle levels.
« Reply #3 on: 01/06/2015 05:55 PM »
FYI in the various Grasshopper thread minimum throttle was consistently calculated at 70% except for the last hop, when it was calculated at 60%.
Sometime after that Elon tweeted that the engine has been improved and can throttle 60%.
This should be actual value, but some people believe 60% is the range and minimum value is 40%.
Oh to be young again. . .

speedevil

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Re: Falcon 9 R landing decelleration / Minimum Merlin throttle levels.
« Reply #4 on: 01/08/2015 08:36 PM »
As some context.

This is the second stage oxygen tank.
Assuming that the base of the tank is hemispherical, and that it is full diameter - I make this about 60% or so of the diameter of a hemisphere covered with fuel.
Or around a 30cm depth.
Of the order of 300l. (please note, this was a rough measurement in which I used a bowl I had just finished using for yogurt and a cracker. More accurate guesses using other foodstuffs welcome.)

Assuming it has the same burn duration - 28s as in the ORBCOMM, and the thrust level is 40% (again, see above) - and that the sea level impulse is 250 (10% down on max thrust), this is 2900kg of propellant - 3200L.

1250l of RP1, and 1900 of LOX.
Or in US gallons - 310 and 475.
To leave only 15 and 150 as discussed in the filings referred to in http://forum.nasaspaceflight.com/index.php?topic=36513.15 is quite astounding.

It clearly can't be metered down from the ~400000l or so loaded into the tanks initially.

I guess in principle, you can measure to a few kilos - with differential pressure transducers in the tank - and a lot of averaging on the 'smooth' portions of the descent into atmosphere.

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