Author Topic: SpaceX Reusable Falcon 9 (Grasshopper ONLY) DISCUSSION Thread (3)  (Read 665735 times)

Offline mlindner

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It can be argued that GH is not an aircraft.

Good luck with that ... ;)

Well we can expand it then. Is Grasshopper the tallest craft that is heavier than air that has ever risen into the air under its own power with no external attachment and landed? (No external attachment to avoid including cranes lifting other objects.)
« Last Edit: 02/11/2013 05:49 pm by mlindner »
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Offline kirghizstan

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Saturn 5 would fit the description of your question. Try rephrasing it

Offline StephenB

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Saturn 5 would fit the description of your question. Try rephrasing it
Not the landing part.

Offline R7

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Is Grasshopper the tallest craft that is heavier than air that has ever risen into the air under its own power with no external attachment and landed?

Probably. Stole the crown from Mercury-Redstone 1 ?  ;D
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Offline Lar

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Is Grasshopper the tallest craft that is heavier than air that has ever risen into the air under its own power with no external attachment and landed?

Probably. Stole the crown from Mercury-Redstone 1 ?  ;D
I don't think that was a "planned" landing.  So maybe

"Is Grasshopper the tallest craft that is heavier than air that has ever risen into the air under its own power with no external attachment and landed on purpose?"

Also in response to an earlier post

Phase 2 to 670 ft will probably have the first restart attempt with the altitude to allow for the free fall time.  If they need to stick the landing there based on fuel availability, they'd test that part first, right?

I had assumed that Grasshopper I was never going to land with dry tanks, or at least not for a very long time... most of the fuel/ox on board I thought was extra mass for ballast rather than planned to be consumed. A GH I, with just enough fuel for a 90 second burn, would presumably jump off the pad from being so light
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Offline Okie_Steve

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That brings up a question. I know that the F9 throttles two engines to maintain G limits on the first stage. Anyone have any idea what the limits are and whether Grasshopper would be anywhere close to them at full throttle on a single engine and nearly empty tanks? Given the massive landing gear I'd expect not for V1.0 but how about a V1.1 stage with the presumably smaller/lighter landing gear if there's even a WAG about it's mass?

Offline Jason1701

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That brings up a question. I know that the F9 throttles two engines to maintain G limits on the first stage. Anyone have any idea what the limits are and whether Grasshopper would be anywhere close to them at full throttle on a single engine and nearly empty tanks? Given the massive landing gear I'd expect not for V1.0 but how about a V1.1 stage with the presumably smaller/lighter landing gear if there's even a WAG about it's mass?

I think I heard that M1D can throttle down to 70%, but Grasshopper might have a modified one. Also, F9 v1.0 turns off two engines - M1C doesn't throttle.

Offline joek

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That brings up a question. I know that the F9 throttles two engines to maintain G limits on the first stage. Anyone have any idea what the limits are and whether Grasshopper would be anywhere close to them at full throttle on a single engine and nearly empty tanks? Given the massive landing gear I'd expect not for V1.0 but how about a V1.1 stage with the presumably smaller/lighter landing gear if there's even a WAG about it's mass?

One of the reasons for the Grasshopper propellant load is ballast.  (If you're speaking to more general F9 v1.1 mass and reuse, that's been addressed in other posts; edit: see the SpaceX First and Second Stage Re-usability thread.)
« Last Edit: 02/12/2013 02:53 am by joek »

Offline joek

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Another question is throttle response speed, which I'd think would not be terribly fast for a pump-fed engine?  That would seem to have a significant impact on the control problem.  What would be reasonable for such as M1D for small and large throttle changes?  Small fractions of seconds?  Seconds?

Online Comga

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That brings up a question. I know that the F9 throttles two engines to maintain G limits on the first stage.
(snip)
IIRC, the F9 V1, doesn't throttle two Merlin 1C engines to limit the acceleration, it turns them off completely. That is evident in the acceleration curves.
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Online meekGee

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Another question is throttle response speed, which I'd think would not be terribly fast for a pump-fed engine?  That would seem to have a significant impact on the control problem.  What would be reasonable for such as M1D for small and large throttle changes?  Small fractions of seconds?  Seconds?

From a control perspective, if the flight model is accurate, the only reason you'd need fast changes is to respond to external outputs.

Wind is mostly horizontal, and its effect over the last seconds of descent is very small.   Even so, tilting the thrust vector gives you a sine effect laterally, and a 1-cosine effect vertically, so as long as the angle is small, vertical speed is not effected and so again, no need for throttling.

There might be last-second ground effects, but those should be repeatable and thus predictable.

It's like when you get really good at lunar lander. One long smooth burn and you're done.

I had one on my phone a few years ago. I got real good at it.  When waiting for my luggage one time, I nailed 100 straight landings with single continuous burns under "hard" setting.  Just sayin' - if they ever need a pilot, what a rush it will be!  :)
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Offline mlindner

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From a control perspective, if the flight model is accurate, the only reason you'd need fast changes is to respond to external outputs.

Flight models are never 100% accurate, an isolated system doesn't exist. Thrust itself is not 100% constant or even 100% linear as the burn isn't completely even. Then there are all sorts of effects like vibration and tank sloshing which can't be modeled perfectly. In regards to repeatability of the landing, the _average_ is repeatable, but what happens at any moment is quite unpredictable.

Feedback is required and the tuning of that feedback is difficult and can induce further problems if done improperly, so this is the chief development that is happening under Grasshopper testing.
« Last Edit: 02/12/2013 09:27 am by mlindner »
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Offline CuddlyRocket

Another question is throttle response speed, which I'd think would not be terribly fast for a pump-fed engine?  That would seem to have a significant impact on the control problem.  What would be reasonable for such as M1D for small and large throttle changes?  Small fractions of seconds?  Seconds?
Rather than throttling the main engine another option is to have a secondary engine or engines for fine thrust adjustments. Disadvantage is the additional mass (unless you use something that may be there already, like stability and manouvering thrusters) and complexity of control.

Online meekGee

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From a control perspective, if the flight model is accurate, the only reason you'd need fast changes is to respond to external outputs.

Flight models are never 100% accurate, an isolated system doesn't exist. Thrust itself is not 100% constant or even 100% linear as the burn isn't completely even. Then there are all sorts of effects like vibration and tank sloshing which can't be modeled perfectly. In regards to repeatability of the landing, the _average_ is repeatable, but what happens at any moment is quite unpredictable.

Feedback is required and the tuning of that feedback is difficult and can induce further problems if done improperly, so this is the chief development that is happening under Grasshopper testing.

All true, but those are qualitative.  I didn't say anything was 100%.  You clearly need a closed loop system, you need good flight rules, and obviously GH is testing all that.

What I said is that I don't think there will be a requirement for fast throttle changes, since all of these variable conditions are either small in relation to the >1g acceleration the stage is under, or that their variability is low.

These things will get worse if the stage will dilly dally in hover mode before touch down.
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Offline billh

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It's been raining the past several days in McGregor, but should be good weather today and tomorrow. Maybe we'll see a Johnny Cash Hover Slam soon!

Offline R7

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Should raining even be a launch constraint if the aim if for fast-paced space access? IIRC Arianes can be launched into rain from Kourou, and we all know Apollo-12.
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Offline LegendCJS

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Should raining even be a launch constraint if the aim if for fast-paced space access? IIRC Arianes can be launched into rain from Kourou, and we all know Apollo-12.
In a developmental program at this stage and for testing in general you want to control as many variables as you can, just have a little patience.  And making sure all the vegetation in the area is well hydrated before doing this test might be a good idea as well ;)
« Last Edit: 02/13/2013 01:23 pm by LegendCJS »
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Offline simonbp

Should raining even be a launch constraint if the aim if for fast-paced space access? IIRC Arianes can be launched into rain from Kourou, and we all know Apollo-12.

Yeah, because it's never not raining in Korou. West Texas is dry enough that they can afford to not bother with the rain.

Offline IRobot

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Should raining even be a launch constraint if the aim if for fast-paced space access? IIRC Arianes can be launched into rain from Kourou, and we all know Apollo-12.
They also have to be careful when landing on Tuesday's banana-truck-unload days! Rain and bananas slippage are the reason vertical landing rockets fail.

Offline R7

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Rain and bananas slippage are the reason vertical landing rockets fail.

Actually rain and bananas would increase landing safety. Less friction under landing legs, less torque to tip over if you have lateral speed left. And the post-landing skidding would look awesome.

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