Author Topic: Possible Mini Grasshopper?  (Read 9374 times)

Offline Zed_Noir

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Possible Mini Grasshopper?
« on: 01/01/2014 06:19 AM »
Will SpaceX build a Mini Grasshopper? An upper stage landing testbed.

Offline Lourens

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Re: Possible Mini Grasshopper?
« Reply #1 on: 01/01/2014 02:15 PM »
Hmm, I'm not sure it would be worth the investment. By the time they start trying to land a second stage they will have plenty of experience with first stages returning. The second stage will probably be different in a few aspects, but nothing that would require a separate vehicle to test repeatedly. Engine restarts, final guidance to a landing, and ground handling will have all been worked out with the existing Grasshopper and first stages, and they'll have one second stage to experiment with for every launch. They should be able to do quite a bit of testing by bringing those down somewhere over an ocean without building a special vehicle.

Offline sojourner

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Re: Possible Mini Grasshopper?
« Reply #2 on: 01/01/2014 05:10 PM »
I would imagine an "engineering example" being built to test the various pieces of hardware involved, but I doubt there would be much flight envelope testing.

Offline laika_fr

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Re: Possible Mini Grasshopper?
« Reply #3 on: 01/01/2014 05:13 PM »
Molly McCornick (spacex) said :
"The vehicle is evolving." "Yeah, pretty soon it's going to start walking and bearing live young."

I don't know what it means exactly but since the F9 is a grown-up, there is maybe room for a smaller very low cost launcher.
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Offline sojourner

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Re: Possible Mini Grasshopper?
« Reply #4 on: 01/01/2014 05:18 PM »
Molly McCornick (spacex) said :
"The vehicle is evolving." "Yeah, pretty soon it's going to start walking and bearing live young."

I don't know what it means exactly but since the F9 is a grown-up, there is maybe room for a smaller very low cost launcher.

We're not talking about a "smaller very  low cost launcher".  We're talking about a test article for Falcon 9 stage 2 reusable.
« Last Edit: 01/01/2014 05:19 PM by sojourner »

Offline rst

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Re: Possible Mini Grasshopper?
« Reply #5 on: 01/01/2014 05:33 PM »
Hmm, I'm not sure it would be worth the investment. By the time they start trying to land a second stage they will have plenty of experience with first stages returning. The second stage will probably be different in a few aspects, but nothing that would require a separate vehicle to test repeatedly. Engine restarts, final guidance to a landing, and ground handling will have all been worked out with the existing Grasshopper and first stages, and they'll have one second stage to experiment with for every launch.

Their second stage reuse plans may have evolved, but their concept video showed a second stage that landed on dedicated thrusters instead of its main engine.  So, I'd expect they would want to test that part out at ground level at least.  Likewise for the leg design, if there are serious differences (as again, there seemed to be in the video).

Offline laika_fr

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Re: Possible Mini Grasshopper?
« Reply #6 on: 01/01/2014 05:52 PM »
Oh sorry it was about the piggy-back, in that case of course it needs to be tested quickly, the lean approach would ask for it.
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Offline Zed_Noir

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Re: Possible Mini Grasshopper?
« Reply #7 on: 01/01/2014 09:12 PM »
Hmm, I'm not sure it would be worth the investment. By the time they start trying to land a second stage they will have plenty of experience with first stages returning. The second stage will probably be different in a few aspects, but nothing that would require a separate vehicle to test repeatedly. Engine restarts, final guidance to a landing, and ground handling will have all been worked out with the existing Grasshopper and first stages, and they'll have one second stage to experiment with for every launch. They should be able to do quite a bit of testing by bringing those down somewhere over an ocean without building a special vehicle.

You do realize that the Falcon 9 ver 1.1  (aka F9R) upper stage will be nothing like the 1st stage.

Landing approach will be on SuperDracos thrusters firing in burst mode likely with no hovering. As the Merlin engine is too powerful for the landing burn.

Completely different landing gears from the first stage.

According to the SpaceX promo video the Merlin engine assembly is suppose to move up to change the center of gravity to a more favorable position.

Offline rst

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Re: Possible Mini Grasshopper?
« Reply #8 on: 01/01/2014 09:23 PM »
According to the SpaceX promo video the Merlin engine assembly is suppose to move up to change the center of gravity to a more favorable position.

I'm not sure it's clear what the video shows --- a lot of people think it's showing only the nozzle retracting, or folding away, or something, to keep it out of trouble on reentry.  But whatever it is, they'll probably want to give it a fair bit of testing as well before putting a payload at risk.

Offline Lourens

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Re: Possible Mini Grasshopper?
« Reply #9 on: 01/02/2014 06:33 AM »
My point is not that the first and second stages are identical and that there needs to be no further testing therefore. My point is that they'll have enough testing opportunity on ordinary flights, and won't need a separate vehicle.

The reason for Grasshopper, as I see it, was to provide a hardware-in-the-loop testbed for the approach software, sensors, and engine control. Can you control this accurately enough to land at >1 thrust to weight? That's the question they answered with Grasshopper, and they couldn't answer it with existing stages because F9 v1.1 wasn't ready, they didn't have reentry figured out yet, and the flight rate was too low for rapid experimentation. Also, they could add ballast to Grasshopper and start with lower thrust to weight, then work up to a more realistic scenario.

Another question regarding reusability is how often you can fly that first stage; how do the engines hold up, what do the aerodynamic loads do to the structure, and so on. You can't test that on real launches, because no one is going to fly their payload on a reused stage until after you've got that information. So, that's what Grasshopper II is for.

Now, for the second stage, provided that you have enough low-mass payloads, you can do tests on real launches, because all the testing will be after the payload has been delivered. With flight rates increasing, there may well be more such opportunities this year than Grasshopper flights last year. The biggest problem is reentry, which they'll investigate by post-payload-delivery testing. Once they've got that, they can add the landing thrusters and try a "landing" on the ocean, with or without legs. Finally, bring it in over land. The only thing missing then is whether the legs will support the weight, but I'd think there are easier ways of testing that than an entire special vehicle.

Offline Falcon H

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Re: Possible Mini Grasshopper?
« Reply #10 on: 01/02/2014 02:14 PM »
Guys, remember that Elon said the video wasn't accurate, so we really don't know anything for certain. SpaceX will probably make changes to their original design as they get into development and find problems with it. I personally think that a mini Grasshopper is very likely. A reusable second stage will be very complex, and very different from Grasshopper-1. SpaceX would be crazy not to build a prototype.
« Last Edit: 01/02/2014 02:16 PM by Falcon H »
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Online guckyfan

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Re: Possible Mini Grasshopper?
« Reply #11 on: 01/02/2014 03:08 PM »
Guys, remember that Elon said the video wasn't accurate, so we really don't know anything for certain. SpaceX will probably make changes to their original design as they get into development and find problems with it. I personally think that a mini Grasshopper is very likely. A reusable second stage will be very complex, and very different from Grasshopper-1. SpaceX would be crazy not to build a prototype.

What would it be good for? The problems will be in reentry from Orbit. A mini Grasshopper will not be able to duplicate that. Grasshopper is great for first stage development. But it won't do any good for the big second stage problems. IMO of course.



Offline StephenB

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Re: Possible Mini Grasshopper?
« Reply #12 on: 01/02/2014 08:56 PM »
If so, it should be named the cricket. Remember, you heard it here first.

Online TrevorMonty

Re: Possible Mini Grasshopper?
« Reply #13 on: 01/25/2014 06:58 AM »
The reusable 2stage will be more expensive than current expendable version. I see them flying both versions in future as there would be no point in using the more expensive reusable version on a expendable max performance mission. The is
a need for a GH version to test landing stage, this could launch using Merlin then land with SDs.

Offline wannamoonbase

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Re: Possible Mini Grasshopper?
« Reply #14 on: 05/03/2014 07:12 PM »
I can see a flying test bed for a second stage. Use a Merlin 1D to get to altitude and practice the landings with the Dracos or whatever thruster assembly they use.

Flight dynamics will be different as the stage is smaller. It will be easier because of the experience with the first stage but still lots to work out. They have fuel tanks and Merlin's so may as well use them to get it to altitude.

Lots to work out with legs and a shifting engine assembly if they are going to do that (sounds like added weight to me).

I think they will start with burns and re-entry of production stages after the primary mission is complete.

(P.S. No need to develop a smaller falcon if you have a reuseable F9. Just don't put in as much fuel and reuse both stages.)
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Offline Adaptation

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Re: Possible Mini Grasshopper?
« Reply #15 on: 05/03/2014 09:05 PM »
Their second stage reuse plans may have evolved, but their concept video showed a second stage that landed on dedicated thrusters instead of its main engine.  So, I'd expect they would want to test that part out at ground level at least.  Likewise for the leg design, if there are serious differences (as again, there seemed to be in the video).

Oh come on... throttling a kero-lox engine below 10%, and with a vacuum expanded nozzle to boot, what could possibly go wrong.  You've seen the size of the nozzle, just park it on that thing, no legs needed.   ;D ;D

http://www.spacelaunchreport.com/f91s2.jpg

Makes me wonder though if they could use pyrotechnics to ditch expanded portion of the nozzle. 
In all actuality I fully expect them to use the super draco or something along those lines if and when they ever want to purposively land the second stage. 
« Last Edit: 05/03/2014 09:23 PM by Chris Bergin »

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