Author Topic: SpaceX Reusable Falcon 9 (Grasshopper ONLY) DISCUSSION Thread (4)  (Read 293274 times)

Offline IRobot

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They will get PLENTY of data to analyse. That is gold.

Offline Roy_H

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We've heard that the legs will also be used as flight control surfaces (right?), but this stage doesn't have any legs. Does it mean that the legs are only used for flight control during the last portion of the trajectory, to steer the stage to the pad (or away from the pad, should the engine fail)?

Not exactly "flight control" but IIRC Musk said they would aid in aero braking so as to reduce fuel requirements to slow decent. This test then without the legs would require slightly more fuel. Not an issue on this flight. I don't believe there is any attempt to use them for steering.
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Offline mlindner

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We've heard that the legs will also be used as flight control surfaces (right?)

No we haven't heard that. I believe we have heard that they will be used as passive drag surfaces, but no confirmation of control. Control is also skeptical because of the added complexity and weight needed.
« Last Edit: 09/06/2013 09:49 pm by mlindner »
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Offline A_M_Swallow

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I'm not sure what he means about the radar's ability to detect water; this isn't exactly the first time someone has pointed a radar at the ocean! On the other hand, restarting the engine sounds like a much larger risk.

False returns are common with radar pointing at a fluid surface. 

Sea water moves up and down.  So the system has to solve there is water there now but since the wave is moving will their be water there in 2 seconds time?

Offline douglas100

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Grasshopper has already proved they can land accurately on a solid surface. What happens when the CASSIOPE stage reaches the ocean doesn't matter unless they have a boat waiting to try to retrieve it.

If not, the most important things this experiment can demonstrate are braking before entry and controllability and accuracy during entry and descent through the atmosphere. These are the major unknowns at the moment.
« Last Edit: 09/06/2013 10:51 pm by douglas100 »
Douglas Clark

Online Joffan

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Sea water moves up and down.  So the system has to solve there is water there now but since the wave is moving will their be water there in 2 seconds time?

No, it doesn't need that ability. This test isn't aiming for a touchdown, just a controlled speed-reduction burn that approximates a landing profile. If it even gets that far - the return through atmosphere is the biggest test. v=0 at h=0 can wait for land landings.


EDIT: And this is already an outrageously ambitious test plan. Were it to succeed fully, it would be a major coup.
« Last Edit: 09/06/2013 11:03 pm by Joffan »
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Offline Danderman

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Remember that previous attempts to do something with the first stage have all resulted in missing first stages.

Offline douglas100

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Remember that previous attempts to do something with the first stage have all resulted in missing first stages.


True, but none of their previous first stages were braked before entry. I think they have a fair chance of getting it intact as far as the surface. We'll see.
Douglas Clark

Offline A_M_Swallow

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Sea water moves up and down.  So the system has to solve there is water there now but since the wave is moving will their be water there in 2 seconds time?

No, it doesn't need that ability. This test isn't aiming for a touchdown, just a controlled speed-reduction burn that approximates a landing profile. If it even gets that far - the return through atmosphere is the biggest test. v=0 at h=0 can wait for land landings.

Just allow for h=0 being 3 metres above the surface of the water.

Offline Robotbeat

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Remember that previous attempts to do something with the first stage have all resulted in missing first stages.


True, but none of their previous first stages were braked before entry. I think they have a fair chance of getting it intact as far as the surface. We'll see.
Plus, there's a lot more events on this flight... For v1.0 recovery, they pretty much just prayed that it deployed chutes. Here, post-sep, they can flip the stage around, restart the three engines, burn for a certain length of time, then do whatever attitude control they plan on doing on the way down, then look for the water, then restart the middle engine for just the right length of time until soft splashdown. They are presumably getting telemetry the whole time, so they'll know what's going on. Because they're attempting active recovery instead of nearly passive, they have a lot more things they can do to fix issues on later flights.

They'll know if the stage flips around properly, if the three engines relight, if it ends up decelerating the right amount, if it stays in the right attitude or has some stability issues on the way down, etc. So even if it doesn't work this time (probably won't), my bet is that they'll be able to make it work after a few tries.
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Offline llanitedave

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The water's not that big a deal anyway, since they're developing it to return to a solid pad on land.
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Offline Retired Downrange

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...seems like this thread has gone off "Grasshopper only" to a discussion of first trial of Falcon "softer impact" on water.

Offline Robotbeat

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...seems like this thread has gone off "Grasshopper only" to a discussion of first trial of Falcon "softer impact" on water.
That's because it's now apparent that F9 v1.1 (which I believe Elon calls F9R, as in Falcon 9 Reusable) is essentially just Grasshopper 2 without legs. Heck, Grasshopper 2 (the next legged VTVL test vehicle to be flown outside of Texas) won't even be called Grasshopper, and Grasshopper 1 may well have flown its last flight.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Retired Downrange

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...change the tittle to:

When The Grasshopper Becomes The Sensei

?

: )

Offline ChrisWilson68

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Yes, but it's something that is easily handled by statistical signals processing. Ocean-hugging anti-ship missiles have been doing this for decades. There are probably numerous AIAA and IEEE papers on the subject. Again, relative to the Merlin relight, the radar is of minimal concern.

The issue isn't whether it's possible to create a radar system that can accurately track the surface of the ocean.

Remember, F9R is not designed for water landing.  It's designed for landing on a pad.  When designing the radar system, they wouldn't have tried to make it capable of landing on water.  The whole water landing thing is just a test, and the final burn to soft landing is only a small part of it.  You don't design a system for a totally different environment than you expect to use in production just because you want to test it that way once or twice.

Offline go4mars

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Is it possible to eject an octocopter or two just before the final middle engine ignition, and if necessary, just before it slips into the pond?  Likely, the camera part of these would need to be buoyant and waterproof, and would probably need little model rocket type parachutes.   

Also, if this all goes smoothly on this first "Legless Grasshopper 2" test, will any competitors reallocate $? 
Will there be any reaction at competitor shops (analogous to Chevy Volt existing because of Tesla's existence)?
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Offline Robotbeat

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If anything, it's the other way around: Grasshopper is a response to other VTVL ventures like Blue Origin and the failure of SpaceX's previous reuse efforts.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline mlindner

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Is it possible to eject an octocopter or two just before the final middle engine ignition, and if necessary, just before it slips into the pond?  Likely, the camera part of these would need to be buoyant and waterproof, and would probably need little model rocket type parachutes.   

Also, if this all goes smoothly on this first "Legless Grasshopper 2" test, will any competitors reallocate $? 
Will there be any reaction at competitor shops (analogous to Chevy Volt existing because of Tesla's existence)?

That requires designing a new optocoptor model. While certainly "possible", its certainly not worth the effort. They were just using stock octocoptors from commercial companies that were manually controlled.
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Offline Ronsmytheiii

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Remember, F9R is not designed for water landing.  It's designed for landing on a pad.  When designing the radar system, they wouldn't have tried to make it capable of landing on water.  The whole water landing thing is just a test, and the final burn to soft landing is only a small part of it.  You don't design a system for a totally different environment than you expect to use in production just because you want to test it that way once or twice.


Most of the flight back will be over water, only the last few minutes and miles will be over land.
« Last Edit: 09/08/2013 12:43 am by Ronsmytheiii »

Online kkattula

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Remember, F9R is not designed for water landing.  It's designed for landing on a pad.  When designing the radar system, they wouldn't have tried to make it capable of landing on water.  The whole water landing thing is just a test, and the final burn to soft landing is only a small part of it.  You don't design a system for a totally different environment than you expect to use in production just because you want to test it that way once or twice.


Most of the flight back will be over water, only the last few minutes and miles will be over land.

But will be at high altitude, so relying on GPS & Inertial Navigation. Radar's only needed (if at all) for the last few 10's of metres.

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