Jump to content


Photo

SpaceX Grasshopper Sets Record

  • Please log in to reply
40 replies to this topic

#26 Ira

Ira

    Mercury-Atlas

  • *****
  • Posts: 2624
  • Joined: 22 Aug 2010
  • Loc: Mitzpe Ramon, Israel

Posted 25 April 2013 - 07:40 PM

I think Danny doesn't believe we're going anywhere soon.

/Ira

#27 seryddwr

seryddwr

    Innocent Bystander

  • *****
  • Posts: 3319
  • Joined: 19 Feb 2010
  • Loc: La-la land.

Posted 25 April 2013 - 11:25 PM


I still do not understand what good will come of such a vehicle.


Did you read what I wrote in my third post in this thread?


It's a pretty simple concept. As Musk said, "You don't throw away a brand new 747 after every flight."

Wait... You mean I've been doing it wrong all this time?! I'd better cancel my next 20 orders with Boeing.

#28 David Knisely

David Knisely

    Hubble

  • *****
  • Posts: 15446
  • Joined: 19 Apr 2004
  • Loc: southeastern Nebraska

Posted 26 April 2013 - 01:09 AM

deSitter posted:

I still do not understand what good will come of such a vehicle.


Uh, like maybe figuring out how to re-use a costly booster by seeing if you can fly it back to the launch site? What is so hard to understand about that?

"The 60's weren't good to you, were they?" (Sarge from the movie CARS)

#29 Shadowalker

Shadowalker

    Apocaloptimist

  • *****
  • Posts: 10867
  • Joined: 23 Nov 2004
  • Loc: Bay Saint Louis, Mississippi, USA

Posted 26 April 2013 - 03:36 AM

The obvious disadvantage is that having the booster land under its own power requires propellant that won't be available to lift the payload. However, there is usually extra margin for most payloads. Also, the returning booster hasn't all the mass of the upper stage and payload and the spent propellant to lift. I haven't run any numbers, but I'd make a SWAG that returning to launch site or other recovery area would need about 15 to 20% of the propellant.

A tradeoff. Sacrifice some payload mass to recover a 150 million dollar booster? Worth it, I'd say.

#30 Centaur

Centaur

    Vendor

  • *****
  • Vendors
  • Posts: 2551
  • Joined: 12 Jul 2004
  • Loc: Chicago

Posted 26 April 2013 - 09:29 AM

The obvious disadvantage is that having the booster land under its own power requires propellant that won't be available to lift the payload. However, there is usually extra margin for most payloads. Also, the returning booster hasn't all the mass of the upper stage and payload and the spent propellant to lift. I haven't run any numbers, but I'd make a SWAG that returning to launch site or other recovery area would need about 15 to 20% of the propellant.

A tradeoff. Sacrifice some payload mass to recover a 150 million dollar booster? Worth it, I'd say.


Of course gravity and not thrusting does the lowering, while atmospheric friction slows a falling object to its terminal velocity. For a person that is about 120 mph, and I imagine less for a nearly empty rocket. So the braking thruster only needs to become activated near the end of the descent.

#31 Jarad

Jarad

    Fly Me to the Moon

  • *****
  • Posts: 6354
  • Joined: 28 Apr 2003
  • Loc: Atlanta, GA

Posted 26 April 2013 - 09:37 AM

From the video, I suspect it will need to be thrusting continuously to maintain attitude control. I think if it cut thrust completely and fell at terminal velocity it would probably start tumbling, then resuming controlled thrust would be difficult.

Maybe in combination with some drag chutes or something...

It was impressive how it maintained balance on top of just a single engine.

Jarad

#32 llanitedave

llanitedave

    Humble Megalomaniac

  • *****
  • Posts: 22175
  • Joined: 25 Sep 2005
  • Loc: Amargosa Valley, NV, USA

Posted 27 April 2013 - 11:42 AM

The obvious disadvantage is that having the booster land under its own power requires propellant that won't be available to lift the payload. However, there is usually extra margin for most payloads. Also, the returning booster hasn't all the mass of the upper stage and payload and the spent propellant to lift. I haven't run any numbers, but I'd make a SWAG that returning to launch site or other recovery area would need about 15 to 20% of the propellant.

A tradeoff. Sacrifice some payload mass to recover a 150 million dollar booster? Worth it, I'd say.


I think what I'm reading is that they're looking at about a 40% performance hit for re-usability, but I think that includes the second stage as well. If each stage can be re-used 10 or more times with a minimum of refurbishment (gas and go, more like an airliner than the Space Shuttle), that will lead to a huge reduction in launch costs.

And if they have a customer that absolutely requires maximum payload capacity, then they can use an end-of-life stage for that particular launch and expend it. They can charge a premium for that flight while still undercutting the expendable-only competition.

#33 llanitedave

llanitedave

    Humble Megalomaniac

  • *****
  • Posts: 22175
  • Joined: 25 Sep 2005
  • Loc: Amargosa Valley, NV, USA

Posted 27 April 2013 - 11:44 AM

From the video, I suspect it will need to be thrusting continuously to maintain attitude control. I think if it cut thrust completely and fell at terminal velocity it would probably start tumbling, then resuming controlled thrust would be difficult.

Maybe in combination with some drag chutes or something...

It was impressive how it maintained balance on top of just a single engine.

Jarad


I believe the re-usable booster will have cold gas thrusters at the top for stability control.

#34 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10490
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 03 May 2013 - 09:05 AM

Might not gyroscopes be installed in any recoverable stage so as to control, or facilitate control of attitude?

#35 llanitedave

llanitedave

    Humble Megalomaniac

  • *****
  • Posts: 22175
  • Joined: 25 Sep 2005
  • Loc: Amargosa Valley, NV, USA

Posted 03 May 2013 - 09:31 AM

Each stage would have to have its own avionics system.

#36 Jarad

Jarad

    Fly Me to the Moon

  • *****
  • Posts: 6354
  • Joined: 28 Apr 2003
  • Loc: Atlanta, GA

Posted 03 May 2013 - 09:33 AM

I'm sure they have gyroscopes, but the gyro's usually just measure the attitude, you still need thrusters or something to control it.

I couldn't see any thrusters at the top in the video, but they could have been there and just not visible compared to the brightness of the main thruster. I just assumed that they were controlling it with fine adjustments to the main thruster, but I don't actually know that to be the case... Maybe one of the JPL guys knows?

Jarad

#37 llanitedave

llanitedave

    Humble Megalomaniac

  • *****
  • Posts: 22175
  • Joined: 25 Sep 2005
  • Loc: Amargosa Valley, NV, USA

Posted 03 May 2013 - 09:37 AM

From what I understand the current Grasshopper has no thrusters, its attitude control comes only from engine gimbaling. Grasshopper 2 will need them, because it will experiment with shutting off and relighting its engine during free fall.

#38 Ravenous

Ravenous

    Viking 1

  • -----
  • Posts: 518
  • Joined: 14 Nov 2009
  • Loc: UK

Posted 03 May 2013 - 11:23 AM

Aerodynamic braking for a part-empty rocket stage with the engines shut down... that would be an interesting problem. There would probably be a quarter or so of the fuel sloshing around in the tanks, the off-centre weight would make attitude control tricky, unless they either use a big drogue chute or a "don't care" attitude.

Having said that perhaps the stage return and landing could be done with an extra, central, fuel & oxidiser tank. Extra complexity but would allow a separate pressurisation system as well as better control of balance.

Not saying aero braking is what they plan to use (because I don't know) but it's an interesting idea, and an interesting problem...

#39 Whichwayisnorth

Whichwayisnorth

    Apollo

  • *****
  • Posts: 1436
  • Joined: 04 Jul 2011
  • Loc: Southern California

Posted 06 May 2013 - 02:43 PM

You folks are acting like they don't already know what they are doing and how they are going to do it. This isn't just "hey wouldn't it be cool if" and "lets do some experiments to see if". This is "we are doing this so step aside".

#40 Jarad

Jarad

    Fly Me to the Moon

  • *****
  • Posts: 6354
  • Joined: 28 Apr 2003
  • Loc: Atlanta, GA

Posted 06 May 2013 - 03:19 PM

And this is us speculating about how they are planning to do it.

We don't expect them to listen to us or change what they are doing, but it's both fun for us to speculate about it, and occasionally some of the guys who are rocket scientists read this and enlighten us about what they are actually doing.

As for the "let's do some experiments to see if", the video in the OP was one of those experiments. The rest of the thread is us speculating about what's next in their plan, and how it would be used in a real launch.

As far as stepping aside, we're already standing on the side, we're just cheering them on.

Jarad

#41 llanitedave

llanitedave

    Humble Megalomaniac

  • *****
  • Posts: 22175
  • Joined: 25 Sep 2005
  • Loc: Amargosa Valley, NV, USA

Posted 06 May 2013 - 06:20 PM

If they knew exactly how they were going to do it, they wouldn't need to be doing all this testing. Elon Musk has already said that they expect to fail several times before they get a successful recovery. They have a working model of how it should be done, but SpaceX is in a learning mode too, and it's fun to share the suspense with them.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics