Space X CR6 First stage landing • Hacker News Board

Re: Space X CR6 First stage landing

#161

Post by justin66 » Thu, Apr 16, 2015, 2:23 AM UTC

zaroth wrote: post #100

It was jaw dropping as-is, but when they finally do stick a landing I imagine the engineers will be running around like little kids screaming with the pure joy and exhilaration of it all. I hope they share the video at mission control as well when the day does come! The sheer drama of how fast it comes down, the awesome power of the Merlin engine just blasting away delta-v... I wonder how many g's it takes on that fi…

> Would a drougue have simply added too much weight or too much complexity to deploy?

Never mind the weight: the problem sited with this landing was excessive lateral velocity. A drogue chute would have the potential to make that problem worse rather than better.

Re: Space X CR6 First stage landing

#162

Post by ezegolub » Thu, Apr 16, 2015, 2:25 AM UTC

If any of the engines has a TWR above one when landing, even at it's lowest setting, i can't help but wonder why don't they use a smaller engine which can be more effectively used to land.

I'm sure theres a perfectly good reason for this, there's a reason SpaceX has its reputation, but i'm curious as to why.

Re: Space X CR6 First stage landing

#163

Post by jfoutz » Thu, Apr 16, 2015, 2:30 AM UTC

bane wrote: post #118

Pretty inspiring after all this time that the problem is not so much getting stuff up, but landing it successfully. I kind of feel like this is where we should have been in the 70s.

Pretty sure the original plan was mars after the moon. The cold war shifted away from the space race, and we built skylab instead. Some of the mars plans were pretty crazy. send 3 rockets. 1 with crew and supplies, 2 for the return trip. 2 just in case something went wrong.

Re: Space X CR6 First stage landing

#164

Post by pixie_ » Thu, Apr 16, 2015, 2:31 AM UTC

paulddraper wrote: post #127

That's tricky stuff. What was the intention behind not doing a water landing, like with the Space Shuttle boosters/tank?

Mars doesn't have water.

Re: Space X CR6 First stage landing

#165

Post by hydrogen18 » Thu, Apr 16, 2015, 2:33 AM UTC

TeMPOraL wrote: post #140

hydrogen18 wrote: post #93

Earlier quoted context omitted.

This how NASA recovered the SRBs. They landed upright, much like SpaceX's rocket. They were bouyant and floated vertically. The tender would approach the SRB, hook onto it, pump more water into it so it would sink. It would then carry it horizontally back to land. Seawater is incredibly corrosive and as it evaporates it leaves behind a fine layer of sea salt. Since these were SRBs, they had few moving parts. They wer…

Correct me if I'm wrong, but AFAIR SRBs weren't really reused, it was cheaper to make new ones than refurbish those they fished out.

https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Boo...

Some of the parts from the first missions were re used on the last. The SRBs weren't disposable, but they were essentially refurbished after each launch.

Re: Space X CR6 First stage landing

#166

Post by unspecified » Thu, Apr 16, 2015, 2:38 AM UTC

TeMPOraL wrote: post #137

jccooper wrote: post #115

Earlier quoted context omitted.

The rocket is, at the very least, a thin pressurized tube full of kerosene vapor and oxygen, and probably some decent amount of liquid kerosene and liquid oxygen. Crack it open and it's gonna make a pretty good flash. But as rocket explosions go, that wasn't very big. Get to YouTube if you want to see some real rocket explosions.

Check out, for instance, Proton-M going full Kerbal in 2013: https://www.youtube.com/watch?v=Zl12dXYcUTo .

Thank you so much for this. The anticipation while waiting for the sound of the explosion to arrive got a cackle out of me.

Re: Space X CR6 First stage landing

#167

Post by will_hughes » Thu, Apr 16, 2015, 2:38 AM UTC

justin66 wrote: post #158

toomuchtodo wrote: post #153

Earlier quoted context omitted.

When near fuel exhaustion, the Merlin engine has a thrust to weight ratio above 1. Its firing is timed in order to land at a distance above the landing surface that will allow for a reusable vehicle (close enough to not snap off the landing legs). It carries only enough fuel to do this while also using that same fuel for hydraulically actuating grid fins for aerodynamic control while in a controlled fall to the groun…

> When near fuel exhaustion, the Merlin engine has a thrust to weight ratio above 1. Wouldn't it have a thrust to weight ratio well above 1 all the time? The ratio will be highest just as it's about to run out of fuel. About 160 to 1?

For the final stage of landing only a single Merlin engine is lit.

They're also limited in how much they can control the throttle on it.

Even at minimum throttle, it's not possible for it to hover.

Re: Space X CR6 First stage landing

#168

Post by will_hughes » Thu, Apr 16, 2015, 2:40 AM UTC

darkmighty wrote: post #150

fleitz wrote: post #141

Earlier quoted context omitted.

I have a feeling their engineers may have considered this. This is after all rocket science.

I also have this feeling, but from my limited experience with control systems you might want to try the simplest first. There is a tremendous amount of variables to consider, and the analysis of how to take each into account is non trivial -- if you don't measure some variable precisely enough it might make the whole system less stable, the solution might get worse due to increased computational load, the conditionin…

The thrust to weight ratio of a single engine firing is so high that they have no option but to fire at the last moment. Firing for longer would result in the rocket going back up, it can't hover.

Re: Space X CR6 First stage landing

#169

Post by shostack » Thu, Apr 16, 2015, 2:40 AM UTC

TeMPOraL wrote: post #148

shostack wrote: post #132

Earlier quoted context omitted.

Can you or someone else please explain what that means in slightly less technical terms without dumbing it down too much?

A control system is a feedback loop that connects measurements (orientation, speed, acceleration) and steering (grid fins orientation, engine power and gimbaling, RCS/other means of attitude control) in order to steer the entire system into a desired state (in this case, upright orientation and 0m/s velocity exactly at the point of touchdown). The control system measures the state of the rocket and directly adjusts t…

This was exactly what I was looking for. Thanks for taking the time to go into this level of detail.

So looks like if they can solve for modeling this latency (likely a software problem), there's a decent shot they'll nail it the next time around?

Re: Space X CR6 First stage landing

#170

Post by dlgeek » Thu, Apr 16, 2015, 2:43 AM UTC

ezegolub wrote: post #162

If any of the engines has a TWR above one when landing, even at it's lowest setting, i can't help but wonder why don't they use a smaller engine which can be more effectively used to land. I'm sure theres a perfectly good reason for this, there's a reason SpaceX has its reputation, but i'm curious as to why.

Because the primary job of the engine is to get as massive a payload as they can to orbit, not to land. That's just a bonus.