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Space X CR6 First stage landing

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Re: Space X CR6 First stage landing

#171

Post by sfeng »

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.

For one, developing reliable rocket engines takes a long time, you can't just whip up a new one. The current Merlin has a very large throttling range already when compared to existing engines. The size of the engines they use now is defined by the need to get payload's to space, the landing has to be secondary.

Re: Space X CR6 First stage landing

#172

Post by TeMPOraL »

shostack wrote: post #169
TeMPOraL wrote: post #148

Earlier quoted context omitted.

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?

If this turns out to be the only problem then yes, they have a decent chance of doing a proper landing the next time around.

Re: Space X CR6 First stage landing

#173

Post by will_hughes »

jccooper wrote: post #101
monk_e_boy wrote: post #71

Earlier quoted context omitted.

How high did it get? Does it burn the whole way down (I assume it uses those little X-wings to 'fly down?) When does it start the descent burn? How fast is it going at this point? How much does it weigh, what sort of forces are we talking about? How hard is landing this puppy in terms of human achievement? Top 10% of tricky things humans have so far achieved? [edit] wouldn't it be easier to hover it a few meters abov…

Stage separation is somewhere around 100km and Mach 6 or more. The exact figures will depend on the mission profile. There's a "boostback burn" at altitude to slow the horizontal velocity right after separation, an entry burn to put it into the atmosphere, and a landing burn just before it hits the deck. The guidance in between those last two burns is aerodynamic from the grid fins. The first stage mass is ~18,000kg…

According to a (now deleted?) tweet from Elon, it was stiction in a control valve that caused a lag in response time, causing repeated over corrections.

Re: Space X CR6 First stage landing

#174

Post by mikeash »

melling wrote: post #159
mikeash wrote: post #154

Earlier quoted context omitted.

The explosion isn't going to cause significant damage to the landing ship or the landing pad. There isn't much fuel left onboard at that point anyway, so it's fairly superficial. The rocket is scrap if it falls over regardless of whether or not it explodes, and the explosion won't hurt much, so there's not much reason to put effort into reducing the risk of an explosion in a crash. The plan is to eventually land at t…

Ok, just thought the rocket could be salvaged if it didn't get blown to pieces. That's the goal. Reusable rockets are going to lower the cost.

I think that once it falls over, it's scrap. There's so little margin that once you even suspect things are bent, you pretty much have to rebuild it anyway.

Re: Space X CR6 First stage landing

#175

Post by justin66 »

will_hughes wrote: post #167
justin66 wrote: post #158

Earlier quoted context omitted.

> 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.

Odd responses. I guess I shouldn't have phrased my response in the form of a question.

Here's another try:

> When near fuel exhaustion, the Merlin engine has a thrust to weight ratio above 1.

The rocket will always have a thrust to weight ratio well above 1.

Re: Space X CR6 First stage landing

#176

Post by jccooper »

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.

Mostly, I think, because they'd prefer to avoid having several types of engine involved. SpaceX is big on cost savings through commonality. The development and tooling and testing etc. for creating a smaller landing-only engine would probably cost darn near as much as for the main engines, so if you can get it done with them, it makes sense to do so.

If they can't get them on the ground reliably with just the Merlins after trying enough, then maybe they'll do something like that. But if it's a simple matter of software and a handful of tests, as it seems may be the case, then they've saved a lot of effort and cost.

The other answer is that the F9 architecture was designed (and flying) before this mode of recovery was "the one". And they'd prefer to not redesign the thing if it can work as-is. So they'll try it and see before spending gobs of money on a re-design. Rockets aren't software (except to the extent they are) and changing things about them is dangerous and expensive.

Re: Space X CR6 First stage landing

#178

Post by MrSourz »

rtpg wrote: post #177

this might be a silly question but why can't they use parachutes? wouldn't a mostly empty rocket float? or is there an issue with the salt water and whatnot

They intend for it to land on land in the future, not out at sea. For that they cannot use parachutes.

Re: Space X CR6 First stage landing

#179

Post by jccooper »

31reasons wrote: post #103
waterlesscloud wrote: post #41

Earlier quoted context omitted.

Watching it made me realize I only had an abstract idea of how difficult it was to even attempt this. It's insanely hard, and they're so close!

Yes its a very hard problem but what makes it hard ? Is it engineering of the components, algorithms or expensive tests and economics of it ? Maybe people before Elon Musk were not interested in solving this problem not because its technically hard problem but because their existing business model is much more lucrative.

From a technical standpoint, it's tough. You've got big expensive machinery that operates at or past any reasonable limits in extreme conditions and moreover gets destroyed with every test if you make the slightest mistake. I know that when I end up with an iteration interval of an hour it seems like development takes forever. SpaceX is able to test, what, every couple months?

But the economics is probably more important. The incumbents have such sky-high development costs that reusability seemed like an impossible task to do economically. Last time anyone tried they came up with STS, which was highly capable but ultimately dangerous and ruinously expensive. A new reusable system seemed like it would take half a trillion dollars in development, and would probably never have the flight rate to make it worthwhile. Elon thought that he could do it better. NASA believes designing a reusable system would be too expensive for them. They're probably both right.

Re: Space X CR6 First stage landing

#180

Post by jccooper »

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 .

And here's what it looks like when a rocket of similar class, but fully-fueled, explodes: https://www.youtube.com/watch?v=bx1CeHFeea0
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