SpaceX to Debut Upgraded Falcon 9 on Return to Flight Mission | Hacker News Reader

SpaceX to Debut Upgraded Falcon 9 on Return to Flight Mission

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82 pointssituationista20 comments

"I want to stick a landing this year"

That's a gymnastics phrase for landing on your feet (with no bounce) during a dismount and is a pretty appropriate analogy for what they're trying to do with the Falcon 9 first stage.

To carry the analogy further, the second landing attempt could have been a "one hop" landing if they'd had a bigger drone ship. The first landing (which needed a bit more propellant to keep the stage upright) would have succeeded as a "one step" dismount - if the stage could actually take a step with it's legs.

I wouldn't take that phrase too literally. I take it to mean that she wants them to actually land a rocket (not necessarily on all feet with no bounce). The other attempts were "landings", but they blew up. Bounce or no, if they land the rocket upright, I think that's sticking it.

The problem as I understand it, and I think what smoyer was talking about, is that there are no other viable landing methods for the rocket than "feet" first.

Sufficient thrust is only available from the main engines, and those only point in one direction.

So if it starts to tilt off vertical then the rocket has no way to right itself after a point (and certainly no way to counteract gravity). Gravity + tall enough rocket + landing on side = boom (or crumple).

So "stick it or bust"?

There are cold gas thrusters on top of the first stage that fire to keep its balance. If it lands in an uneven manner and starts to tip, those should be enough to maintain the landing. You can see them working in the video embedded in the link below. They only didn't work in that instance because the main rocket motor fired too long at it was too much for the small thrusters.

http://www.spacex.com/news/2015/06/24/why-and-how-landing-ro...

I think the bigger issue is that idle thrust of a single Merlin engine (which is used to land the first stage) produces more force than gravity on the vehicle.

This means it can't "hover" so the final burn must be timed with exact precision to have ~zero velocity on impact. Doesn't give a lot of room for error.

Thanks for the link! I was aware there were thrusters on top of the stage (otherwise the difficulty would seem impossible), but that description and the longer video were a lot better than what I'd seen previously.

I couldn't dig up any specs on the cold-gas thrusters' actual output. I mean, how much tilt can they recover from on landing? Best I could find was this Musk response.

"Q. In addition, can you talk more about the grid fins that will be flying tomorrow? How do they compare to maneuvering with cold-gas thrusters?

The grid fins are super important for landing with precision. The aerodynamic forces are way too strong for the nitrogen thrusters. In particular, achieving pitch trim is hopeless. Our atmosphere is like molasses at Mach 4!"

The other side to the upgrade to the Falcon 9 is chilling down the fuel more than normal so that it shrinks and they can fit more of it in the rocket's tanks.

Do you have any reference for that? It seems to me that further cooling already liquified fuel doesn't give you any noticeable difference in volume.

r/SpaceX has a lot of good info. Check out the "What are the differences between the Falcon 9 v1.1 and the upgraded Falcon version popularly called the v1.2?" FAQ here: https://www.reddit.com/r/SpaceX/wiki/faq/falcon

This Wikipedia page has a mention of super-chilling: https://en.wikipedia.org/wiki/RP-1

Why not? According to a random page I found on the internet, kerosene has a coefficient of thermal expansion of 0.00099 per Kelvin. If you can cool it by 50 degrees then you'll be able to store about 5% more fuel in the same volume of tank, which is a nice little upgrade.

I just want to see a Klondike Bar in middle of ocean and a pencil on top of it.

Looks like they are taking the software approach to fixing things.

Still an improvement to the Von Braun approach, I'd guess.

What's the Von Braun approach? I mean, his work was a model for hardware reuse and improvement every launch. (I think considering the Atlas, Titan, and Saturn 1.)

V2.

They've been working on the Falcon 9 1.2 for a long time. It wouldn't make sense to go back to 1.1 to fix a known problem with a strut.

>that features engines with increased thrust, providing an increase in performance of about 30 percent.

I'm not a rocket engineer but this sounds like a mind-blowingly good increase. Is it just that they're early in the development of the engines and since they're taking an iterative approach this is just low hanging fruit? Even if that's the case, 30% increase seems impressive.

It's not low hanging fruit so much as it's a matter of systematically working the way up to higher and higher branches, and taking the time to build higher and higher ladders. You'd be surprised how little iterative development there is in traditional aerospace must often.

By careful selection of metric goals, impressive results can be achieved with little change. (this is a summary of modern american corporate process LOL).

So if your metric is pounds of payload delivered to orbit, for example, rockets are usually pretty heavy compared to the payload delivered so a rather small improvement in thrust to weight ratio can have seemingly extreme improvement in pounds delivered. You're still launching a million pounds sitting on the pad, its just removing a mere 300 pounds of insulation over than million pound booster means you get 1300 of payload delivered instead of 1000.

Rockets have all kinds of interesting problems with thermal limits and combustion instability and its very easy to defeat those problems at considerable weight penalty and then when telemetry consistently shows the nozzle temp is fine at full flow rate and projections show 80% flow is fine, then next model has microscopically smaller pipes or whatever. And repeat many times and next thing you know you've gained quite a bit of performance. Its hard to add fluid flow baffles to an existing system, but if it works with existing baffles its usually much easier to slowly remove them.

There is another spacex specific issue that to do the "land on a barge" thing general opinion is they overspec the marketing numbers to about 30% less. I don't know how much, if any, current improvement is lowering that to 27% or whatever. If you were to give up on recycling boosters and one-shot the booster, the marketed performance specs would go up a bit without any engineering changes (well other than software to land wouldn't be necessary, LOL).

I think you're underrating the difficulty of this kind of improvement. Rockets already have to be extremely well-optimized to reach orbit, so there wouldn't be the kind of low-hanging fruit like removing 300 pounds of insulation without side effects. Also, if you want weight reductions to give payload increases in a 1:1 ratio, the weight has to be taken off of the final stage (in F9's case, the second stage); removing it from earlier stages gives you much less benefit.

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