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10 hours ago, magnemoe said:

I was expecting them to do an small jump first. 
On the other hand they don't learn anything new doing so and say the touchdown fail they lost the ship for no purpose while if touchdown fail after an successful belly flop they learned a lot. 
 

SpaceX is arguably the world leader in propulsive rocket landing right now, they have literal metric tons of data on such. The last 10m of the flight is relatively easy, it’s the whole “controlled fall” bit that’s the big unknown, that no one’s really studied yet. They really need to get into that regime ASAP to get the info they need the most.

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23 minutes ago, mikegarrison said:

Oh, I think it would be very difficult to argue this. They point-blank are the leader in this right now.

Well, I mean, there is those other guys. :rolleyes: Y’know, the ones we don’t hear about often, have their own little moon program, even launched & landed a few (sub-orbital-class) things... <_<

Spoiler

We can have a round of nods for Masten Space Systems, right? :sticktongue:

 

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Sure, SpaceX are not the only people doing this. Nor were they the first. I mean, Buzz and Neil landed on the moon, right? They sure didn't use parachutes or wings.

But it's pretty clear SpaceX are currently the leaders in this, with a vast base of experience. (I would guess most people realize I'm not a huge SpaceX fan in particular, but I don't need to be a fanboy to see this obvious fact.)

Edited by mikegarrison
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10 hours ago, CatastrophicFailure said:

SpaceX is arguably the world leader in propulsive rocket landing right now, they have literal metric tons of data on such. The last 10m of the flight is relatively easy, it’s the whole “controlled fall” bit that’s the big unknown, that no one’s really studied yet. They really need to get into that regime ASAP to get the info they need the most.

Agree, now raptors handles different than merlins but you can simulate an powered landing with an static fire using the trust and trust vector you get out in your simulation. 
An 3 engine jump is a bit different but again doing it with 5 or 6 would make more sense as they are pretty disposable at this time. 

And yes the  flip will be interesting. now one way to make it much easier is to flip higher, the same way high attitude is safer in an plane as you have more time to recover,  now unlike an plane starship has very limited flight time after the flip. If they have serious problems getting this to work adding extra header tanks and do the flip higher on SN 14-15 might help here. 
I see the real problem is to get the control of the flaps ant the engine burn correctly. Getting the 20 ton trust hot gas trusters to work will also help a lot stabilizing starship here

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Great music video by Scott Manley of the Sentinel 6 launch. Seeing the continuous shot footage on the left is a real treat, and it gave me a new sense of how much these boosters utilize lift while on approach to landing.

Spoiler

 

 

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Well, when they finaly make the high hop that will change for sure.

Its kinda strange that the legs are a tough problem, but the more i think about it the less i see an easy solution. The first one to land on Mars will be especially hard, i guess everyone after that will at least have a flat landing site.

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On 11/27/2020 at 3:14 AM, magnemoe said:

I was expecting them to do an small jump first. 
On the other hand they don't learn anything new doing so and say the touchdown fail they lost the ship for no purpose while if touchdown fail after an successful belly flop they learned a lot. 
 

Yeah, I agree. The short hops without the nose cone made sense for the sake of getting the leggy touchdown correct, but once they go higher, they're going to want to test the belly flop.

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2 hours ago, RealKerbal3x said:

Seems like I've been the only person posting anything here for a while :P

 

That's clearly why he preferred the tripod design earlier on -- those three legs had a ridiculously wide stance. You could have landed anywhere. But building the flaps to actuate along the medial axis while ALSO taking a transverse load at touchdown would have been way, way harder. I'm sure the flaps are much lighter and stronger with lighter actuators since they don't have to take any transverse load.

The current leg design uses a fold-out-and-telescope design so that there is no need for shielding. In contrast, the lunar starship legs (as depicted) drop straight down. I wonder if it would be possible to telescope through the skirt:

legs.png

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9 minutes ago, sevenperforce said:

That's clearly why he preferred the tripod design earlier on -- those three legs had a ridiculously wide stance. You could have landed anywhere. But building the flaps to actuate along the medial axis while ALSO taking a transverse load at touchdown would have been way, way harder. I'm sure the flaps are much lighter and stronger with lighter actuators since they don't have to take any transverse load.

The current leg design uses a fold-out-and-telescope design so that there is no need for shielding. In contrast, the lunar starship legs (as depicted) drop straight down. I wonder if it would be possible to telescope through the skirt:

legs.png

For lunar starship, it has no shield it also does not need to retract the legs, you could go falcon 9 style legs just with better suspension. 
Else yes you need an door in the heatshield, but you will not extend the legs until ready to land. 
Another idea is to use the current rotating legs but curve them and let them rotate more increasing the footprint. 
Now not sure how much an issue this is outside of mars or then you start flying starship on an moon landing route, yes a bit wider legs gives an bit more margin. 

Unlike planes there the landing gear is very important part of the structure and hard to change as changes will also require changes to the wings, starship legs are mounted on the skirt who is the load bearing structure. 
If needed drop the bottom cargo holds for more room for legs. 

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4 minutes ago, sevenperforce said:

The current leg design uses a fold-out-and-telescope design so that there is no need for shielding. In contrast, the lunar starship legs (as depicted) drop straight down. I wonder if it would be possible to telescope through the skirt:

I speculated a similar design a while back, though instead of the landing feet being housed in an external pod, they were actually integrated into the skirt, like this:

sdXQNFY.png

Of course then you'd be cutting holes in the heatshield and there probably wouldn't be enough space in the engine bay for RVacs and cargo pods. It's all a trade-off.

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30 minutes ago, RealKerbal3x said:

I speculated a similar design a while back, though instead of the landing feet being housed in an external pod, they were actually integrated into the skirt, like this:

Spoiler

sdXQNFY.png

Of course then you'd be cutting holes in the heatshield and there probably wouldn't be enough space in the engine bay for RVacs and cargo pods. It's all a trade-off.

One thing you have to avoid is the ground contact point being heat-shielded. The tiles on Starship are far, far too fragile to have any ground contact. The legs on Falcon 9 deal with much less entry heat and so they can be exposed to the airstream, but whatever actually touches the ground for Starship cannot be shielded.

38 minutes ago, magnemoe said:

Another idea is to use the current rotating legs but curve them and let them rotate more increasing the footprint. 

The good thing about the current legs is that the angle is fairly low, so the force is transferred pretty directly to the skirt. There's a little torque at the hinge, but not a lot. If the current rotating legs curved out more, the torque on the base of the skirt would be too high.

Falcon 9 style legs are awesome because the telescoping piston transfers the force up higher at a more narrow angle, which avoids extreme torque at the hinge. And it works well because the legs themselves act as a heat shield for the piston. But trying to fit a telescoping piston under an orbital-class heat shield is NOT an easy task.

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4 minutes ago, sevenperforce said:

One thing you have to avoid is the ground contact point being heat-shielded. The tiles on Starship are far, far too fragile to have any ground contact. The legs on Falcon 9 deal with much less entry heat and so they can be exposed to the airstream, but whatever actually touches the ground for Starship cannot be shielded.

Yeah, I can see why Elon said designing legs for Starship was a challenge! Maybe the landing feet could have transpirational cooling, but that seems like it's introducing unnecessary complexity. The best part is no part.

Though, interestingly, on the 2016 ITS design the landing feet did appear to have some kind of thermal protection material on them. I'm not sure how that would have worked.

spacex-its-lander-engine-layout.png

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13 minutes ago, Silavite said:

Paging @mikegarrison, since I believe that he was the one who most acutely foresaw this issue.

I can not claim great insight here. It's just that I've taken some classes in aerospace vehicle design (in school and after that), and it was drilled into me that landing gear is a surprisingly difficult problem to fix if you don't get it right *very* early in the design process.

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6 minutes ago, RealKerbal3x said:

Yeah, I can see why Elon said designing legs for Starship was a challenge! Maybe the landing feet could have transpirational cooling, but that seems like it's introducing unnecessary complexity. The best part is no part.

Yeah, that would be unnecessarily complicated for sure. 

7 minutes ago, RealKerbal3x said:

Though, interestingly, on the 2016 ITS design the landing feet did appear to have some kind of thermal protection material on them. I'm not sure how that would have worked.

The 2016 ITS would have used a PICA-X heat carbon fiber heat shield, which is much more robust (but not nearly as reusable). The glass tiles on Starship are tougher than the ones on the Shuttle, but if you recall the Shuttle tiles could be crushed with your fingertips.

The only way to do it without a seam in the tiles, I think, would be to use fairly large nacelles that enclose legs telescoping out at an angle. Like my example above but with a heat shield cowling that maintains its angle and doesn't move -- the legs would need to protrude out under it. The only problem there is that you're lifting the vehicle higher off the ground, which raises the center of gravity and obviates some of the advantages of that wide stance.

17 minutes ago, mikegarrison said:

I can not claim great insight here. It's just that I've taken some classes in aerospace vehicle design (in school and after that), and it was drilled into me that landing gear is a surprisingly difficult problem to fix if you don't get it right *very* early in the design process.

Yeah. Fortunately the legs here are largely decoupled from the rest of the design so they can iterate a bit but they are running out of room.

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4 hours ago, sevenperforce said:

One thing you have to avoid is the ground contact point being heat-shielded. The tiles on Starship are far, far too fragile to have any ground contact. The legs on Falcon 9 deal with much less entry heat and so they can be exposed to the airstream, but whatever actually touches the ground for Starship cannot be shielded.

The good thing about the current legs is that the angle is fairly low, so the force is transferred pretty directly to the skirt. There's a little torque at the hinge, but not a lot. If the current rotating legs curved out more, the torque on the base of the skirt would be too high.

Falcon 9 style legs are awesome because the telescoping piston transfers the force up higher at a more narrow angle, which avoids extreme torque at the hinge. And it works well because the legs themselves act as a heat shield for the piston. But trying to fit a telescoping piston under an orbital-class heat shield is NOT an easy task.

I agree that my design would need an pretty hefty joint who would probably not be the one with the suspension. 
And an falcon 9 design is not very suited for starship. 

With legs poking trough the heatshields you need doors like the space shuttle landing wheels had. Doors who can open and who latches close so more systems although not an very critical one outside the latching as the leg would just push it open if it did not open beforehand.

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16 hours ago, magnemoe said:

With legs poking trough the heatshields you need doors like the space shuttle landing wheels had. Doors who can open and who latches close so more systems although not an very critical one outside the latching as the leg would just push it open if it did not open beforehand.

I think Elon really wants to avoid heat-shield seams if at all possible, but that may end up being the only solution.

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