Author Topic: Landing Lunar Starship nose down  (Read 5210 times)

Offline mikelepage

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Landing Lunar Starship nose down
« on: 10/18/2023 02:01 pm »
Edit for context: The first few posts were originally on the Lunar HLS thread and have now been separated out into this thread. After the initial fierce mocking which was expected, I think that there may be some merit to this. I've attached my <second> quick and dirty animation from down-thread to this OP, so there can be more thoughtful discussion.

Cards on the table - this started as a joke at first. But the more I explore it, the I more I think there might be something to it.

Idea: Land Lunar Starship Nose-Down <in order to keep landing thrusters maximum distance from the ground>.

As it stands, the only places where starship will need landing legs, are in low-gravity, more-or-less airless worlds like the moon and Mars. On Earth, starship will swoop in and be caught by some variant of mechazilla. On Mars, starship still needs the flaps to get down to terminal velocity, but the final part of landing will be comparable to the completely propulsive landing of lunar starship.

In both cases the lack of atmosphere means that - if starship lands with its powerful engines, ~5m off the surface - it's going to cause regolith particles big enough to do serious damage to be picked up off the ground and ejected at hypersonic/orbital velocities towards any exisiting base infrastructure. In order to be sure of not damaging existing assets, starship would likely have to land at distances from the base that are impractical (10s of km).

This recent Off-Nominal episode with Dr Phil Metzger was fun/excellent (and is the inspiration for my paragraph above). I'd recommend the whole thing, but two key sections are:
Apollo 12 and Surveyor 3 at 13'27"
Starship elevated thrusters discussion at 45'52"



Long story short: The best solution is to elevate the thrusters off the ground, and for spacecraft the size of starship, the further, the better (i.e. Elon's musings about not needing the elevated thrusters would seem to require some explanation, because it is a real problem).

So the initial appeal of nose-down landing concept comes down to three factors:
1) Ideally, we want the landing thrusters to be well above the surface, and the higher, the better. 
2) Ideally, the crew compartment should to be closer to the surface, eliminating having the hatch >25m above the surface.
3) Space is at a premium at the base of the rocket, and most wide-stance landing leg designs tend to interfere with engines or prop tanks in some way. At the nose, it's easier to recess large landing leg structures/crush cores next to the crew compartment.

So, putting aside the overall wackiness of the idea for a second, what would have to happen to implement this nose-down config?...

I think the only way to do it by putting two header tanks in each of the prop tanks (at top and bottom). Add to that, the necessary second set of plumbing to send propellant to the landing thrusters - which I'm imagining would be incorporated into the pre-existing thrust structure at the rear of the rocket, but on the outside of the skirt facing forward/down (so close to 50m off the surface at landing in nose-down config). A starship-scale sky crane.

Landing legs with a wide stance (akin to F9) would fold down out of their stowed positions next to the crew compartment and - having no nose header tank - we could have a nose-airlock + elevator arrangement which can lower crew and supplies the handful of meters to the ground. No high crane/platform required.

The landing maneuver would be similarly complex as with the landing swoop on Earth. Lunar starship would kill all horizontal velocity and most vertical velocity whilst still a significant height above the landing site, then flip to nose-down config for final descent and landing. Ascent would reverse the maneuver. A decent amount of gravity losses during the flip obviously, but I'd be surprised if any crewed vessel is going to be doing a hover slam anyway.

As to the internal layout of the starship, it's important to consider that for landing on low-gravity bodies like the moon or Mars, the g-forces are not going to be significant once the main retro-propulsive burn is done. The acceleration couches would be oriented nose-up, while the rest of the living space would be oriented nose-down and secured during launch/landing. This way, the acceleration couches would be out of the way, on the ceilings of each room while the starship is landed. During the flip maneuver, the crew can get out of their seats and then stand upright for the rest of the landing.

Anyway, that's the idea. Curious for your thoughts.
« Last Edit: 11/24/2023 11:02 am by mikelepage »

Offline cuddihy

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Re: Landing Lunar Starship nose down
« Reply #1 on: 10/18/2023 02:49 pm »
1. How does the HLS take off from the lunar surface with the engines pointed to the sky?
2. How do you keep it from tipping over with 90% of the weight in the top 10 m of the rocket?

Offline Barley

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Re: Landing Lunar Starship nose down
« Reply #2 on: 10/18/2023 04:03 pm »
If you're opening this up, you should probably consider all orientations.  How about horizontal with the heatshield up?

Offline mikelepage

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Re: Landing Lunar Starship nose down
« Reply #3 on: 10/18/2023 04:25 pm »
1. How does the HLS take off from the lunar surface with the engines pointed to the sky?

Same way it landed, with the engines that are pointed at the ground. Think of it as taking the same additional canted engines they’re currently planning to put on the “waist” of HLS, but instead put them on the outside of the skirt, adjacent to the main engine cluster, and face them upward (downward when nose-down).

Quote
2. How do you keep it from tipping over with 90% of the weight in the top 10 m of the rocket?

It’s carrying 50+ ton of crew supplies, plus sufficient prop to get back to NRHO, so total mass is a little over 600 ton or so. Still top heavy but not as bad as you describe, and beefy F9 style legs with wide stance can have mechanisms which intrude into crew compartment in ways not possible with prop tanks/engine cluster,

Also, main chance to tip over is regolith subsidence, and there are self-levelling type mechanisms that can take care of that in the first seconds after landing.

Offline mikelepage

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Re: Landing Lunar Starship nose down
« Reply #4 on: 10/18/2023 04:28 pm »
If you're opening this up, you should probably consider all orientations.  How about horizontal with the heatshield up?

The only reason to consider this idea is to keep the landing engines as far off the ground as possible. That the crew compartment ends up close to the ground in the nose-down orientation is a bonus. Horizontal landing puts the landing engines within 9m of the ground.

Edit: just reread your post - ha :P but there was a rationale to the notion.

« Last Edit: 10/18/2023 04:31 pm by mikelepage »

Offline whitelancer64

Re: Landing Lunar Starship nose down
« Reply #5 on: 10/18/2023 04:52 pm »
Cards on the table - this started as a joke at first. But the more I explore it, the I more I think there might be something to it. Requesting feedback please before I waste any more time on this.

Semi-crackpot Idea: Land Lunar Starship Nose-Down.

As it stands, the only places where starship will need landing legs, are in low-gravity, more-or-less airless worlds like the moon and Mars. On Earth, starship will swoop in and be caught by some variant of mechazilla. On Mars, starship still needs the flaps to get down to terminal velocity, but the final part of landing will be comparable to the completely propulsive landing of lunar starship.

In both cases the lack of atmosphere means that - if starship lands with its powerful engines, ~5m off the surface - it's going to cause regolith particles big enough to do serious damage to be picked up off the ground and ejected at hypersonic/orbital velocities towards any exisiting base infrastructure. In order to be sure of not damaging existing assets, starship would likely have to land at distances from the base that are impractical (10s of km).

*snip*

By the time there are surface bases on the Moon / Mars, there will be prepared landing pads nearby that will pose little to no risk to them.
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Offline Brigantine

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Re: Landing Lunar Starship nose down
« Reply #6 on: 10/18/2023 06:44 pm »
Sounds like a very sporty ascent - how are you going to get the propellant settled for the raptors to light? Cold gas ullage thrusters? At what altitude? Header tanks? Where?

I note (a big part of) the reason the garage is above the tanks, is because they wanted the extra stability you get by having the tanks that much lower, and resulting low CoM.

What happens when you want to take the rover for a spin, but you still need it in the garage for ballast?

Offline notthebobo

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Re: Landing Lunar Starship nose down
« Reply #7 on: 10/18/2023 08:55 pm »
Semi-crackpot Idea: Land Lunar Starship Nose-Down.

Andy Griffith already thought of that in 1979.


Offline mikelepage

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Re: Landing Lunar Starship nose down
« Reply #8 on: 10/19/2023 05:00 am »
Sounds like a very sporty ascent - how are you going to get the propellant settled for the raptors to light? Cold gas ullage thrusters? At what altitude? Header tanks? Where?

Not particularly, thanks to the low-G environment. If we're trying to minimise disruption to the moonbase, then none of those landing/launch sequences can be high g-force anyway. Suppose ascent starts with 20 seconds of 1xG . At this point we're ~2km above the surface and travelling 196m/s. Under lunar gravity with no further thrust it would take another 120 seconds to get to apolune, which is at ~12km. After stage sep, the Falcon 9 booster flip takes what, 10-20 seconds? So if Starship flip takes a similar time before lighting up the main engines, there will be some gravity losses, but nothing too extreme. Likewise for descent.

I think settling the propellant requires two header tanks in each of the two prop tanks (one header tank at each end of each prop tank, for each direction of thrust). HLS probably has a more complex arrangement of tanks anyway due to needing to keep cryo propellants cold during loiter time in NRHO and on the surface. I'm not sure what the penalty of these two extra header tanks and plumbing would be, maybe another 5-10 tons?

If it means being able to land within sight of the base without sandblasting it every time, that trade would have to be worth considering. Remember, the problem is not just the lack of prepared landing pads, but the lack of atmosphere (this is why I posted the youtube link above).

Quote
I note (a big part of) the reason the garage is above the tanks, is because they wanted the extra stability you get by having the tanks that much lower, and resulting low CoM.

What happens when you want to take the rover for a spin, but you still need it in the garage for ballast?

I'm not sure what kind of disturbances are being imagined on the moon that make that extra stability important? No wind, and the moonquakes are pathetic. Ongoing lunar regolith subsidence? Say the center of gravity is 40 meters off the ground, but the leg span at the base is 18m. Plenty of objects in our daily lives have a worse aspect ratios than that (e.g. book cases, wine glasses), and we're not worried about them falling over unless there's something to push them. Even a 1 ton rover will be insignificant in mass compared to starship itself, so what's doing the pushing?

I mean I expected to get roasted for this idea, but I think a lot of it so far is 1) not appreciating the magnitude of the problem being addressed , and 2) people's Earth-trained intuitions getting in the way. Things can be taller and narrower on the moon. Thanks for the non-sarcastic response  :)

ps. The actual criticism I expected to get was on abort modes for lunar landing, especially when switching between engines. But then you have that to a certain extent with the elevated landing thrusters anyway.

Offline Slarty1080

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Re: Landing Lunar Starship nose down
« Reply #9 on: 10/19/2023 07:46 pm »
If you're opening this up, you should probably consider all orientations.  How about horizontal with the heatshield up?
shhh pleeeease no! Not the horizontal Starship lander again. We know what comes after that...
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Offline redneck

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Re: Landing Lunar Starship nose down
« Reply #10 on: 10/19/2023 10:28 pm »
If you're opening this up, you should probably consider all orientations.  How about horizontal with the heatshield up?


https://selenianboondocks.com/2021/06/layover-lunar-lander/

Offline lykos

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Re: Landing Lunar Starship nose down
« Reply #11 on: 10/20/2023 08:25 am »
There is no heatshield on the HLS!

Offline redneck

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Re: Landing Lunar Starship nose down
« Reply #12 on: 10/22/2023 08:56 pm »
It seems to me that if one were going for the nose down approach, permanent legs and landing engines could be attached after the vehicle is in orbit. A vehicle that will never again deal with atmosphere shouldn't have to worry about a sleek profile. It would eliminate one failure profile of things not extending during the landing maneuvers.

Offline mikelepage

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Re: Landing Lunar Starship nose down
« Reply #13 on: 10/23/2023 04:21 am »
It seems to me that if one were going for the nose down approach, permanent legs and landing engines could be attached after the vehicle is in orbit. A vehicle that will never again deal with atmosphere shouldn't have to worry about a sleek profile. It would eliminate one failure profile of things not extending during the landing maneuvers.

Perhaps, but isn't the complication of assembling this in orbit an even bigger QA issue?

Part of this thread concept was inspired by the problems seen on the Starship landing leg thread.

If you're not so volumetrically constrained by the engines and prop tankage, you can afford to make the legs quite a lot bigger.

Quick and dirty animation of the concept attached.


Offline daedalus1

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Re: Landing Lunar Starship nose down
« Reply #14 on: 10/23/2023 06:45 am »
If you're opening this up, you should probably consider all orientations.  How about horizontal with the heatshield up?

There isn't a heat shield.

Offline mikelepage

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Re: Landing Lunar Starship nose down
« Reply #15 on: 10/23/2023 07:52 am »
If you're opening this up, you should probably consider all orientations.  How about horizontal with the heatshield up?
shhh pleeeease no! Not the horizontal Starship lander again. We know what comes after that...

There is no heatshield on the HLS!

There isn't a heat shield.

I'm pretty sure Barley was making a joke, and knows there is no heat shield on HLS.
Also, since I'm not proposing to lower the starship to horizontal, can we leave this tangent alone please?

Here's a question for the rocket nozzle specialists: to what extent can the landing thrusters (whether nose down or not) get away with having a small bell as depicted in the video?

Offline daedalus1

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Re: Landing Lunar Starship nose down
« Reply #16 on: 10/23/2023 08:12 am »
Yeah, it's about as much of a joke as nose down landings. I never assume someone knows something about the question they ask.

Offline Paul451

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Re: Landing Lunar Starship nose down
« Reply #17 on: 11/24/2023 03:25 am »
If you're not so volumetrically constrained by the engines and prop tankage, you can afford to make the legs quite a lot bigger.
Quick and dirty animation of the concept attached.

Why is the nose so far off the surface? Bring it as close to the ground as you can. Get rid of the elevator entirely.

Hell, if the engines are way-up-the-other-end, why not use the nose itself as the primary landing leg. Then the outer legs are purely for stabilisation rather than supporting the entire mass, hence their deployment systems can be reduced in size/mass.

And given the way you've folded out the landing legs, those outer legs can also serve as ramps/stairs, leading to openings in the sides of the nose to the unpressurised cargo deck. (Hmmm, you could have two longer, two shorter legs, leading to two cargo decks. Increasing available cargo volume.)


I mean, if you're going to do this, go all in.

Offline InterestedEngineer

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Re: Landing Lunar Starship nose down
« Reply #18 on: 11/24/2023 04:01 am »
Why not like Space 1999?

Put an engine and landing legs in the stubs where the 4 Elonerons would normally go.  Each with a nominal thrust of 25tf or so.

Do a flip a kilometer or so off the surface, land horizontally.

Only need a small ramp to get stuff down.

Online MickQ

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Re: Landing Lunar Starship nose down
« Reply #19 on: 11/24/2023 05:02 am »
Nothing beats the Eagle in design, versatility and just plain Cool.

Actual technical feasibility is another thing entirely.

Offline zodiacchris

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Re: Landing Lunar Starship nose down
« Reply #20 on: 11/24/2023 05:19 am »
That’s a great idea, let’s call it Steagle! The Steagle approach definitely beats the headstand landing…😎

Offline kkattula

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Re: Landing Lunar Starship nose down
« Reply #21 on: 11/24/2023 06:09 am »
One disadvantage of the nose down concept, with landing thrusters pointing in the opposite direction of the main engines, is having both lit at the same time wastes propellant and maybe impractical due to settling.

This means you likely have to commit to shutting down one set before starting up the other, on both descent and ascent. 

With the high-mount ring of landing engines, or even side-mount thrusters for a horizontal lander, you can easily start up the second set while the first is still running, and if they won't start, abort to orbit if descending, or abort back to the surface if ascending.

Offline kkattula

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Re: Landing Lunar Starship nose down
« Reply #22 on: 11/24/2023 06:19 am »
It seems to me that if one were going for the nose down approach, permanent legs and landing engines could be attached after the vehicle is in orbit. A vehicle that will never again deal with atmosphere shouldn't have to worry about a sleek profile. It would eliminate one failure profile of things not extending during the landing maneuvers.

No need to wait for landing to deploy them.  Once the ship is in LEO, the legs could be unstowed from a launch position, and permanently locked in place.  As long as they don't interfere with required docking. 


Offline mikelepage

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Re: Landing Lunar Starship nose down
« Reply #23 on: 11/24/2023 10:59 am »
If you're not so volumetrically constrained by the engines and prop tankage, you can afford to make the legs quite a lot bigger.
Quick and dirty animation of the concept attached.

Why is the nose so far off the surface? Bring it as close to the ground as you can. Get rid of the elevator entirely.

Hell, if the engines are way-up-the-other-end, why not use the nose itself as the primary landing leg. Then the outer legs are purely for stabilisation rather than supporting the entire mass, hence their deployment systems can be reduced in size/mass.

And given the way you've folded out the landing legs, those outer legs can also serve as ramps/stairs, leading to openings in the sides of the nose to the unpressurised cargo deck. (Hmmm, you could have two longer, two shorter legs, leading to two cargo decks. Increasing available cargo volume.)


I mean, if you're going to do this, go all in.

To be honest I didn't even think of using the nose as the primary landing structure. That's a much better idea, thanks for actually working the concept. I figure you could eject a small nose cone and use that entire payload space for some kind of shock absorber structure.

I've tried mocking up an animation of nose-as-landing-leg, with an unpressurised cargo area which is basically at ground level (actually about a meter up). I went with three legs here in order to maximise the width of the openings (the max dimensions for a habitat module here would be 3.5m diameter, 5-ish meter height, assuming you arrange the pistons better than I've done here). I've run out of time tonight to try to implement something with stairs, but I'm not even sure stairs are the right choice. With a 25m stance width, the stairs would have to be pretty steep.
« Last Edit: 11/24/2023 11:01 am by mikelepage »

Offline mikelepage

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Re: Landing Lunar Starship nose down
« Reply #24 on: 11/24/2023 11:11 am »
One disadvantage of the nose down concept, with landing thrusters pointing in the opposite direction of the main engines, is having both lit at the same time wastes propellant and maybe impractical due to settling.

This means you likely have to commit to shutting down one set before starting up the other, on both descent and ascent. 

With the high-mount ring of landing engines, or even side-mount thrusters for a horizontal lander, you can easily start up the second set while the first is still running, and if they won't start, abort to orbit if descending, or abort back to the surface if ascending.

Yes, the abort modes are the thing that seems most problematic to me. But having said that, the flip itself can be pretty leisurely in lunar gravity. I played with the math upthread, and found the ship can decelerate to a near halt at a staging altitude of say 12 km, commence the flip, and if there are any issues starting landing engines, one can let the flip turn into a 270, and proceed to abort to orbit. The decision to make a final approach for landing or not isn't a hurried one.

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