@nyrath I hope the SpaceX team is aware of Metzger's work. Elon is a genius sometimes and at others one wants to slap him for being so intentionally ignorant or disrespectful of prior work in a topic area. hope lunar landings for their Starship craft is not the latter kind
@nyrath
BTW has anybody done work on how realistic spacecraft can land on garden worlds without wrecking the landing site?
I’m thinking of “Scout Service” type craft—if you don’t have magitech, what’s the best propulsion to use if you’re going to land explorers and need to be able to return to orbit? This kinda has to be SSTO, of course...
Landing is the easy part - parachute. Launching is tougher. Depending on the specifics, balloon or some sort of skycrane can at least get you off the ground, but reaching orbital velocity can be tough.
Since for an exploration ship, it is more efficient to send down landers than to land the entire blasted ship, the BDM Corporation proposed laser propulsion. The mothership has a huge power reactor and a gigawatt laser. It sends laser energy to the landers which they utilized to climb back into orbit.
So the landers don't require huge amounts of propellant for the return.
@michael_w_busch
“any sort of atmosphere”: I thought the key thing here was moisture?
That is, you’ll have big dust issues any time there’s no moisture to bind the regolith. I kinda assumed, actually, that the worst dust situation would be on a waterless world with thick atmosphere relative to its gravity, because that would maximize the time the dust takes to settle after disturbance...
@michael_w_busch
what I’m thinking of as super-Mars planets—waterless, lifeless, but with substantial atmosphere—wind up being fairly common in my current project, so I’ve been thinking about what their conditions will be like... @nyrath
@tkinias@nyrath For Mars surface pressure & gravity; impact depth for a 1-mm dust grain is about 100 meters. Far more than Earth, where it is only a few meters. But sandblasting the surroundings is way less of a problem than in vacuum.
@tkinias@nyrath For a first approximation of the scale over which particles stop acting like projectiles and start moving with an atmosphere; you can go all the way back to Newton:
Impact depth = diameter * density of particle / density of atmosphere .
The details get complicated. Per the papers linked above.
@tkinias@nyrath The Moon's lack of atmosphere means that small dust grains, which can get accelerated to very high speed by an exhaust plume, do not slow down (given that they are high enough up that the grains are no longer colliding much with each other).
So they can both travel a very long way and potentially cause a lot of damage to sensitive things like solar panels and spacesuits if they hit them.
So, thinking through this, airless means that nothing slows them down and they travel long distances, whereas atmosphere potentially suspends them. The latter can cause problems of visibility, for example, but less damage because they’re not high-velocity particles? @nyrath
@Sevoris
hmm, that’s an interesting idea—basically your orbital interface shuttle isn’t a lander but just delivers an aircraft to the upper atmosphere?
then the shuttle doesn’t have to be able to land and the lander doesn’t have to be able to reach orbit... @nyrath
@tkinias@nyrath Yeah, and specifically - you can optimize. A (shielded) fission reactor is heavy, but that doesn’t matter when you don’t plan to land outside of big runways (or emergency touchdown). But you can have enough power to burn back out to orbit. The tilt-rotor can have big rotors for efficient hover and exploration flight + efficient wing lift. Jet pusher gets it up to transonic speeds for rendevouz with orbital carrier.
@tkinias@nyrath and if we want to be a bit fantastical, the nuclear carrier can be optimized for all sorts of missions. Flying in drone aircraft; with Enough Variable Geometry™ it can serve as a hypersonic explorer of the upper atmosphere of gas giants or other such places.
Or just drop cargo pods with auto-rotor landing systems for your research site.
@RogerBW@tkinias@nyrath if we want to get extra extreme and have enough dV on your deep-space exploration craft… the mother kills her orbital velocity, idk, 300 kilometers above ground level, then just uses continuous thrust to hover against gravity while the landing party gets winched down to the ground…
@Sevoris
I’m imagining that I might not enjoy being winched down through the exhaust plume of whatever kind of rocket yields the kind of delta-vee that makes that possible... @RogerBW@nyrath
with the semiautonomous Mars rovers Curiosity & Perseverance using this scheme recently, the skycrane had 8×thrusters angled slightly outboard from vertical in each of the cardinal directions so that net thrust was UP but plume was NOT down onto payload.
With sufficient redundancy this could be human-rated?
@n1vux
oh, I was just imagining that the ‘safe area’ is likely to be rather smaller if we’re talking about a terawatt fusion drive rather than such chemical thrusters @Sevoris@RogerBW@nyrath
@tkinias@n1vux@RogerBW@nyrath tractor configuration, and inverse-square law. The fusion reaction occurs several kilometers away from the mothercraft, which has directional shielding. You get winched down in the shadow of that directional shielding.
This sort of configuration, with multiple parallel lines, is plausibly an unworkable nightmare if attempted in practice. There's nothing to prevent the slightest amount of spin from twisting the lines, and if any line breaks you've got some big problems.
Also, the fact that the lines are close to the exhaust can be a problem.
Now, my preferred artificial spin configuration makes the lines almost perpendicular to the thrust exhaust...
@n1vux@tkinias@Sevoris@nyrath I guess the safe angle for a thruster plume is going to vary quite a bit with atmosphere—building one for Mars is one thing and I don't want to diminish that, but building one for a wide range of planetary gravities and atmospheres would be harder.
You don't need a single universal skycrane. It can be modular, with tether length chosen for safe distance, optional parachute/parasail system, optional heat shield system, optional rotor system, optional balloon lifter ...
@RogerBW
Yes, flexible enough to handle any away-mission in any atmosphere is quite an order for skycrane, air-raft, or ship's boat.
In SciFi (books/movies/games) we traditionally dismiss some of those engineering challenges as "solvable with sufficient something": fusors provide nearly free power, FTL works ...
(In context of this thread, flexible enough for only handling part of the mission is easier than flexible enough for entirety of any mission.)
@n1vux
yeah, it feels like the universal shuttle or ship’s boat only really works with magitech contragrav or non-fuel-consuming ‘thrusters’ or the like... @RogerBW@Sevoris@nyrath
@n1vux@tkinias@RogerBW@Sevoris@nyrath AIUI the anti-missile lasers various navies are testing only really work because ships float on top of an essentially infinite heat sink—laser weapons dump about 3x as much energy as waste heat at the emitter end as they deliver to the target, and that's after decades of efficiency improvements. (Vacuum is a very good insulator: so much for laser weapons on spaceships ...!)
@cstross@n1vux@tkinias@RogerBW@Sevoris@nyrath Microwave lasers can get up to about 50% efficiency with resonant cavity vacuum tubes; which are extremely annoying to work with. But that still means sinking as much waste heat as the power going out.
Which some people who still promote beamed solar power as a replacement for panels on the ground are pleased to not acknowledge.
@michael_w_busch@cstross@n1vux@tkinias@Sevoris@nyrath I am reminded of the proposal for a laser sniping rifle that would be powered by a 100kW polonium RTG. Never mind the laser, the power source alone is going to cook you, not to mention light you up for anyone with an IR scope.
I seem to recall a David Brin story where a science vessel was using a laser to dump excess heat while operating in close proximity to a star. It didn't seem right at the time but I dismissed it as sci-fi tech.
Funny fact: laser energy is so ordered they begin converging towards having no entropy at all. It‘s the precise inverse of a heat radiation reaction, which is defined by an unordered emission process.
But the Navy just buys commercial off-the-shelf technology and rigs it up to produce powerful enough beams. The "laser engines" for contemporary laser systems are only so special.
The laser weapon became possible because the market wanted better welders and cutters, basically…
@tkinias@nyrath BTW, You do NOT need SSTO. You just need to look at the stages upside-down.
You're used to thinking of a bunch of stages on the ground, and discarding them on the way up.
But you can have multiple stages in orbit, which rendezvous on the way up. So the first stage aerobrakes enough to pick up the payload, and rockets up until it's spent.
The second stage only aerobrakes enough to meet the first stage. Then it rockets to orbital speed.
@tkinias@Sevoris@nyrath No, he's still thinking in SSTO terms. His nuclear rocket is an SSTO. I'm talking about 2 or 3 stages to orbit.
(I explained the simpler 2 stage version, but if you actually run the numbers for Earth it requires an uncomfortably large second stage. If you want chemical rockets with identical stages, you really need at least 3 of them.)
@tkinias@nyrath I call this concept "Hypersonic Skyhitch", because it's a bit like hypersonic skyhook but with little rocket ships rather than some long tether.
It is complicated. Dr. Busch has the straight dope.
Yes, water landing does avoid both the "uneven landing site" problem and the "vaporizing the landing site with nuclear exhaust" problem. But you'll need to launch and recover some kind of boat.
Presumably they will want to ferry an equipment load to shore as least the size of the Apollo landings. Which are awkward to carry when you are swimming.
how to avoid hammering a giant hole in the water column with your rocket exhaust (and what happens when all that water comes back to fill that hole) if you do a vertical landing
if your engine exhaust touches the water, you have the risk of torching aquatic life or shattering their eardrums with the loudness of the rocket engines.
@nyrath@Sevoris@tkinias much as I can like the image of sea-launch super heavies, these days it makes me cringe.
I guess you can argue the environmental impact is overall small compared to global shipping noise, but considering the effort we go through with rockets so the engine noise doesn’t shatter windows, uuuh… yeah.
@Sevoris
Doing it in deep water probably means a lot less ecological damage than on land or coastal waters, but then of course you have the issue that your spaceship just landed 200 km from the shore @nyrath
@tarheel@nyrath@tkinias I don't think water would generally do much to the nozzles, but the combustion chamber and plumbing may be a different story.
However, lots of nozzles are cooled by cryogenics like lox, which may make icing a problem.
Anyway, there's enough problems that you probably want to keep your main rockets raised above the water (like how floatplanes keep their engines above water).
Hmm, so, designing a craft with engines up high but a center of gravity not too high (at least, not while floating in water (or liquid methane??)) sounds like a fun challenge.
Suddenly, we get marine design issues in a "space" ship.
@tarheel@isaackuo@nyrath@tkinias if your engines are mounted at the top, you could design the entire bottom of the craft as a flotation structure (if need be, have parts fold out). Bonus points, you can place the heavy cargo bay at the bottom and make it open to sea water.
It lands on inflatable cushions (terrain adaptable) and has a nice big cargo bay at the bottom.
Add a plasma sail and it becomes kinda a hard sci-fi Serenity!
(I have considered that ships like this may be a mainstay interface/short-flight interplanetary vehicle for my independent space communities in my writings)
The engines don't need to point straight down. You could have a forward bridge ship shape (like X-bow), with two rocket engines to the sides of the bridge.
These engines are pointed rearward and a bit downward rather than directly downward. Thus, it lifts the nose up into the air before the vessel fully lifts off.
This elongated hull sails around well, while also being adequately aerodynamic.
I feel like something that looks like a tail sitter ... four floats on tail legs. But it floats flopped over on the side. The nose floats on the water, and two of the leg floats keep the engines above water.
To launch, a solid rocket or kite tethered to the nose lifts the nose up at least 30 degrees, and then you fire up the main engines.
@isaackuo@tarheel@nyrath@tkinias Some mega-economy of scale rocket proposals used water launch and landing, if only because out at sea was the only practical place to launch a rocket with the takeoff thrust of a couple of tactical nukes.
If the planet has an atmosphere, you will need some kind of Supersonic Retro-Propulsion technique. You have to be careful spitting into the wind, but you have to be doubly careful firing a flame thrower into a hurricane.
@michael_w_busch
LOL in re “Dr.”—it’s funny how this works, because other than my students I prefer people to call me by my first name, but if you’re gonna use title+family name, it really is Dr and not Mr @nyrath
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