Things that go crump in the night: Watch Musk's mighty missile go foom Yet another of SpaceX's Starship prototypes, SN3, was left in pieces last night following tank testing. After the initial optimism of the Starhopper flights in 2019, SpaceX has struggled with its tanks. The Mk1 was destroyed in a pressure test, SN1 also failed, and now SN3 – the stage planned to demonstrate Raptor-powered low- …
"Starship could be able to replicate the Space Shuttle trick of capturing satellites in orbit and returning them to Earth."
One imagines that the nose fairings will remain in place until the most opportune moment, when they will open like a giant sharks mouth and swallow the enemy friendly satellite for return to the service depot. No extinct volcanic lairs needed.
"Slightly trickier in reality than on paper"
Indeed - many sats have outrigger type solar panels that will need to be discarded/detached some how or retracted.
Likewise if the sat is coming back for servicing then each one will require a slightly different "docking" connector onto which the Musk rocket will need to align itself and then lock onto. (otherwise the sat will be rattling around inside the return vehicle and cause damage).
Then the issue will be the re-entry as the sat will need to be protected from the high temperatures generated when it comes back - opening/closing "jaws" (a la James Bond) or some form of shroud around the Musk rocket will help but it'll just make things trickier.
Given the cost of putting the sats up there in the first place, surely a better option is to place them into a "parking orbit" so that at a later date, an orbiting "recycling station" can be put into space which can re-use the metal - or at least reduce the "volume" of the sat and return precious/expensive materials back to earth.
Any surplus materials can be shredded. put into a captured old 2nd or 3rd stage rocket (and there's plenty in orbit to choose from) along with any hazardous materials - Then attach a small Payload Assist Module send it to the Sun (not the former Murdock old chip-wrapping red top).
> Then attach a small Payload Assist Module send it to the Sun
Although it seems to be "downhill" to the sun it is as hard to slow an object down from Earth's orbital velocity so it can "fall" into the sun as it would be to launch it from the sun to Earth orbital velocity.
That's the problem with returning stuff from orbit to Earth. You have to make stuff go really fast horizontally to make it to orbit and so you have to spend the same energy/fuel to get it to stop orbitting.
The Shuttle and Apollo capsules used friction with the atmosphere to dump this speed, but a returning SpaceX rocket (or that junkyard rocket from the 70s TV show) needs to carry 2x as much fuel to use engines slow down.
This means that the 2nd stage weighs 2x as much, and so the first stage needs to be bigger to lift it, then the first stage needs to be even bigger to lift that extra fuel, which takes more fuel - that way madness (and the Saturn V) lies
It takes nowhere near the same amount of fuel to slow a down a returning rocket, it's mass is far less and it still uses friction of the increasingly dense atmosphere on the way down.
It should be bleeding obvious watching a SpaceX launch - 9 merlins at full tilt on the way up, and on the way down a quick 3 engine burn to slow it, deceleration due to friction, and an even quicker single engine landing burn.
The first stage doesn't go to orbit, it accelerates straight up in the atmosphere and then falls back down, slowing in increasingly dense air until the engines stop it near the ground. Getting it back down is no harder than the bloke that "jumped from space".
The upper stage and payload are doing 10km/s horizontally, to get them to fall down you have to apply -10km/s. Yes you can use some aerodynamic braking if you want to carry the mass of a heatshield, and you can arrange to enclose the captured payload inside it safe from multi-1000 deg plasma.
It would still end up at the height and speed that the 1st stage released it - but that is the easy bit
"Yes you can use some aerodynamic braking if you want to carry the mass of a heatshield"
A lot of the issues with heatshields have to do with the shield being the same size as the spacecraft (meaning the energy of the mass*velocity scrub off has to be dissipated over a small surface area)
If you make it considerably larger then the issues are reduced considerably. NASA has been working on this for a long time (so have other agencies) - the easiest way being an _inflatable) heatshield - which is nowhere near as crazy as it sound and has been tested a few times already
https://www.youtube.com/watch?v=ojgDZZIsWA4
not quite twice the fuel...
a) tanks half full weigh half as much, need half as much thrust from that
b) tanks empty as you burn off fuel, even less required at the very end
c) going up you're accelerating upwards, which has a higher fuel requirement, since it's 1G thrust plus acceleration needs. Coming back, it's less than 1G thrust.
even if the payload coming back is the same (or slightly higher for that matter), it's a fraction of the weight of the fuel. And so the limiting factor here is fuel weight for the return trip, which will be considerably LESS than half of your fuel.
In an ideal scenario I would venture to guess that the going up fuel would be 3 or 4 times the coming back fuel. Then you add a safety margin. So maybe 1/4 to 1/3 of the fuel is for coming back SAFELY, a bit less than 1/2. But still significant.
"You have to make stuff go really fast horizontally to make it to orbit and so you have to spend the same energy/fuel to get it to stop orbitting."
Most of the effort is about decrowding the orbits. The actual incremental cost of a spacecraft is very low compared with the mission costs (all those models and manhours) and if it's end of life all you really need to do is make the orbit eccentric enough that it will graze the atmosphere. Friction will do the rest for you very quickly.
If you _really_ need millions of tons to LEO, there's always Project Orion
Yes returning a satelite is almost never economic.
It was done twice IIRC for the shuttle, although that had the extra restrictions that the satelite had to have been originally designed for the shuttle (eg. no hypergolic fuel) and be in a low enough orbit (ie booster failed). The bigger issue was that a shuttle flight cost $1Bn so apart from something like Hubble it would never be cost effecive
The original USAF plan for the shuttle was to 'retrieve' enemy craft. But it was obvious that as soon as the shuttle was announced the opposition would fit all their interesting satellites with 100g of C4, a bag of nails and a pressure switch.
you might be able to duplicate what happened with a simple experiment
a) pressurize a plastic 2L bottle somehow [specia cap?] using something that's close to its condensation point, like a refrigerant gas.
b) release the pressure rapidly [it will cool the bottle slightly]
c) quickly seal it after venting (a check valve might help)
d) watch the fun
note that gas velocity through the valve will draw a slight vacuum if the valve closes rapidly, due to momentum, depending on the length of the pipe involved. And it cools the place where the gas once was, even more effectively if it can try and condense into a liquid afterwards (like refrigerant).
similarly, (don't try this at home - gutless disclaimer) lighting off alcohol vapor within a 2L bottle through a 9mm hole in the cap might give you a similar effect (it also makes for a fun rocket - distance and safety precautions apply). The velocity of the escaping exhaust through the cap literally draws a vacuum behind it, and the bottle (which quickly cools down the remaining gasses and they have lots of water vapor in them) implodes a bit.
Also...
Rocket fuel tanks are designed to be pressurized because that gives the entire rocket more structural strength, gives positive head pressure [stop it you with dirty minds] for the fuel pumps, and also PREVENTS the 'implode when the pressure drops too low' problem, particularly on launch where compressive stress would otherwise 'accordion' the rocket.
Option a) Cheating: Starship is 70m tall, Superheavy is 50m. Not sure how big the flame diverter under construction at Kerbal Kennedy Space Center is but as they are starting a few meters above sea level the top of Starship will probably reach 150m while being stacked on top.
Option b) Raptor engines can throttle down to less than the weight of a mostly empty Starship. Either only put a little fuel in before take off or hover around until the ship is light enough not to crush the landing legs.
Actually, it's highly unlikely that Raptor engines can throttle down that far. The current Merlin engines certainly can't. Plus it takes extra fuel to ease down to a 'kiss the ground' landing.
Instead SpaceX rockets do something called a hover slam manoeuver to land. Very roughly speaking, a rocket descending at 20 m/s from a height of 20 m, applies 2 g's of deceleration for 1 second and cuts the engines at the moment of touchdown.
I am guessing that as the cryogenic fuel is pumped in, what atmosphere there is inside must contract quite rapidly, so it must be a tricky balance of influx versus pressure drop.
I would be inclined to deduct one of the hundred tons of payload and use it to beef up the tanks.
But then I'm more of a builder than a rocket scientist.
What they had here was tank full of liquid sitting on top of a tank they were trying to keep stiffend with pressurised gas. Under normal circumstances both tanks would either be empty or filled with cryogenic fuels. However, we are yet to see a fully built rocket, which will have to support a fully loaded payload section on top of an empty tank section until fueling takes place on the launch pad. This means figuring out how to keep those tanks stiffened until then.
Taking 100 Ton payload to the moon, as if we have not already put enough crap up in space. It best use would be collect some of the crap that is already in orbit but that does not make headlines. Whilst exploration in space is commendable, we have (and continue to) treat space as a dumping ground because it is too expense or too difficult to get the stuff back with it is finished with. The efforts to ensure biological contamination is minimised are commendable but we still just leave the space craft to orbit or crash.
Just like anything else that is too difficult, nuclear waste fast-food packaging or single-use coffee cups. Just bury it and make it someone else's problem.
The aerospace community is now suffering from exactly what was predicted by older engineers. The companies are now becoming populated with mediocre calibre engineers who are too young & have zero experience, but huge self belief. The old guys have left & the middle management are just as complacent as the 12 year old engineers they are supposed to supervise.
As a friend said humanity has a 30 collective memory, Apollo 13 was in flight 50 years ago, you do the math.
Space-X crushing their own rocket is beyond belief. Boeing, who after all are well known for their recent programming & risk analysis skills (anyone for a 737max8 ?) being allowed to launch a defective vehicle, just shows how far the rot has set in. When your 12 year old programmer fucks up an app, it’s embarrassing. As Gus Grissom said, when asked how it felt just before launch of his Mercury mission, “How would you feel knowing your sitting on the efforts of a 100 low bidders”. As regards Apollo 13 O2 tank issue showed, assumptions & lack of QA lead to fuck ups.
The Boeing 777, would not fly. The new boys used computers & said all good to go. The old boys who were still around when it first flew insisted on some empirical testing. The engine stalled it’s compressor on first take off due to a bad “computer modelled” inlet lip. Luckily it was attached to the 747 test bed airframe that was able to fly on its other three engines. If the old boys had not prevailed, then the first 777 test flight would have crashed & probably killed the program.
You have been warned, supervise your 12 year olds, QA their work & ensure a robust & comprehensive risk analysis is performed on all designs, whether physical or virtual. Lives may depend on it.
The Register 
Biting the hand that feeds IT
Copyright. All rights reserved © 1998–2024
