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69 votes
Accepted

Why doesn’t NASA build its rockets using graphene?

The Technology Readiness Level (TRL) of graphene is at 2 or 3 as far as I can tell. And that is TRL as related to making very tiny stuff. Anything used to build a structure for aircraft or spacecraft ...
BradV's user avatar
  • 3,292
67 votes

Why can't they just drop a solar winch down from a shuttle and have planes fly up and clip things on?

Because space isn't about going high; it's about going fast! For example, in a 400 km orbit (like ISS) you need a speed of about 27,500 km/h or 7.66 km per second. So if you would extend a ...
DarkDust's user avatar
  • 12.5k
66 votes

Would it be practical to catch a rocket's exhaust to reuse it as fuel?

Like any perpetual motion machine, it won't work. In this case, there are two major reasons. First, your "send back fuel" arrow is pushing mass forward; every action has an equal and ...
Russell Borogove's user avatar
54 votes
Accepted

Could a space colony 1g from the sun work?

Interesting but no, it wouldn't work for the same reason that astronauts in the International Space Station, other space stations, or orbiting shuttles or capsules do not "feel" gravity with respect ...
uhoh's user avatar
  • 149k
52 votes

Are there any greater risks of traveling significantly faster to another planet?

The biggest risk on a flight to Mars is cumulative exposure to radiation, so a 1-2 month flight would actually be much healthier for a crew than a 7-8 month flight. I don't know of any risks that ...
Russell Borogove's user avatar
51 votes
Accepted

Why isn't SpaceX constructing the Super Heavy launch mount on top of the water?

Salt does all sorts of unpleasant things to just about every building material humans use. Hot salt spray, such as you'd get from a rocket launch, is even worse: spraying something with hot saltwater ...
Mark's user avatar
  • 15.3k
47 votes
Accepted

Why aren't air breathing engines used as small first stages?

Take a look at the SABRE engine. The goal is to achieve single stage runway liftoff/land to/from orbit with a hybrid engine capable of breathing air at low altitude but switching to stored oxidizer ...
Anthony X's user avatar
  • 17.5k
44 votes

Why aren't air breathing engines used as small first stages?

Systems that do this exist and more are being introduced. It's just that they hide their appearance and look somewhat different to what would be expected from what you describe. Orbital Sciences ...
Russell McMahon's user avatar
43 votes
Accepted

Why not use containers to clean up small space debris?

The problem is that every bit of space debris is in its own orbit. If you want to match that orbit, it takes a lot of fuel. If you don't do that, and you just intercept it, you will find that the ...
Hobbes's user avatar
  • 128k
42 votes

Why doesn’t NASA build its rockets using graphene?

Besides the fact BradV pointed out that we don't have the technology to do this yet, the fact is that graphene on a macroscopic scale would not perform as well as the numbers you cited suggest. All ...
Mark Foskey's user avatar
40 votes
Accepted

Wouldn't it make sense to use parachutes for aborting test flights rather than destroying the whole rocket?

The ability to successfully deploy a parachute still requires some level of stability or control, it's simply not an option for a badly out-of-control rocket. In cases where something goes wrong, the ...
Nuclear Hoagie's user avatar
38 votes
Accepted

Exactly why does Starship need to be this big for interplanetary travel?

A lot of launch costs are independent of rocket size. It's no cheaper to clear the flight path for a smaller rocket, for example. It also takes a lot longer to do 10 launches instead of one large ...
Christopher James Huff's user avatar
37 votes
Accepted

Would a grinding machine be a simple and workable propulsion system for an interplanetary spacecraft?

The main engineering challenge in implementing your proposal is that in order to be competitive with a chemical rocket engine, the grinding wheel must rotate at an extremely high velocity. A typical ...
Thorondor's user avatar
  • 486
37 votes
Accepted

Could an electromagnet be used to raise the apogee of a satellite orbiting Earth?

You might like to take a look at Electromagnetic Tethers. A spacecraft moving through a magnetic field (such as the Earth's) can deploy a long, trailing wire and run a current through it. This will ...
DrMcCleod's user avatar
  • 700
37 votes
Accepted

Orbital Supercomputer for Martian and Outer Planet Computing

They act as a massive supercomputer I think you massively underestimate how massive a massive supercomputer is, and most importantly, how massive both the power requirements and the cooling ...
Jörg W Mittag's user avatar
33 votes

Would a duct tape spacesuit be practical?

If the suit would be useful, it has to be inflated. Which is definitively not how it looks like in images. If you could manage duct tape to hold the inner pressure for a moment without rupturing and/...
Martin's user avatar
  • 1,831
31 votes

What are the biggest challenges for high altitude rail-gun launch systems?

What you're describing is (more or less) the StarTram "gen 1" design. The reference design has: 40 tonne unmanned cargo projectile, 25 tonnes of payload, ~2 m wide, ~13 m long. A 130 km maglev ...
Starfish Prime's user avatar
30 votes

Could sheets of stacked graphene be used as part of a heat shield, since its melting point is 3000k to 5000 K

For a non ablative heat shield you need a material with a very high melting point and a very low thermal conductivity. It should not burn in hot air. Unfortunately graphene seems to have a high ...
Uwe's user avatar
  • 49k
28 votes
Accepted

Rocket flywheel instead of battery/generator (crazy idea)?

A flywheel is so efficient because it is big and heavy, both of which you don't want to add to a spacecraft. As for spinning an existing component, the only parts of a rocket that really have any ...
Dragongeek's user avatar
  • 19.1k
27 votes

If someone built a vacuum tunnel through the atmosphere, could you have an orbit with a sea level perigee?

No, unless your structure is located directly on the equator and your satellite follows a perfectly circular orbit, atmospheric "orbits" aren't possible, even in a vacuum tunnel. Because the ...
Dragongeek's user avatar
  • 19.1k
27 votes

Why is it not concerning from a cost perspective to have so many engines on Starship?

Because with current technology, the greatest part of expense in building a rocket motor is not the individual construction, but the research needed in the design of it. And it is simpler, easier and ...
CuteKItty_pleaseStopBArking's user avatar
27 votes

Why do cryogenic fuels want an extra pressure tank?

Rocket fuels are pumped into the rocket engines. Pumping a liquid near or at its vapor pressure is a very good recipe for cavitation. Pumps don't like cavitation. They lose a great deal of efficiency ...
fraxinus's user avatar
  • 2,519
26 votes
Accepted

Why is it not concerning from a cost perspective to have so many engines on Starship?

Economy of scale, strictly. SpaceX focuses heavily on streamlining and automation of production of these engines. High up-front cost, but low unit cost per engine once the process is perfected. The ...
SF.'s user avatar
  • 55k
26 votes

Why not use containers to clean up small space debris?

Sweeping up space debris in orbit is a bit like sweeping up bullets in mid-air in the middle of a machine-gun fight, except the debris is probably moving faster (relative to the sweeper) than the ...
Steve Linton's user avatar
  • 19.6k
25 votes
Accepted

What would be necessary in order for us to achieve a single stage to orbit, reusable rocket?

I am referring to rockets capable of taking supplies and humans to other planets. For an interplanetary single-stage rocket with tens to hundreds of tons of payload capability, no existing propulsion ...
Russell Borogove's user avatar
23 votes
Accepted

Could one actually make a grain silo rocket?

Since the term "grain" is already in use in the solid rocket context, I'm favoring the term "cereal". Cereals contain about 66–76% carbohydrates -- mostly starch (55–70%) plus some ...
Russell Borogove's user avatar
22 votes

What would be necessary in order for us to achieve a single stage to orbit, reusable rocket?

Rockets are basically devices which exploit Newton's Third Law, for every force there is an equal and opposite force. By throwing mass out the back as fast as possible this imparts an equal force that ...
Schwern's user avatar
  • 8,016
22 votes

Would a higher air pressure on the ISS or elsewhere make it easier to "swim" in microgravity?

Would a higher air pressure on the ISS or elsewhere make it easier to “swim” in microgravity? Yes! But what's really important is the density, so instead of pressuring "normal air" you can ...
uhoh's user avatar
  • 149k

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