# Tag Info

## Hot answers tagged design-alternative

67

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 pole, winch or anything else into the lower parts of the atmosphere, it would also move at about 27,500 km/h (if we ignore atmospheric drag and all other ...

55

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 opposite reaction, so pushing that mass forward pushes the rest of the rocket backward. Second, the exhaust isn't fuel; it's already combusted, and the chemical energy ...

54

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 to their station or capsule. When you are inside an object which is in orbit, you are in orbit too! The Earth pulls on the station with nearly 1 g and it pulls ...

51

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 would be increased by a shorter flight. The limitation to making a faster spacecraft is fuel. In order to go a little bit faster, you need to fire your rocket ...

49

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 is one of the techniques used for corrosion testing. Build a launch pad over the ocean, and you'll need to clean it off after each launch to try to keep the ...

46

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 and operating like a rocket when it is not practical to use ambient air. The limitations of an air-breathing engine for space launch are that You can't go very ...

43

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 Corporation (now owned by Northrop Grumman) have been air launching the Pegasus satellite launcher since 1990 (almost 30 years). Virgin Galactic's 'White Knight' ...

36

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 chemical rocket might have a specific impulse between about 250 and 450 seconds; therefore, the exhaust velocity is about 2500-4500 m/s. In a competitive ...

36

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 launch, and spaceflight is full of cases where you have limited launch opportunities. A larger vehicle will have higher throughput and lower costs per kg. Smaller ...

36

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 provide an electromotive force to the wire (and craft) that can be used to raise or lower orbit. It isn't exactly the same as your idea, but it is a practical ...

33

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/or leaking immediately, it would clearly help, but in the same moment, the "suit" would turn so stiff from pressure that it would be impossible to ...

30

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 thermal conductivity, higher than pyrolytic graphite. In-plane thermal conductivity could help to avoid hot spots, but through-planes conductivity should be low as ...

26

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 significant mass (would justify the additional bearings and generator) would be fuel tanks. Spinning a fuel tank with liquid fuel in it would be tricky though, ...

26

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 Earth is on an axis of ~23 degrees and rotates every day, it is not possible to create an orbit which has no ground track precession except for equatorial ...

26

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 larger the engines, the higher the up-front cost would be, as problems of combustion stability, cooling, material durability and so on crop up; this is a well-...

25

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 system can do the job in a practical way. Chemical rockets lack the fuel efficiency; electric rockets don't have the thrust required to leave Earth's surface. ...

25

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 acceleration tunnel, evacuated. An exit point 6000 m up, on a mountain. A plasma window to allow projectile egress into atmosphere without repressurising the entire ...

23

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 sugars and cellulose. Cellulose combusted with gaseous oxygen yields surprisingly good specific impulse, up in the 240 sec range; sugars with potassium nitrate ...

22

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 just make a denser atmospheric mixture and keep the pressure the same. This answer says If you want the air to be 5 times easier to swim, you can just replace ...

21

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 lifts the rocket, engines, payload, and all its own fuel. Single-Stage-to-Orbit can be done, but it's horribly inefficient. This is because of the Tyranny Of ...

21

It absolutely could! First of all, water can be split in to hydrogen and oxygen, which can be enough to launch a rocket. Hydrogen requires a very low temperature, and the rocket engine doesn't have as much thrust as other options out there, but it is the same fuel that ran the Space Shuttle main engine, among others. Water and carbon dioxide, easily ...

21

The underlying aerogel scheme seems to have some serious fundamental flaws: Aerogel is extremely expensive, and you need enough to cover a significant portion of a planet. Aerogel is extremely brittle and easy to pulverize. The poles are dark much of the year. Photosynthetic organisms won't function during the winter, and it'll get cold enough to form heavy ...

20

Unlikely to be plausible. During belly flop descent the forces placed on the horizontal Starship are distributed over the windward side very evenly. The cylinder walls represent the majority of the frontal area and the flaps are fluttered back taking less than their full potential drag - and their mounts are still naturally broad and spread the additional ...

19

I don't know if it has ever been considered by anyone. In my view, this is not a good idea for at least the following reasons: It is equivalent to mechanically throwing things retrograde. See this video for an overly simple example. This is obviously not a good way for propulsion, as the specific impulse is very low. Let's talk just about the impulse p=...

19

There are two major barriers: one is that thrust-to-weight ratio of jet engines is pretty poor (2 J58s massing more than 15 times what 9 Rutherfords do), the other is that it's hard to make an engine that performs efficiently over the wide range of speeds and altitudes that a first stage wants to cover. That said, Boeing at one point toyed with a concept ...

19

Many novel launch schemes need some amount of help from rockets. What kills a lot of them is doing a tradeoff study of just enlarging the rocket part and getting rid of the non-rocket part. Surprisingly often, that works out to be better and cheaper. --Henry Spencer This is a system that needs a rocket part, as one of these two cases would necessary ...

19

Why this wouldn't work? It works for the Earth; the reason why it is not implemented in space is purely in the engineering limitations. Cyanobacteria live in water, humans live in air. Gravity is good at separating water from air, leaving a surface for the gas exchange. Microgravity is very good at mixing everything, so we should think about another ...

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