# Tag Info

110

With a rocket you have to carry the fuel with you. You are not just propelling the mass of the payload, but also the mass of the fuel. Installing a space elevator is a one-time event that can then be used to propel payloads indefinitely. You no longer have to carry the fuel to get to orbit.

82

First stages are generally run to depletion (though not complete depletion - I'll get to that later). First stage ascents often use a preprogrammed, open loop guidance system to get out of the atmosphere with a good chunk of downrange velocity. Because winds aloft and other factors may vary, first stage performance also varies somewhat. Upper stage guidance ...

65

In addition to not requiring fuel: A rocket has to accelerate to orbital speed. This takes a lot of energy. A space elevator can climb at a low, constant vertical speed (albeit for a very long climb), and gets its orbital speed almost for free, from Earth's rotation (see Tom Spilker's answer for far more detail on this). Because a rocket accelerates to ...

63

A big difference is that you wouldn't need to leave someone in lunar orbit. We now have experience and confidence in the remote operation of an uncrewed vehicle. So you could have a crew of two instead of three. Or perhaps a crew of three to the surface with a larger LM. Overall, there would be much more automation, especially for the landing process, ...

61

Here's a simple reason: Most of the rocket's fuel is used just to push the rest of fuel! It sounds strange for those unfamiliar with Rocket equation. The reality is, if we want to accelerate by exhausting something behind us - then we have a problem when the speed we need to reach (8 km/s orbital speed) is greater than exhaust speed (3-5 km/s). In this ...

53

The Delta V requirement to launch is about 14 km/s to low lunar orbit, per Wikipedia. That means that you would have to achieve a speed of 14 km/s in order to orbit the moon. Some of that will need to be done from space, but most of it could theoretically be achieved from the ground. So, what do you need to do to make that happen? In World War II, the ...

53

Monopropellant systems such as catalyzed hydrazine thrusters are attractive at very small sizes, where the simplicity of a single propellant tank outweighs their relatively low performance. According to Wikipedia, Cavea-B requires a small amount of UDMH or a similar hypergolic to begin ignition -- every time you want to fire it, which can be a frequent ...

47

Assuming acceleration is constant, $d=(1/2) a t^2$. So plotted over time, distance traveled is a nice parabola. If you want the time it'd take for a specific distance, it's easy to manipulate $d=(1/2) a t^2$. $t=\sqrt{2d/a}$ If you're using meters and seconds as your units, $a=9.8 meters/sec^2$ To travel half the distance to the moon would take about 1....

43

Not assuming any time taken for orbital maneuvering, turning halfway 180° to decelerate, assuming closest distance of planets (and Luna) to the Earth, and not accounting for fuel burn (i.e. literal constant 1g acceleration): The Moon / Luna: Closest to Earth (Supermoon): 356,577 km Travel time (at 9.80665 m/s2, no deceleration): 2h 22m 12s Travel time (at 9....

43

Short answer: Space stations have been refueled on orbit, as well as some small demonstration missions. Long answer: There is only a limited number of objects that this is even an option. There are 3 types of docking which have generally happened. Those involving manned spacecraft but not a space station, those involving a space station, and those with ...

42

If you watch these videos: ATV boost Zvezda boost ...you can see that the acceleration is quite gentle, but definite. The astronauts do need to hang on to something if they don't want to drift to the back of whatever room they're in. The first video was a reboost performed with the ATV service ship, as described in this article. Depending on what ...

42

The answer to the question, "Do the astronauts feel the station moving?" is yes, definitely, but sometimes in an "indirect" fashion. During Space Shuttle mission STS-109, when floating in my sleeping bag and waiting for slumber to come, I would notice that occasionally my body would softly brush up against one side or the other of said sleeping bag. A ...

41

No, because there's nothing like water for a keel to work against. In water sailing there are two force vectors, the vector from the reaction of the wind against the sail, and the vector from the keel and rudder against the water. These vectors add together to propel the sailboat. This works for almost any direction on the compass except where the wind ...

40

Previously posted comments are correct: in free space (assumed free of any other bodies' gravity fields) there is no way to convert the reaction wheels' angular motion to translational motion. There is one tongue-in-cheek way: throw a reaction wheel off the spacecraft in the direction opposite the direction of the desired delta-V! ;-) If you abandon the ...

38

It boils down to efficiencies of energy conversion and the cost of the technologies doing the conversions. If you have a given mass at Earth's surface that you want in geostationary orbit, you have to raise it to the geostationary radius (or altitude, if you prefer to think in those terms), and you have to accelerate it to geostationary orbit velocity. Both ...

36

You cannot directly propel the solar sail towards the sun. A solar "sail" is basically a mirror. The analogy of wind and sails on ships is not useful for understanding how solar sails work. Each photon from the sun which strikes the sail is reflected. Each photon imparts a small amount of momentum. If the sail is pointed directly at the sun then you get ...

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 ...

35

To answer your title question: By using its engines. However you seems to be quite puzzled by the fact that velocity of an object can decrease and increase over the course of an orbit. If the orbit is perfectly circular, the speed will always remain the same (until thrusters are used). However, as is the case with Chandrayaan-2, most orbits are ...

31

This was one of the questions just now during the Rosetta press briefing. This video was shown during the presentation: The triangular trajectory are hyperbolic orbits with respect to the comet and they'll (also, among other tasks also mentioned in the image you're attaching) serve to establish its mass. In essence ...

28

This is actually somewhat easier than you would think. In the world of Orbital Dynamics, you only have to accelerate or decelerate your orbit to move closer/further away from the object you are orbiting. So, all you have to do is create a net momentum that pushes to slow down your orbital velocity. However, a big part of what makes tacking work is the fact ...

28

Space is basically a vacuum, so there's no air resistance. A probe that's been launched will travel at the same speed indefinitely. Because New Horizons is moving away from the Sun, it loses some speed to overcome the Sun's gravity. New Horizons was launched on the fastest rocket they could get. Then it used a gravity assist from Jupiter to gain some more ...

27

To make a long story short, liquid hydrogen has a very low density of just 70 kg/m3. RP1, on the other hand, has a density very close to that of water - about 1000 kg/m3. This means that for the same mass of hydrogen fuel, you'd need a tank 14 times as large. Couple that with the need to keep LH2 cryogenic or lose it to boiloff, and it becomes a very complex ...

26

You can't do it. It's impossible. Each thruster provides thrust, but each thruster has mass, as do the power sources needed to power them and the tanks to store their fuel. No currently existing ion thruster is able to produce anywhere near that much thrust for its mass, and more significantly, even the best power sources (even speculative ones or those at ...

25

As far as I know, the shock wave in detonating explosives does not go faster than about 2.5 km/s, so a bullet will not be propelled beyond that speed, however many barrels of gunpowder you accumulate. The shock wave can be sped up if the operation occurs in a high pressure environment, though, but reaching enough speed to get to orbit (about 8 km/s) seems ...

23

I found a real world test of this. Dan Barry tried it when STS-96 was docked to the ISS. I've scanned his account from the book "Space Shuttle: the first 20 years." tl;dr - he escaped by throwing his clothes. Transcription: DAN BARRY | Stranded in the middle of the room STS-96 Entering the space station from the orbiter for the first time in ...

23

Ultimately an elevator is going to be more efficient, because it doesn't have to deal with gravity losses. Let me pose a question to you. What does it take for a rocket to hover in place like Blue Origin's New Shepard? If you've watched any of their launches you know they don't shut off the engine completely, but keep them running the whole time while ...

22

While the ISP on Ion thrusters is awesome, the overall thrust is pretty low. Thus the transit time from LEO to GEO would be quite long and slow. In some cases this matters. If it takes an extra year to get in service, that is a year of lost service while in transit. In fact a critique of the Falcon 9's ability to do dual launches is that only the smaller ...

22

Assuming you mean "quite small" in terms of mass as well as thrust output. Fundamentally, current ion drives are limited by the amount of power available to them - it takes many, many kilowatts of input power to provide tiny amounts of thrust. As you know from the answer you linked, the Dawn spacecraft is powered by 3 NSTAR ion engines which ...

22

Propulsion Until someone solves the N-body problem every spacecraft needs some kind of propulsion to correct its course during the mission. New Horizons uses a Hydrazine based propulsion system including four 4.4 N main thrusters and twelve 0.9 N attitude control thrusters. Its 77 kg fuel tank allows a total post launch delta-v of somewhere over 290 m/s (...

22

One reason for separating the RCS and main tankage is the ullage problem; to maintain good flow into the engine inlets, you need to separate the remaining propellant from the pressurant gas in the tank and ensure that the propellant is at the correct end of the tank. As described in an answer to this related question, this is commonly done with either a ...

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