111 votes

Why is using a space elevator cheaper than rocket power?

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 ...
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  • 23.6k
83 votes
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Do booster stages run out of fuel, or are they purposefully shut off?

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 ...
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  • 16.9k
66 votes

Why is using a space elevator cheaper than rocket power?

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)...
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  • 122k
66 votes
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Why can't you travel to Mars in a straight line?

Only answering your first question here, and in qualitative terms: You can't travel to Mars in a straight line for the same reasons you can't throw a ball in a straight line: gravity. If you wanted to ...
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  • 10.1k
65 votes

If the Apollo mandate were delivered today, would the mission vehicle(s) and profile be similar?

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 ...
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  • 57.7k
63 votes

Why is using a space elevator cheaper than rocket power?

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 ...
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  • 8,456
53 votes
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Why don't we use Cavea-B

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 ...
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50 votes
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How fast will 1g get you there?

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/...
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  • 15.2k
44 votes
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Has in-space refueling been done?

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 ...
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  • 119k
42 votes
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When the ISS accelerates, do the astronauts feel it?

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 ...
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42 votes

When the ISS accelerates, do the astronauts feel it?

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 ...
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  • 3,130
40 votes
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Could a complex system of reaction wheels be used to propel a spacecraft?

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 ...
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39 votes

Why is using a space elevator cheaper than rocket power?

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 ...
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36 votes
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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 ...
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  • 476
36 votes
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How does a Spacecraft change its orbit?

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 ...
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  • 12.2k
31 votes
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Is this really Rosetta's orbit around 67P?

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 ...
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  • 75.6k
28 votes
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What powers New Horizons?

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 ...
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  • 122k
27 votes

Why does the Falcon 9 use RP-1/LOx and not LH2/LOx?

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 ...
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  • 1,098
27 votes
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How many ion thrusters would be needed to accelerate a 1000 tonne craft at 9.8m/s²?

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 ...
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  • 8,411
27 votes

Could the Space Shuttle launch with two engines?

If one of the Space Shuttle Main Engines (SSMEs) shut down before liftoff, the countdown would stop and the vehicle would not liftoff. This happened a few times. If the SSME shut down immediately ...
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26 votes
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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 ...
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  • 52.7k
26 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 ...
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25 votes

Tiny emergency propulsive device if stuck floating in a large volume in microgravity

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 ...
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24 votes

Why is using a space elevator cheaper than rocket power?

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 ...
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23 votes
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How are vacuum optimized engines tested without disintegrating them?

Flow separation can occur in a rocket nozzle that is overexpanded. This can cause quite severe turbulence and thus buffeting of the rocket nozzle. The SSME used a special rocket nozzle shape to ...
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22 votes
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Why aren't there any space tugs in use?

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 ...
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  • 76.7k
22 votes
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Can ion thrusters be scaled up?

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 ...
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  • 8,715
22 votes

What powers New Horizons?

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 ...
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  • 7,089
22 votes

Why can't we use the same tank to hold fuel for both the RCS Thrusters and the Main engine for a deep-space mission?

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 ...
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22 votes

Why can't you travel to Mars in a straight line?

Disclaimer: this answer makes some coarse simplifications in order to keep it focussed on the crucial points. If you feel like pointing out something about orbit eccentricities or whatever in the ...
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