112
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 ...
- 23.8k
84
votes
Accepted
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 ...
- 17.2k
67
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)...
- 126k
67
votes
Accepted
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 ...
- 10.6k
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 ...
- 9,041
53
votes
Accepted
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 ...
- 167k
44
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 ...
- 4,175
40
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 ...
- 18.2k
40
votes
Accepted
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 ...
- 18.2k
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 ...
- 486
36
votes
Accepted
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 ...
- 12.6k
28
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 ...
- 1,108
28
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 ...
- 178k
27
votes
Accepted
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 ...
- 8,697
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 ...
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 ...
- 54.9k
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 ...
- 178k
25
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 ...
- 1,196
25
votes
Accepted
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 ...
22
votes
Accepted
Why does the Falcon 9 use RP-1/LOx and not LH2/LOx?
SpaceX looked at the design of a booster with fresh eyes. Their concern was cost and reusability.
Different fuels, while more efficient are much more expensive to manage (two fuel systems), and ...
- 79.3k
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 ...
- 167k
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 ...
- 3,585
21
votes
Why don't we use Cavea-B
John D Clark, Ignition!, p.165, on the testing of a Cavea B motor:
“Well, through a combination of this and that, the motor blew on startup. We never discovered whether or not the [detonation] traps ...
- 1,364
21
votes
Accepted
When has hydrogen peroxide been used in rocketry?
Hydrogen peroxide has a (perhaps not fully deserved) reputation of being hazardous to work with; it decomposes spontaneously, releasing heat, and the decomposition rate increases with temperature, so ...
- 167k
20
votes
Accepted
Why can't we use charged fine metal dust instead of plasma for ion thruster?
In an ion thruster, particles are accelerated because of their electrical charge. The force acting on them is proportional to the charge (and the external field applied, which we can treat as fixed ...
- 14.9k
20
votes
What is the maximum speed an ion engine can propel a spacecraft at?
The limit isn't due to power, but to engine lifetime and fuel limits.
Ion engines produce very little thrust, so in order to reach speeds of 100km/s they must accelerate continuously for months or ...
- 167k
20
votes
Accepted
From which NASA publication originates/where can I find a higher resolution version of this SSME flow diagram?
It's on pdf pages 139-141 here: https://gandalfddi.z19.web.core.windows.net/Shuttle/SSME_MPS_Info/KSC-SSME_System_Eng_Handbook.pdf but you'll have to "glue" it together into one drawing. ...
- 178k
20
votes
Accepted
What is wrong with hydrazine as a monopropellant?
There are a few reasons why a hydrazine replacement is being looked at:
Hydrazine is highly toxic, and carcinogenic so it's dangerous to handle. A leak can put astronauts or technicians at serious ...
- 19.9k
19
votes
Accepted
Splitting water quickly to turn a water rocket into a H/O rocket at launch: could it work?
You'll need a nuclear reactor for that because the process of dissociation of water into constituent hydrogen and oxygen consumes vast amounts of energy and for an ascent stage you'll also need a very ...
- 76.5k
19
votes
Accepted
What is the ortho/para issue with LH2 as a fuel?
The problem is that the transition produces enough energy to boil the LH2. As explained on the old sci.space.history group:
Skipping the gory quantum-mechanical details... there are two energy
...
- 178k
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