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35 votes
Accepted

What was the result of the propellant predictions in the last chapter of "Ignition!"?

Chemical rockets will never have more than 600 seconds specific impulse. Storing free radicals in propellant to defeat this limit is impractical. Validated. Chemical rockets in use top out at 450-460 ...
Russell Borogove's user avatar
33 votes
Accepted

Why are nitric acid and hydrogen combinations not used as rocket fuel?

"Kilojoules per mole" is not the most important measure of a rocket fuel's effectiveness. The most important is effective exhaust velocity, often expressed as "specific impulse". ...
Mark's user avatar
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31 votes
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Why is fuel ratio different for upper stage of a rocket?

The J-2 engine used on the second and third stages of the Saturn V has a "PU valve" (propellant utilization) on the oxidizer turbopump. Adjusting the mixture ratio with this valve primarily ...
Russell Borogove's user avatar
29 votes
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How does Titan have hydrogen in its atmosphere?

How does Titan have Hydrogen in it's atmopshere? Hydrogen is a trace gas in Titan's atmosphere (0.099% per the wikipedia article on Titan's atmosphere). Titan's scant amount of hydrogen in its ...
David Hammen's user avatar
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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 ...
UIDAlexD's user avatar
  • 1,108
22 votes
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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 ...
geoffc's user avatar
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20 votes
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Platinum Colloid in Liquid Hydrogen fuel to increase fuel efficiency

Short answer: no, it won't increase the performance of the rocket. Platinum (or other catalysts) can be used in fuel cells, where the purpose is to get a chemical reaction at (relatively) lower ...
Andy's user avatar
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19 votes
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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 ...
Organic Marble's user avatar
19 votes
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Why is TEA-TEB chemical ignition used instead of spark ignition?

The way you ignite a rocket engine depends a lot on the fuel/oxidiser combination. Some combinations, like Liquid Hydrogen/Liquid Oxygen (HydroLox) ignite very easily and are ignited using a glorified ...
amyinorbit's user avatar
19 votes

How does Titan have hydrogen in its atmosphere?

The chemistry of Titan's atmosphere is actually quite complex. Different reactions occur at different altitudes and there are a wide range of minor constituents in the atmosphere. One of those ...
Slarty's user avatar
  • 9,349
18 votes
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Who built the sparklers used by the Shuttle program during launch?

In the late 70's and early 80's I worked for a company named Unidynamics Defense Systems. We provided this subassembly. We were not the design originators, this design I am quite sure came from NASA. ...
D.S. Harvey's user avatar
15 votes
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Is atmospheric skimming for propellant feasible?

It is easy enough to analyze this in terms of conservation of momentum, we'll assume the velocity through the atmosphere to be 7.8km/s, that is LEO orbital velocity - in actuality it'll be a little ...
Blake Walsh's user avatar
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12 votes

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

I think geoffc is correct. It's primarily about cost and if that is a major concern then Hydrogen is almost immediately off the table. It is good for high efficiency upper stages but as both other ...
Evan Steinbrenner's user avatar
12 votes

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

SpaceX optimizes for cost over performance, and everything SpaceX does with respect to the Falcon architecture works towards that goal. SpaceX will gladly trade a few m/s of ΔV if a) it saves ...
John Bode's user avatar
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12 votes

Why did the Americans abandon kerosene after F-1 while the Soviets continued to develop it to very high sophistication?

Each technology has their own strengths and difficulties. It's really difficult to explain this without going in to a great detail about how rocket engines work, but let me try and give a brief ...
PearsonArtPhoto's user avatar
  • 121k
12 votes

Why are nitric acid and hydrogen combinations not used as rocket fuel?

The "why not this combination" answer: in Ignition! it's mentioned that the main drive for using RFNA as a propellant is that it's an oxidizer that's liquid at outdoor temperatures (say, -40 ...
pjc50's user avatar
  • 605
10 votes

Is atmospheric skimming for propellant feasible?

It can be made to work, but it would be quite different from the typical SF depiction. As Blake points out, you can't gain anything from mass that is already moving faster than rocket exhaust. However,...
Quentin Clarkson's user avatar
10 votes
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Lunar nuclear power system - how can hydrogen generated on the Moon be liquefied?

How would you cool and liquefy this hydrogen on the Moon? I guess one would have to do a serious study to find out for sure which way is more suitable in a given scenario, but I think passive cooling ...
uhoh's user avatar
  • 149k
9 votes
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Has solidifying liquid propellant been considered for space exploration?

The melting point of hydrogen isn't much lower than its boiling point (6K), so the temperature isn't necessarily that much of an obstacle. However, using solid fuel requires either melting it, or ...
Nathan Tuggy's user avatar
  • 4,566
9 votes
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Why does the BE-3 use hydrogen as fuel?

Blue Origin (BO) chosed LH2 for BE-3 rocket engine because of the performance. "Performance drove the decision to use hydrogen fuel in the BE-3," said Rob Meyerson in an interview in 2013, ...
Mark777's user avatar
  • 2,145
8 votes

Why is hydrolox used for first stages?

The problem with your analysis is that impulse per unit of volume isn't the right metric. Impulse is measured in units of force times time. Acceleration is measured in units of force divided by mass, ...
Russell Borogove's user avatar
8 votes

Lunar nuclear power system - how can hydrogen generated on the Moon be liquefied?

Cryogenic cooling typically works by cooling pressurised gas, releasing the pressure to reach even lower temperatures. The cooling itself uses boiling of some other gas with a higher boiling point (...
SE - stop firing the good guys's user avatar
8 votes

Hydrogen-propelled space elevator

Maybe it will be easier to fit all the problems with this in an answer instead of a comment. Some of that includes short answers to the (too many) questions you asked. I'm going to turn on community ...
7 votes
Accepted

Comparison of rocket engines using LH2 & LOX as propellants

There was a large difference in their size, and at the time, they were developed by two entirely different companies. In general, the propellant combination of a rocket engine does not determine its ...
Russell Borogove's user avatar
6 votes
Accepted

How often can liquid hydrogen engines be restarted?

My understanding is that designing for multiple restarts isn't particularly difficult; it's just that most engines don't have the need for it, so they don't bother. The RL-10 mentioned in the Lunar ...
Russell Borogove's user avatar
6 votes

Can we use interstellar hydrogen as a fuel for interstellar travel?

The issue is that the collection mechanism tends to produce more drag than thrust. Robert Zubrin and Dana Andrews showed that this makes most Bussard Ramjet-type starship propulsion impractical. In ...
user19742's user avatar
  • 957
5 votes
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Has (theoretically) metastable metallic hydrogen been considered for use as a propellant?

Considered, yes. See e.g. this study from 2010: We discuss the applications of metastable metallic hydrogen to rocketry. Metastable metallic hydrogen would be a very light-weight, low volume, ...
Hobbes's user avatar
  • 128k
5 votes

Who built the sparklers used by the Shuttle program during launch?

Problems Starting Space Shuttle Engines Prior to the first flight of the Space Shuttle we were test firing the Main Propulsion System Engines at the test site in Mississippi when we experienced a ...
Harvey LeBlanc's user avatar
4 votes

Platinum Colloid in Liquid Hydrogen fuel to increase fuel efficiency

It is possible that the platinum would improve the performance, but not in the way you're thinking. Metal particles in rocket fuels have been used experimentally as a way of increasing the density ...
Peter Taylor's user avatar

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