Cloakedwand72

Thoughts on a Cryogenic & hyperbolic's for engines?

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Are their benefit's on a cryogenic upper stage & for the space capsule service module fuel bye  hyperbolic's? For Earth to moon ops not interplanetary ops?

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Cryogenics get you ~25-50% higher specific impulse than hypergolics, which can ease design considerations greatly. More subtly, the propellants easiest to mine/produce on the Moon would be cryogenic in nature.

I would also strongly recommend against using hyperbole to power your rocket, as that tends to leave us stuck with fantasies instead of space travel.

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You may mean hypergolics, i.e. fuel-oxidizer pairs which ignite on contact.

Unlike liquid oxygen or liquid hydrogen, they often can easily be stored for long periods of time, since how long you can store them is limited by corrosion rather than boiloff. They also are more reliable, since they do not require a separate ignition system, and have flexibility in how many times they can ignite. They also tend to be denser than kerolox or cryogenics. However, they have a lower Isp, are harder to produce, and tend to be unstable toxic carcinogens.

Personally, I'd prefer kerolox or methalox for cislunar operations, except on a very small scale with unreliable tech, such as Apollo. (Unreliable not because of record, but because it simply could not be tested very much.

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There are two issues with taking cryogenics beyond LEO:

1. Boil-off:  The traditional means for keeping cryogenics cold is to insulate a little and let the rest simply boil to keep temperature in a liquid state (presumably we can insulate now better than during the Apollo era, but don't expect hydrogen to stick around).

2. Ignition:  Cryogenics typically involve turbopumps (which further increases Isp) which need to start and are pretty complex.  After that the whole thing needs to be ignited.  Considering just how complicated the ignition systems used on Earth are (and the holding clamps needed to wait for all ignition to happen), you are really betting the lives of the entire crew that the rocket can start.  Hypergolic engines will always work as long as two values open and neither the fuel nor the oxidizer leaked out.

 

I suspect rocket labs may find their electric turbopump more useful as a beyond-LEO rocket engine than a means for launching cubesats.  It has a way to go to prove itself reliable, but I imagine that any rocket that can recharge via the Sun (especially during Pe kicks) and has a lot less turbopump issues will find plenty of buyers.

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Storability is a big mark in favor of certain hypergolics....specifically, the ones always in use. Hypergolics aren't necessarily storable. But we use the ones that are. You could, theoretically, have non-hypergolic props that are readily storable or hypergolic props that are not storable.

Also, hypergolics aren't necessarily pressure-fed. You can pressure-feed cryos and you can pump-feed hypergolics. You could have an electric pump for hypergolics as easily as you could have for cryos. Some pump-fed hypergolic engines have pretty respectable specific impulse. The choice between pump-feeding and pressure-feeding is similar to the choice between cryos and hypergolics (weighing reliability and dry mass against performance) but they are separate questions.

There are roughly six factors to consider in choosing a propellant combination:

  • Toxicity and handling
  • Bulk density
  • Ignition reliability
  • Storability
  • Thrust
  • Performance

When you are dealing with reusable or refuel-able systems then questions like availability (for ISRU) and chemistry (e.g., avoiding coking) also come into play.

Propellant Toxicity/handling Density Ignition Storability Thrust Performance
Hypergolics Very Bad Good Very good Good Good Bad
Kerolox Fair Fair Fair Fair Good Fair
Methalox Fair Fair Good Fair Fair Good
Hydrolox Bad Bad Good Very bad Bad Very good
Solids Good Very good Very good Very good Very good Very bad

 

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So I was interested in bulk densities for the fuels listed (based on common mixture ratios):

  • Hydrolox: 361 kg/m3
  • Metholox: 864 kg/m3
  • Kerolox: 1033 kg/m3
  • A50/NTO: 1202 kg/m3

I wasn't able to find a good density for solid rocket fuel. 

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29 minutes ago, Racescort666 said:

So I was interested in bulk densities for the fuels listed (based on common mixture ratios):

  • Hydrolox: 361 kg/m3
  • Metholox: 864 kg/m3
  • Kerolox: 1033 kg/m3
  • A50/NTO: 1202 kg/m3

I wasn't able to find a good density for solid rocket fuel. 

Usually around 1800 kg/m3.

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Hypergolics means it'll work nearly every time.

Cryogenic means it could work unless it has all boiled off.

Hyperbolics... Have you used the nuclear blast again ? You shouldn't use hyperbolics for Earth-Moon transfers...

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