Why aren't hybrids used more often in spaceflight?

[aquitaine]

What do you guys think about hybrid rocket engines compared with other the other kinds? What are the advantages of using one as opposed to a pure solid or liquid rocket?

 

[Astronuc]

One has to balance the energy requirements, duration, and complexity (and mass) of the fuel system.

Pure solids (fuel and oxidizer are intimately mixed) and the duration is limited by the mass of material and burn rate. They are generally high thrust, low Isp.

Pure liquid requires pressurized and possibly cryogenic storage, which adds to the fixed mass, as well as propellant mass.

Hybrids liquid(oxidizer)-solid(fuel), air(oxidzer)-solid(fuel) are probably desirable where safety is an issue. Liquid hydrogen is problematic and requires a pretty secure storage and handling system. I suspect hybrids have limited application.


The figure of merit for a propuslion system is the kW(thrust) / kg (mass of system) - a kind of specific propulsive energy.

Basically the goal is to minimize mass and maximize energy, but in doing so, one pushes the materials closer to their technical (physical) limits, i.e. decreased margin to failure.

 

[aquitaine]

So it is to get the most energy per mass out of it and reduce costs? If so why aren't hybrids used more today?

 

[Astronuc]

So it is to get the most energy per mass out of it and reduce costs? If so why aren't hybrids used more today?

Hybrids don't necessarily have the greatest specific energy.

They are relatively low tech compared to say a LOX/LH2 or LOX/kerosine systems.

Scaled Composites's SpaceShipOne uses a hybrid rocket - Nitrous Oxide oxidizes passes through a solid rocket motor.
http://www.scaled.com/projects/tierone/data_sheets/html/ox_tank.htm

SSOne only goes suborbital and then it glides after burnout out.

Going orbital is something entirely different, as is going supersonic like the SR-71 or X-15, or hypersonic.