Functional Characteristics:

Chemical Rockets

Chemical rocket engines are used in the smallest vehicles and for specialist high-acceleration applications. They have poor fuel efficiency and are not generally used for interplanetary operations.

Chemical Rocket Data
Type Ve
/ km s-1
/ kN
/ kg kN-1
/ m3 kN-1
Min Max
Solid-fuel 2.8
Liquid-fuel 4.5
Chemical Fuels
Type Density
/ kg m-3
LOX/LH2 330

Fission Rockets

The smallest interplanetary vehicles and most vehicles operating in the Earth-Moon system use fission rockets. Their fuel efficiencies are intermediate between chemical rockets and fusion rockets. The thrust per unit mass of a fission rocket system is much greater than that of a fusion rocket, so fission thermal rockets are also useful for long-range missile systems and fighter spacecraft.

Fission Rocket Design Data
Type TProp
/ K
Thermal Power
/ MW
/ kg MW-1
/ m3 MW-1
Min Max
Solid-Core 4000 200 5000 3 0.003
Liquid-Core 7000 200 10000 6 0.010
Gas-Core 10000 1000 20000 8 0.025

Fission Rocket Performance Data (Hydrogen Propellant)
Type Ve(H)
/ km s-1
/ KN
Max Mass Flow
/ kg s-1
Thrust / Mass
/ kgf kg-1
Smallest Largest Smallest Largest
Solid-Core 9.95 32 800 4 100 5.3
Liquid-Core 13.16 24 1200 2 115 2.0
Gas-Core 15.73 104 2000 8 160 1.3

Fission rockets have a heat transfer efficiency, η of 0.8. The drive will require radiators capable of radiating the other 0.2 &mult; Power.

Fission rockets that are for use on crewed vehicles require additional radiation shielding:

Fission Rocket Shielding
/ kg MW-1
/ m3 MW-1
5 0.0008

Fission rockets are thermal rockets that function by heating a working fluid with the thermal output of a nuclear fission reactor. Typically the reaction mass flows directly through solid-core reactors and some liquid-core designs. In the case of gaseous-core fission drives, the heat transfer is by radiation across a transparent, heat resistant ceramic containment vessel.

The performance of a fission drive is characterised by two numbers: the propellant temperature,TProp and the thermal power, P. These two values together determine the drive system's thrust, T, and fuel consumption, ρ when using a particular working fluid as reaction mass.

The propellant temperature is determined by the physical state of the core in the drive design: solid, liquid or gas. This together with the molecular mass, m, of the working fluid determines the drive's exhaust velocity, ve:

ve = ( 3 kB TProp / m ) ^ (1/2)

where KB is the Boltzman constant, 1.38&mult;10-23 J K-1. Notice that a smaller molecular mass for the reaction fluid will give a higher exhaust velocity, so it is advantageous in high Δv designs to use light-molecule reaction mass, typically hydrogen. Higher molecular masses will give a smaller overall Δv, but with the compensating advantage of a higher instantaneous thrust. Most fission drives dissociate the propellant into individual atoms.

In some cases it may be important to determine the actual thrust of the engine, rather than just its exhaust velocity (for example, during combat). The maximum fuel consumption, ρmax, is first determined from ve, P and the heat transfer efficiency, η:

ρmax = ( 2 η P ) / ( ve2 )

Finally, the thrust of the drive is determined:

T = ρ ve

where ρ is the current mass flow, which is in the range 0<ρ<&rhomax. Note that the thermalised mass flow is proportional to the reactor's current thermal output.

Fusion Rockets

Fusion rockets are huge and heavy devices, but they are the only drive systems that give enough Δv for truly practical for System-wide operations. All the major classes of interplanetary craft use fusion drives.

Fusion Rocket Data
Type Reaction Exhaust energy
/ MeV
/ MW
/ kg MW-1
/ m3 MW-1
Min Max
D + 3He → 4He + p 18.3
3He + 3He → 4He + 2 p 12.9

Working Fluids

Working Fluids
Liquid RMM Density
/ kg m-3
Hydrogen, H2 2
Methane, CH4 16
Ammonia, NH3 17
Water, H2O 18 1000
Carbon dioxide, CO2 44 70

Drive Performance

The exhaust velocity of the drive system and mass ratio

μ = ( mship + mreaction ) / mship

determine the Δv of the spacecraft:

Δv = Ve ln ( μ )

This is the maximum change in the velocity of the vessel. The Δv in turn determines the manoeuvres that can be carried out by the ship.

The standard drive system is the rapid-pulse, semi-continuous fusion drive. Such a drive is somewhat like riding on a continous stream of mini-nukes, perhaps equivalent to a 10kt bomb every second; the power output is in the regions of tens of terawatts. The exhaust velocity is as much as 6000 km s-1, and the drive plume is highly dangerous out to a distance of tens of thousands of kilometres. Clearly such a system is only reasonable for operations well away from planets or habitats; indeed the drive is of considerable use as a weapon. A ship using a fusion drive is also extremely un-stealthy, and will be easily visible on the far side of the system.

Note that for a given drive power, the acceleration will be lower for a higher exhaust velocity. Therefore, in selecting an exhaust velocity there is a tradeoff between maximum Δv and acceleration. Drives, especially those of warships, often allow this balance to be adjusted to some degree to adapt to different operating conditions.

The future of Ad Astra

Site Meter