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PDF versions of YAGS can be downloaded for free from the links below. See Downloads for the full list of rules.


YAGS is released under the GNU Public License, and as such the full set of source documentation is available, as well as the final PDFs above.

YAGS Source

All content on this site is copyright Samuel Penn, © 2019. All textual content is licensed under the GPL version 2. Original photographs may be considered public domain.

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Chemical Rocket Engines

Fuel Types

For a typical chemical rocket, the fuel and propellant are the same thing. For Nuclear rockets the fuel and propellant are different things. The fuel powers the reactor, and the propellant is thrown out the back.



Density: 14
Max Impulse: 4.7

Liquid Oxygen and RP-1. Denser and requires less cryogenic cooling than liquid hydrogen.


Density: 4
Max Impulse: 5.3

Liquid Oxygen and Liquid Hydrogen. Very low density but high efficiency without being insanely dangerous. The cryogenic needs complicate things though.


Density: 10
Max Impulse: 4.6

Liquid methane and liquid oxygen.

Liquid Hydrogen

Density: 1
Impulse: ~

Generally used as propellant in nuclear or plasma engines.

TL 8 Engines (Modern Day)

The following drive types are based on real technology, simplified for purposes of the game.

Engine dT Mass (t) Thrust Impulse Fuel Type Density Environment
Merlin/a ¼ 0.5 850 2.8 Kerolox 14 Atmosphere
Merlin/v ½ 1.0 980 3.1 Kerolox 14 Vacuum
Raptor/a ½ 1.5 1,960 3.3 Methalox 10 Atmosphere
Raptor/v 1 2.5 2,150 3.5 Methalox 10 Vacuum
F-1/a 2 8.4 6,770 2.6 Kerolox 14 Atmosphere

TL 9 Engines (Plausible Future)


Small reaction-control thrusters designed for attitude control and fine manoeuvring. They often have poor impulse and very low thrust. There will often be multiple such engines arranged around a craft.

Linear RCS Thruster

Engine dT Mass (t) Thrust Impulse Fuel Type Density Environment
Linear RCS 1 200 3.4 Mono 12 Vacuum
RCS 1 100 3.2 Mono 12 Vacuum

Standard Engines

Typical bell nozzle designs. There are two variants - vacuum and atmospheric. The latter are optimised for use at sea level.

Engine dT Mass (t) Thrust Impulse Fuel Type Density Environment
RD-4/a ¼ 1 900 3.8 Kerolox 14 Atmosphere
RD-4/a ¼ 1 900 4.1 Kerolox 14 Vacuum
RD-4/v 1 2 1,000 4.5 Kerolox 14 Vacuum
RD-2/a ½ 2.5 3,000 3.7 Methalox 10 Atmosphere
RD-2/a ½ 2.5 3,000 4.0 Methalox 10 Vacuum
RD-2/v 2 5 3,200 4.3 Methalox 10 Vacuum


Engine dT Mass (t) Thrust Impulse Fuel Type Density Environment
Aerospike 1 15 1,000 3.8 Methalox 10 Atmosphere
Aerospike 1 15 1,000 4.5 Methalox 10 Vacuum
Aerospike 3 30 2,500 3.8 Methalox 10 Atmosphere
Aerospike 3 30 2,500 4.5 Methalox 10 Vacuum

TL 10 Engines

Standard Engines

This is the pinnacle of chemical engines using classic fuels. More exotic fuels are required for any further improvement.

Engine dT Mass (t) Thrust Impulse Fuel Type Density Environment
RX-2/v 2 4 4,000 4.5 Methalox 10 Vacuum
HX-A/v 1 2 2,000 5.2 Hydrolox 4 Vacuum

TL 11 Engines

Monatomic hydrogen engines become available. These are not as efficient as nuclear engines, but are significantly cleaner. The only issue is fuel density is very low. By this point, pretty much all engines are of the aerospike design, since it provides a lot more flexibility and the material/heating problems have been solved.

Engine dT Mass (t) Thrust Impulse Fuel Type Density Environment
H1-A/v 2 4 1,000 18 Hydrogen (H1) 1 Vacuum
scifi/spacecraft/drives/chemical.txt · Last modified: 2020/04/27 20:41 by sam