(Spaceships) Help me stat drive

[Clusterfrak]

If anyone has read the Aliens Universe: Colonial Marines Technical Manual, the dropships engines had three phases starting out as a turbo jet, then scram jet once it reached top jet speed, and then the intakes closed and it became a chemical rocket. How would I stat out this drive for orbital velocities?

[omeyer]

I haven't read the Manual, which has been OOP for quite some time, but then Aliens isn't exactly unknown ...

However, I have several answers:

• Spaceships has jet engines which cover everything from turboram to scramjets. One or two of these, the fuel tanks, plus chemical reaction engines and tanks for that, and you're in business. More detailed stats, like the exact changeover, are not appropriate for the Spaceships system.

• The engines could be reconfigurable systems, if the TL is high enough.

• Either way, chemical rockets will have insufficient delta-V to go down, fly combat missions, and go up again. So make it a fusion torch, and quench the temptation to run them in atmosphere.

Regards,
Onno

[SuedodeuS]

From your description, it sounds like it starts as a high-thrust air-breathing engine, changes to a medium-thrust air-breathing engine, then when atmosphere runs out becomes a low-thrust fuel-burning engine. You could approximate this by using reconfigurable systems, where the three systems are a high-thrust (2x thrust, 1/2 delta-V - or in this case, 1/2 time) Jet Engine, a regular Jet Engine, and a low-thrust (1/2 thrust, 2x delta-V) HEDM Engine. Note the high- and low-thrust options for these drives are not RAW but rather extrapolated from the rules of other drives. For simplicity, we'll allow the Jet Engine to use HEDM fuel as though it were jet fuel (we're dealing with ^ anyway). The total cost of each such engine would be, for an SM +8 craft, 69M (3M for HEDM, 10M for each Jet configuration; x3 for 3 possible configurations). Each engine would give 4G on take-off (burning through 1 fuel tank per half hour), 2G on atmospheric flight (going through 1 fuel tank per hour), and 1G thereafter (with delta-V 0.45 per tank).

[Clusterfrak]

Originally Posted by SuedodeuS

From your description, it sounds like it starts as a high-thrust air-breathing engine, changes to a medium-thrust air-breathing engine, then when atmosphere runs out becomes a low-thrust fuel-burning engine. You could approximate this by using reconfigurable systems, where the three systems are a high-thrust (2x thrust, 1/2 delta-V - or in this case, 1/2 time) Jet Engine, a regular Jet Engine, and a low-thrust (1/2 thrust, 2x delta-V) HEDM Engine. Note the high- and low-thrust options for these drives are not RAW but rather extrapolated from the rules of other drives. For simplicity, we'll allow the Jet Engine to use HEDM fuel as though it were jet fuel (we're dealing with ^ anyway). The total cost of each such engine would be, for an SM +8 craft, 69M (3M for HEDM, 10M for each Jet configuration; x3 for 3 possible configurations). Each engine would give 4G on take-off (burning through 1 fuel tank per half hour), 2G on atmospheric flight (going through 1 fuel tank per hour), and 1G thereafter (with delta-V 0.45 per tank).

Thanks this really looks like the best way to go. I had thought about having an HEDM powered jet but I was also wondering if it was possible to create the Aliens drop ship true to the technical manual using the spaceship rules. I didn't consider low thrust for the scram jet since it is requires such high speeds but I have to remember acceleration isn't speed.

[SuedodeuS]

Originally Posted by Clusterfrak

Thanks this really looks like the best way to go. I had thought about having an HEDM powered jet but I was also wondering if it was possible to create the Aliens drop ship true to the technical manual using the spaceship rules. I didn't consider low thrust for the scram jet since it is requires such high speeds but I have to remember acceleration isn't speed.

Well, from your description, I figured the point of the scheme was to start out with a huge push to get it airborne (thus high thrust), then let it tone down and work more efficiently while still within atmosphere, and finally once atmosphere runs out go to a chemical burn. Now that I've read a bit more on the engine schemes, I see that another bit is that scramjet engines need to already be moving at a decent clip before they start working, hence the use of turbojets for the beginning. To make it a bit less superscience, you could probably have it use rocket fuel (liquid hydrogen + liquid oxygen) rather than HEDM (I'm not certain getting oxidizer from the atmosphere does any good with HEDM, as I'm not very familiar with how they function). I suspect liquid hydrogen would be able to work with decent efficiency using atmospheric gases, which is what the first two stages would be.

Humorously, it should be noted that the turbojet is actually an older, less-efficient design than the scramjet used for the second "stage." I suspect they opted for a turbojet scheme because it sounds like something crazy powerful. They are more powerful (albeit less-efficient) than the turbofans that have apparently replaced them, however, so when you want to lift off a hostile world in a hurry turbojets are probably a good way to go.