Reader JP emailed to ask about the practical sequence of spaceship design, where 'practical' means suited for created settings, stories or games, not spaceworthy for actual travel. In other words, do not try these tricks anywhere but at home.
And if you haven't done so already, this is a good time to consult relevant sections of Atomic Rockets, including the handy page of equations (from which I also swiped the image above).
I get the feeling that there's a specific order that I have to go in in order to determine the following:The short answer is there is no one 'right' way to attack this interlinked web of performance traits. Acceleration (thrust), specific impulse, and propellant flow are all very closely tied together, along with propellant fraction, which in turn constrains payload. Drive power density is also in this mix, constraining acceleration on the one hand and payload fraction on the other.
-Propulsion system specific impulse
-Available payload capacity
-Propulsion system mass flow
Define one parameter and all the others can vary around it. Define two parameters and the rest become much more constrained. Each defined parameter reduces the degrees of freedom for the remaining ones, until you lock it down. Great in principle, not so helpful in practice. I have my own approach and rules of thumb, shaped by my workflow habits and biases as well as mission requirements. But I gotta say something, so here goes:
The first parameter of all is mission delta v - how fast you want the ship to go - because that will drive everything else. After that, start is with parameters that are fixed by your techlevel, because that sets the ground rules that possible designs have to play by. And the parameters that are most fixed by techlevel are specific impulse and drive power density (power/mass ratio).
For chemfuel, both of these are sharply defined. Specific impulse (for H2-O2) is about 450 seconds, or 4.5 km/s exhaust velocity, and power density is on order of 1 MW/kg. Chemfuel engines put out such prodigious thrust relative to their weight that you can pretty much ignore drive power density (and therefore engine mass) unless you intend multi-g acceleration.
Example 1: Suppose an orbital 'gunship' with 5 km/s mission delta v, and an H2-O2 chemfuel drive. Total mass is an arbitrary benchmark (you can have big ships or small ones), but let us say 100 tons full load mass.
Right off the bat we know that full load mass ratio must be 3.04, a value determined by mission delta v, specific impulse, and the basic rocket equation. This means that out of our 100 tons departure mass, 67 tons will be propellant, the remaining 33 tons everything else - engines, fuel tankage, ship structure and equipment (including crew), plus payload.
Old Sir Isaac tells us that 1 kg of propellant burned in these engines in a second produces 450 kg of thrust (~4500 Newtons, for the picky), with a thrust kinetic energy of 10.125 MJ. Suppose we want a maximum acceleration of 2 g, requiring 200,000 kg or 2 MN of thrust. We need to burn 444 kg/second of propellant to get, which will produce 4.5 GW of effective thrust power. By my simple rule of thumb the engines will have a mass of 4.5 tons.
This performance requirement happens to be very close to that of a Space Shuttle Main Engine, SSME, now verging on retirement. It has a mass of 3.2 tons, but turned out much less rugged than hoped, so I'd stick with 4.5 tons for a combat capable engine.
That leaves 28.5 tons for the rest of your ship. Say 7 tons for fuel tankage and fittings and 5 tons each for overall structure, equipment, and crew pod including the crew, leaving 6.5 tons for additional payload such as a weapon pod.
A couple of things to note. As you burn off propellant, performance increases. Once this ship has burned off half its fuel, acceleration is 3 g, and the ship has about 3 km/s of delta v remaining. Assuming your mission involves going somewhere, blowing someone up, and returning, your combat acceleration will be higher than full load acceleration, something to remember in design.
The other thing to note is that you cannot overload a ship in space. It will not sink, crash at the end of the runway, or even bottom out its suspension. It will merely be sluggish. Thus full load, maximum load, or whatever you call it, are all really terms of art, and for some ships will be almost meaningless - for example a drive bus, that can be mated to a small payload section for fast travel or a big payload for slow hauling.
Example 2: Now suppose a deep space ship, such as the one I outlined last post for travel to Titan. In that case I first specified a 'travel speed' of 100 km/s, corresponding to a mission delta v of 200 km/s.
I assume some sort of 'plausible midfuture' nuclear-electric plasma drive, without going into details, even fission v fusion. Drives of this type need not have a fixed exhaust velocity (specific impulse). The maximum is very high, hundreds or even thousands of km/s, but these drives lend themselves to VASIMR style variable specific impulse. The key techlevel parameter is power density, how much oompf you can fit into a given mass.
But for this discussion I leave techlevel itself a bit open. Instead I pick an exhaust velocity of 200 km/s (~20,000 seconds Isp), equal to mission delta v, and specify an average acceleration of 1 milligee. Departure mass is, arbitrarily, 1000 tons.
Since mission delta v and exhaust velocity are the same, we know that the mass ratio is e, 2.72. Thus our ship departs with 632 tons of propellant, and the ship itself (plus payload) has a mass of 368 tons. Acceleration will increase as fuel is burned off, so let us say that full power delivers 600 kg of thrust, or 6 kilonewtons. Departure acceleration is thus 0.6 milligee, while acceleration at fuel exhaustion is 1.63 milligee.
Burning 1 kg/second in the engine described produces 20 tons of thrust, and requires 20 GW of power. So to meet our requirement we will burn 0.03 kg/second, and our drive engine puts out 600 megawatts of thrust power. (Last post I said 10 GW, because I was working really quick & dirty.)
Suppose that our drive tech can develop 10 kw/kg - much less than chemfuel rockets, but comparable to jet engines, and about 100 times better than a first generation nuke electric drive might put out. Thus our drive engine must have a mass of 60 tons, including shielding and radiators, leaving 308 tons for the rest of the ship. Hydrogen propellant is bulky, and you'll need a cryo system for long missions, so let the fuel tankage be 20 percent of fuel mass or 74 tons, leaving 234 tons. Say 34 tons for structure, 50 tons for equipment and fittings, 100 tons for the hab, including crew/passengers, leaving 50 tons for additional payload.
We can also cross check a few numbers. With fuel consumption of 0.3 kg/second it will take 2.11 million seconds to burn through all the propellant and reach 200 km/s, and our actual average acceleration is 0.95 milligee. Close enough for quick & dirty!
I previously wrote about spaceship design in general, and life support. (Connoisseurs of SEO spam may note that this last post gets more spam comments than any other - perhaps the combination of design and life.)
Update: Commenter nqdp did some head scratching about my deep space ship's drive engine, and when I checked I found I'd committed multiple brain dead errors, adding or dropping zeroes, that ended up with a correct result - so long as you didn't check too closely. I have fixed the errors in the main text, but for the sake of completeness here is the original section, with errors lined out:
Since mission delta v and exhaust velocity are the same, we know that the mass ratio is e, 2.72. Thus our ship departs with 632 tons of propellant, and the ship itself (plus payload) has a mass of 368 tons. Acceleration will increase as fuel is burned off, so let us say that
Burning 1 kg/second in the engine described produces 20 tons of thrust, and requires 20 GW of power. So to meet our requirement we will burn
Suppose that our drive tech can develop 10 kw/kg - much less than chemfuel rockets, but comparable to jet engines, and about 100 times better than a first generation nuke electric drive might put out. Thus our drive engine must have a mass of 60 tons, including shielding and radiators, [and miraculous save, dropping a zero to end up with the correct result] leaving 308 tons for the rest of the ship ....
Well, I did say only to try these tricks at home!