"Low" Efficiency High Power Fusion drives for weapons and "High Efficiency" lower power drives for ships?

Question

I'm currently trying to develop 2 different "classes" of drives for space ship, Expanse style combat with "Low" Efficiency High Power Fusion drives for weapons and "High Efficiency" lower power drives for ships. When I say expanse stay, I mean with a 2 distinct flavors of engines with a "torpedos fusion torch" engines and "ship grade Epstein drive" division, with the fusion torch just being like a solid rocket motor, powerful and quick, but inefficient, with a Epstein drive being not as powerful, but a lot more efficient. While I want both to be relatively high power(compared to today's rocket, eg higher than 1g acceleration), but for the "weapons style" drives, I want them to be higher power, yet significantly less efficient(like a solid rocket motor, but with a longer burn duration(for longer range combat) and quicker to power up(no long spool up)) and for ships(less powerful, but maybe lower to speed up(kind of like how a larger ship takes some time to reach max power)). Any tips for how to describe the science differences between the two? I was hoping to have some sort of fusion, but I'm not sure how to write in it to make sense.

Answer 1

This is relatively straightforward

The power, thrust and efficiency of a rocket engine are linked by the following equations:

The rocket equation: $\Delta V=v_e ln (\frac{m_0}{m_f})$

Thrust: $T= \dot{m} V_e$

Rocket power: $P = \frac{1}{2} \dot{m} V_e^2 = \frac{1}{2} T V_e$

Where $\Delta V$ is the maximum change in velocity the rocket can impart, $V_e$ is the exhaust velocity, $m_0$ is the mass of the rocket fully loaded with propellant, $m_f$ is the mass of the rocket empty of propellant, $P$ is the power of the engine, $\dot{m}$ is the mass of propellant ejected per second, and $T$ is the thrust.

High Efficency

So, imagine you have a fixed amount of power from the reactor, a fixed sized tank for fuel, and you want to get as much $\Delta V$ as possible. This means maximising $v_e$. The maximum $v_e$ you can get from a fusion rocket is the speed of the fusion products from the specific fusion reaction used (probably highly energetic helium nuclei, but it depends upon the chosen fuel). To make a high efficiency fusion rocket, you want to design your reactor to directly exhaust the fusion products out the back of your spacecraft. The downside of this is that although our $v_e$ is high, the actual mass of propellant exhausted ($\dot{m}$) is going to be quite low. For example, if your reactor is rated for 3 Gw (i.e. a large modern fission reactor today), and was 100% efficient (obviously not possible, but it sets an upper limit), and running on a perfectly aneutronic D-He3 fusion cycle (many challenges to overcome with this), it would be exhausting only 8.5 mg of fuel per second* to produce thrust. The efficiency of this would be excellent, but you aren't going to get much thrust.

To get "The Expanse" levels of thrust (Multi-g for a reasonably sized spacecraft) you are going to need a reactor that operates in the multiple TW range. That's an enormous amount of power, and will come with a great many engineering challenges to solve. However, you can trade off efficiency for thrust.

High Thrust

If you want higher thrust, you need a higher $\dot{m}$. All you need to do is design the rocket such that the hot fusion products heat up some sort of working fluid. This is heated to plasma, expands drastically, and exhausted through the rocket nozzle. Example working fluids could be liquid hydrogen, water, methane or some denser hydrocarbon. By pumping more propellant into the rocket, you can increase the mass flow rate, and hence the thrust. The down-side is that this means that the fusion products will have had to collide with all those newly introduced propellant molecules, and will now be slower. The propellent molecules will also be travelling much slower than the theoretical maximum velocity of the fusion products. Consequently, the average exhaust velocity of the rocket will be less, and it will be much less efficient. It will burn through the additional propellent at a higher rate, but the thrust will be much more impressive.

Summary

Most ships will run predominantly with a purely fusion product exhaust. They will generally operate on a low cruising acceleration (perhaps up to a couple hundred milli-g) to minimise fuel burn. They will be able to maintain this small thrust for days and weeks, which is still enough to get around the solar system in respectable times.

Some ships (Military vessels, express couriers and "business class" passenger ships) will have additional propellant tanks on board. This will cut down the available space for payload, but it will mean they can inject propellent into their rocket nozzles, and substantially increase thrust. They may spend most of their time cruising on low acceleration, but can execute burns of multiple gs when they need to for a much more limited amount of time.

Missiles take this approach to the extreme. They are essentially a warhead, a fusion engine, and a big stack of fuel tanks in the middle. They can pull off accelerations that would kill any human, but not for very long. Consequently, they need to be carried around the solar system on larger ships, in order to get them to the point where they can be launched and able to keep up with their targets before they run out of fuel.

*All calculations are very back of the envelope, and may contain errors. Please do not use for actual rocket science.

Answer 2

Fusion propulsion versus throwing nuclear bombs out the back

Here are two technologies that could give you your desired results that have been heavily studied by serious scientists.

High efficiency: fusion-driven rockets

A fusion rocket would convert propellent to power in a linear and predictable way. It would also be gentle on the structure of the rocket. Here's a description from NASA:

The propellant is rapidly heated and accelerated to high exhaust velocity (> 30 km/s), while having no significant physical interaction with the spacecraft thereby avoiding damage to the rocket and limiting both the thermal heat load and radiator mass... The energy from the fusion process is thus utilized at very high efficiency.

Immediate power: finish Project Orion

So you want power and you want it right now? Don't care about efficiency? Try throwing a nuclear bomb out the back of your rocket and riding the shockwave. Repeat as needed to change speed or direction. Believe it or not, this was a serious project by the US government called Project Orion. If you want to see the case for building the thing, check out this article from after the project's cancellation.

Answer 3

The ship power source might direct a percentage of the energy from each fusion event into the igniting the next reaction and the weapons might direct all that energy into a laser. Eventually using the fusion reaction as the most stupidly powerful laser pump.

The current thinking for fusion reactors is to ignite a small deuterium pellet using lasers or magnetic fields. Then finding a way to channel some of the energy into igniting the next pellet and capture the resultant thermal energy to create electricity. Think of it as similar to the way that a gasoline engine is just a series of tiny controlled explosions.

In what I have described above there is no difference in efficiency (at least in terms of you get out the same amount of energy per unit fuel) but instead you are changing where the energy is going, is it all going to doing external work or to sustaining the chain of fusion reactions. So you have one of the continuous output engines producing power to charge capacitors, which initiate the fusion reactions for the weapons.

Answer 4

Do you really need fusion drives for your missiles?

Given ships equipped with 'realistic' fusion drives drives the setting would dictate ships with relatively low DV but potentially high final velocities. Basically long slow accelerations (compared to a chemical rocket) but with very much higher fuel efficiency given the extremely energetic nature of fusion reactions.

So why not equip warships with missiles using some form of highly energetic (variable thrust) liquid or solid fuel booster? Its space warfare so you don't necessarily need continuous burns to reach your targets. Nor do you need the high accelerations of rockets lifting of from Earth. Even 1G of acceleration would be far more than most of any realistic fusion drive could manage. (Or if they can manage it it would only be for very short time periods because they will go through their fuel reserves every quickly.

Also in space missile combat would be decided by differences in the velocities of the two sides and their respective orbits or angles of approach/departure. This means there can be 'encounters' where due to velocity differences etc even though the two sides seem to be relatively quite 'close' to each other neither side (or perhaps only one) has a chance of hitting the other and other encounters here even though the range is extremely long both can launch and hit.

What this means is that the type of drive is not so important as you may think given the rockets starting velocity (compared to the target) will be that of the fusion powered ship which launched it. So after a fashion it gets the benefits of fusion power even if not equipped with one.

And the benefits of a chemical/solid fuel rocket?

• cheapness
• simplicity
• compactness (more can be carried)
• harder to spot (if cooled sufficiently and coasting, something you have no hope of doing with a fusion drive - since nothing screams 'here I am' so much as a small sun accelerating on towards you on an intercept course.)