fusion powered capital missiles?

[Korzon77]

Let's say you have a slightly more advanced power that instead of the traditional missiles we see, is wealthy enough to simply equip them with full up fusion drives.

How big an advantage would that pose, and how would you model it?

[Daryk]

Use the SDS fighter rules from IO?  ???

[The_Caveman]

Existing capital missiles must be fusion-powered already, they couldn't have the delta-v necessary otherwise. Even with fission rockets the missiles would need to be over 95% fuel. Chemical rockets, over 99% fuel.

Because they're designed to be expendible and produce huge thrust-to-weight ratios, I would imagine capital missiles run almost-unshielded fusion reactors that aren't designed to last more than a few minutes in operation. XXXL engines, if you will. Hence why a Barracuda has an effective thrust rating of 50, which would be impossible with standard engines. Maybe their fuel consumption is dramatically worse than a fighter engine too, or maybe the missiles just have very small fuel tanks.

If you really wanted to put a standard engine in a missile, you'd end up with something similar to a cross between SDS fighters and the Bullet Suicide Drone. Basically a very slow tele-operated capital missile with more fuel points.

[Korzon77]

Are they? I recall some claims that they were chemical-fueled units.

[The_Caveman]

Are they? I recall some claims that they were chemical-fueled units.

The math just doesn't work.

A super-optimistic case for chemical rockets would give a maximum range of less than 8 space hexes. And that requires both a huge fuel fraction and access to propellants well in advance of anything currently thought possible. To get capital missile ranges the missile would have to be like a bare space shuttle SRB with no real warhead or targeting radar.

It is possible to do kinetic damage with just the mass of the empty shell, but from what I've read the "sweet spot" for a kinetic impactor is about twice the exhaust velocity--any faster than that and the increased speed actually reduces impact energy because so much mass gets thrown away as propellant. A chemical capital missile would need to be going 7-8 times its exhaust velocity or more.

A realistic fusion rocket, nevermind BT's super-fusion engines, could get capital missile ranges with a fuel fraction under 75%, which supports the descriptions of large warheads and powerful tracking systems. Based on that I'd have to say capital missiles are powered by at the very least tech C fusion drives.

[VhenRa]

Honestly, I half suspect they are nuclear lightbulbs.

[Talen5000]

Let's say you have a slightly more advanced power that instead of the traditional missiles we see, is wealthy enough to simply equip them with full up fusion drives.

How big an advantage would that pose, and how would you model it?

You mean...AeroSpace fighters with a robotic control system?

[Giovanni Blasini]

I don't think you could realistically have a fusion-powered missile with the kind of accelerations capital missiles must produce. Meanwhile, there have been solid-fuel rockets able to handle 100+ Gs.

[Empyrus]

I don't think you could realistically have a fusion-powered missile with the kind of accelerations capital missiles must produce. Meanwhile, there have been solid-fuel rockets able to handle 100+ Gs.

Don't think realism really matters here considering humans have evidently mastered proton-proton fusion with magnetic fields in BattleTech universe.

However, given that even within BTU, fusion is somewhat expensive, it seems doubtful it is used for capital missiles as it is. A Barracuda unit cost is by rules 7000-8000 C-bills (per Sarna, one price might for tele-operated version), highly doubt you can cram a high-power fusion reactor to that price.

Seems more plausible to me that the capital missile utilize some kind of maybe-physics-defying chemical fuel/propellant. Wouldn't be really out of place with magic armor and magic fusion. (EDIT Metastable metallic-hydrogen or metallic-helium perhaps? IIRC they'd theoretically make good chemical propellants when they uncompress, assuming they're possible and assuming they can be made, but i don't think either would be out of place in BattleTech, if they have very limited use as space missile fuels.)

Not sure they have unreasonable delta-v though, given that space combat in BattleTech seems to happen in relatively low velocities in general.

[Maingunnery]

Would it help if the launchers themselves were coil-guns to give the missiles some initial delta-V?

[Empyrus]

Would it help if the launchers themselves were coil-guns to give the missiles some initial delta-V?

Only if the initial launch direction is to proper direction, which would require turret-mounted system probably. If the missile needs to counteract launcher-granted delta-V, it is only a nuisance at best. But such system would make the launcher more complex, which might eat in ship's payload or volume restrictions, but on the other hand the missiles might be smaller and lighter.
Of course, BT systems are abstracted enough some launchers might be turret-mounted accelerators, while others could be VLS-style cells (cold or hot launch).

[The_Caveman]

I don't think you could realistically have a fusion-powered missile with the kind of accelerations capital missiles must produce. Meanwhile, there have been solid-fuel rockets able to handle 100+ Gs.

Thrust scales with the mass flow rate of the exhaust, which is basically determined by the size of the nozzle throat and the upstream pressure. BT fusion rockets have no problems here--though it should be noted that their exhaust velocities are almost nonsensical as dictated by designer fiat. Their problems are more related to the impossibility of generating enough energy to get the exhaust moving as fast as it does. BT fusion rockets have the efficiency of nuclear photonic rockets with the thrust of nuclear thermal rockets, an impossibility IRL.

However, given that even within BTU, fusion is somewhat expensive, it seems doubtful it is used for capital missiles as it is. A Barracuda unit cost is by rules 7000-8000 C-bills (per Sarna, one price might for tele-operated version), highly doubt you can cram a high-power fusion reactor to that price.

There's a huge difference between a fusion reactor that has to run for decades and one that only has to run for a couple of minutes before destroying itself. In the latter case you can use cheaper materials and simpler designs, as efficiency and durability are basically irrelevant. With the reactor at the tail end of the missile you only need a small disk of shielding material to protect the electronics in the nose from the radiation flux; everything else will be fine with being irradiated for a minute before the warhead goes off.

Seems more plausible to me that the capital missile utilize some kind of maybe-physics-defying chemical fuel/propellant. Wouldn't be really out of place with magic armor and magic fusion. (EDIT Metastable metallic-hydrogen or metallic-helium perhaps? IIRC they'd theoretically make good chemical propellants when they uncompress, assuming they're possible and assuming they can be made, but i don't think either would be out of place in BattleTech, if they have very limited use as space missile fuels.)

A physics-defying chemical propellant would only get you into fission rocket territory for exhaust velocity. Chemical rockets are just that damned inefficient.

[Giovanni Blasini]

Has anyone ballparked them based off teleoperated missiles?

[The_Caveman]

Not sure they have unreasonable delta-v though, given that space combat in BattleTech seems to happen in relatively low velocities in general.

We're about to science 7th grade math the shit out of this.

Considering the case of an on-board engagement (not going to get into high-speed closing engagements here because the math is way more complex and it requires even more crazy-high delta-v on the part of the missiles), the range of a Barracuda is 50 hexes. A space hex is 18,000m and a space turn is 60 seconds. Putting those together, we find the minimum average speed of the missile is 15,000 m/s. It's actually higher than that because the missile almost certainly doesn't have the full 60 seconds to reach its target, but we only need the minimum possible speed for the sake of argument.

We're going to ignore the velocity of the mothership because the rules don't adjust weapon ranges based on speed in on-board engagements, and a ship with velocity 0 can still engage any target within the 50-hex range bubble.

The average velocity of an object accelerating from a standstill is half the final velocity, so the required final velocity is 30,000 m/s. That gives a minimum acceleration of just under 51g, which is an entirely reasonable acceleration value for a missile. The upper bound of acceleration for a missile that won't crush itself like an empty beer can is probably around 150g. By increasing the acceleration you can hit top speed sooner and then coast, which does reduce the final velocity requirement. With 150g and a boost-and-coast profile you could get the final velocity as low as 18,000 m/s (12 seconds spent accelerating followed by 44 seconds of coasting).

So, depending on the acceleration our rocket motor can provide, we're looking at a minimum delta-v of between 18 and 30 kilometers per second (that's 2-3 times the total delta-v budget required to get from the ground to low Earth orbit).

Knowing this, we can plug the information into the rocket equation, dV = Ve * ln(launch mass/final mass), and solve for various propulsion methods.

Exhaust velocity for a chemical rocket is in the range of 2500 m/s, for solid-fuel rockets, to 5000 m/s for the most efficient liquid-fuel combination ever studied. To reach 18 km/s the liquid-fuel rocket would need a mass ratio of ~36.5, that is to say it would be 97.2% fuel. At the other end, for a solid-fuel rocket to reach 30 km/s it would require a mass ratio of 163,000 (!). That's a missile which is 99.9993% fuel!

Fission rockets have exhaust velocities as high as 10,000 m/s, which gives a mass ratio between 6.1 and 20.1 (83.6% to 95% fuel). A gas-core fission rocket could get the mass ratio as low as 2.46 (59.3% fuel) for the lower-bound velocity, but the radioactive mess created would make them problematic to use anywhere near a settled planet.

Metallic hydrogen would have similar performance to a fission rocket, if it's even possible--but it would still require tons and tons of the (expensive) stuff per missile, and the properties that make it a good candidate fuel also make it a good explosive and it's theorized to detonate spontaneously due to quantum effects, so making it stable enough to use as a fuel without the whole missile exploding in the magazine would be no mean feat. And if they had access to something that energetic, they wouldn't just be using it as space missile fuel.

BattleTech's magic fusion engines have dramatically higher exhaust velocities than any of the above technologies. One ton of fuel provides a 30-ton fighter with 40 g-minutes of delta-v, that is about 23,500 m/s. An effective exhaust velocity of ~695,000 m/s! Even if capital missiles are using fusion engines just 10% as efficient as the ones in aerospace fighters, that's still 3x the performance of the best fission rockets and 20x better than typical chemical rockets.

The BT fusion engine is also efficient enough to raise the missile's velocity to the point where it can hit a max-range target in just a few seconds rather than a full minute. None of the other technologies make that practical. Even gas-core fission engines couldn't deliver the performance to hit a target at 900km in under 30 seconds.

[Empyrus]

Duhh, forgot how long aerospace turns are, along with the hex size. Thought both smaller.