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Strap-on water tanks with ground-started fusion drive, in place of strap-on solids (or other high-thrust boosters) and air-start fusion.It doesn't matter whether it's antimatter, fusion, fission, or whatever, the very basic physics preclude launching with high Isp propulsion systems.
The two equations of interest:
Energy Rate = 1/2mdot*v2
Force = mdot * v
where mdot is the mass flow rate and v is the exhaust velocity.
Assuming we have a 1000t fuel+mass vehicle and it needs a TWR of 1.5 and a deltaV of 9km/sec, and just for fun set the exhaust velocity so that the mass ratio is 2.7:1.
Let's see what variables in the above equations are fixed, and we'll solve for the rest
Your force needs to be 15MN. the exhaust velocity is 9.2km/sec.
That gives us an mdot of 1630kg/sec and an energy rate of 69GW.
Now there is no propulsion system I can find (chemical, nuclear, ion) that has a kinetic energy efficiency of better than about 0.6, so you divided by that to get the total energy rate and that's 115GW
Now you are in Starship Booster territory which by the same equations above has an mdot of 23,000kg/sec and a total energy rate of 200GW
leaving aside what kind of damage exhaust traveling at 3 times the exhaust velocity of Starship will do your launch pad, that leaves us with about 10km/sec as about the highest exhaust velocity a launchpad could handle, given that Starship's launch pad can barely handle its 200GW of output at 1/3 the exhaust velocity. And you are still expending 2.7 mass ratio to get to LEO.
Now let's take our exhaust velocity up to 100km/sec, which is more what you want to speed around the solar system in reasonable amounts of time, no staging required, no refuels for a typical journey to Mars or the outer planets, etc
now your mdot is 150kg/sec, but your energy rate is a whopping 750GW/.6 = 1.25TW
I doubt you can supply enough water at high enough pressure to counter 1.25TW of plasma screaming at the concrete launch pad at 100km/sec.
While I agree it's very impractical, there IS a way around this ... Don't use a solid launch pad. Like the Sea Dragon concept (or Heinlein's torchships launching from the Pacific).
There's another potential way around this -- have your propulsion system be able to adjust its exhaust velocity -- ie for takeoff and near the surface, maybe you use extra reaction mass to lower your exhaust velocity and power levels to something a pad can handle, but once you're above a certain altitude, you shift to something more optimized Isp-wise. By that point you also won't need as much thrust, which will help. Still no getting around that ve^2 term, definitely puts an upper limit on the Isp you can realistically use for ground launch.
~Jon
One of the Sci-Fi space elevator books (Fountains of Paradise?) briefly discussed a Mars elevator. The big problem was that that the orbits of Phobos and Deimos are lower than the Aerosynchronous orbit. The Sci-Fi solution was to induce an oscillation of the elevator so it swings out of the way of the moons. This seems infeasible to me. I think you need to de-orbit the moons.
