If your rovers were small enough, you might be able to make a gun-launched or gun-assisted scheme work. If you could engineer your probe/launch vehicle to withs

If your rovers were small enough, you might be able to make a gun-launched or gun-assisted scheme work. If you could engineer your probe/launch vehicle to withstand 20 Gs, for example, a linear acceleration at that level would get you to Mach 1.2ish in 400 meters or so. You come out of the barrel cleanly in ramjet territory and, what's better past the point where your shock wave angles are highly dependent on Mach number. You could start out in subcritical mode for the engine and progress through critical and supercritical, buying hardware simplicity with some loss of efficiency. You might even be able to shed some of the diffuser cowling in stages at the lip for a Q&D variable geometry engine. By the tme you hit M = 5-7, you'd probably be out of the atmosphere and would have to switch to rocket mode anyhow. Your mass fraction for the booster stage would be pretty amazing because you'd shaved 2000-2500 m/s off your delta-V with the gun/air-breathing phase. (That matters a lot to the rocket equation. I'll leave it as an exercise for the reader to work out the actual benefit.) The limiting factors here are obtaining a guidance and control system that is small enough to fit down your (admittedly large bore) low-pressure launch gun. Also, your gun would have to be pretty dang straight or your vehicle would have to be wrapped in a vibration-absorbing sabot. The benefits are: 1) Once you have the gun, you can launch as many as you like; 2) Mass fraction as mentioned above; 3)The possibility of production-lining launch vehicle construction. Not that I've given this any thought. BTW, Bruckner et. al. at the University of Washington have been working on a ram accelerator, a type of launch gun, for years. The only thing slowing them is funding. They omit an airbreathing stage in favor of obtaining all the Delta-V in one shot, subjecting the payload to 700-1e3 Gs.