High-Energy Exhaust Shielding for Far-Future Drive
Imagine a far-future spacecraft drive that accelerates its reaction mass to a small fraction the speed of light (say, 1,000 km/s). Say the mass flow rate to the drive were 1kg/s, and say the reaction mass were hydrogen. what effect would the resulting exhaust have on humans? On other spacecraft? If fired within an atmosphere comparable to Earth's at sea level, how would these effects change? What sort of shielding would be needed to protect a spacecraft or EVA suit and the people inside from damage or injury? How far would you have to be from the nozzle to safely remain inside the exhaust jet in a vacuum? In an atmosphere comparable to Earth's at sea level? How would this change with higher exhaust velocity, say 30Mm/s?
Such a jet would put out 500GW of power, and assuming the jet diverges, the intensity drops with the inverse square of the distance. This question was discussed here, but the provided answer deals primarily with the force exerted on spacecraft in the jet and is more focused on less powerful engines with a much lower exhaust velocity (40-100km/s). This question deals more with considerations that must be made with exhaust velocities at a significant fraction of the speed of light.
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You have mass flow rate and velocity, but the area this is spread over will say a lot -
$$\dot{m} v = F = 1 {{kg} \over {s}} \cdot 1,000 {{km} \over {s}} \cdot 1,000 {{m}\over{km}}= 1 \times 10^{6} N $$
If your engine design spreads this over a 2 meter (6.6 ft) diameter (1 m radius), you'd have $$ {{1 MPa} \over {\pi r^2}} \approx 333 kPa$$
Compared to atmospheric pressure (101 kPa), three atmospheres is still a lot for unprotected civilians. This relationship scales with $$ {{1}\over{r^2}} $$
Your design for atmospheric operation could double the exhaust area, which would make a tolerable gauge pressure.
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