Interface rates for laser rockets

[AlexanderHowl]

Gravitational and atmospheric drag tends to average 1 mps after accounting for the Obearth effect, so total delta-v to LEO is around 6.6 mps (7 mps gives you a safe margin).

[Michael Thayne]

Originally Posted by AlexanderHowl

Gravitational and atmospheric drag tends to average 1 mps after accounting for the Obearth effect, so total delta-v to LEO is around 6.6 mps (7 mps gives you a safe margin).

It's more the closer your thrust-to-weight ratio is to 1G though. Hence my doubts about the viability of a vehicle than can only manage 1.5G.

[AlexanderHowl]

I generally prefer 4g-6g acceleration in my designs for interface spacecraft.

[Flyndaran]

6g does not sound at all safe for most humans. Great for cargo, I guess.

[AlexanderHowl]

6g is safe for most humans in good health, it is just impossible to steer (use automated pilots)

[Michael Thayne]

Passenger rocket ships might want to accessible to people in less-than-great health, though. What's reasonable for those purposes? 3G? 4G?

[Flyndaran]

Originally Posted by AlexanderHowl

6g is safe for most humans in good health, it is just impossible to steer (use automated pilots)

I'd say survivable by most humans in good health. But that's not "safe", and who's to say that most humans are in "good health"?
Seconds of 6g isn't the minutes of burn time this rocket will experience.
Does anyone have data on how normal people handle minutes of high g force?

[Anthony]

I'd be a bit leery of 6g rockets. Your reach max drag at about 8 km altitude, max thermal load at about 12 km. A 6G rocket accelerating straight up (net +5G up) will be traveling at 1100m/s at 12 km (mach 3.3 at 40,000'), which is reasonably scary aerodynamic requirements.

[Fred Brackin]

Originally Posted by AlexanderHowl

I generally prefer 4g-6g acceleration in my designs for interface spacecraft.

Peak acceleration for the Gemini space program was over 6 and almost to 7 Gs. Same for the fastest flights of the X-15. The 3 G limit for the Shuttle was based on the vehicle's limits and not the passengers.

What can "normal" people take and for how long? No one has ever needed to know that very badly.

[weby]

Originally Posted by Michael Thayne

NTR does seem to allow much cheaper interface. I wonder if such designs realistically account for gravity drag, though. I'm not sure what the most realistic formula is, however. Pyramid #3/79 had an article that tried to give better formulas, but the formula for liftoff appears to include 1 mps delta-V required per atmosphere of pressure, which I'm pretty sure isn't right.

The one I suggested was NTR ram-rocket, so if it works as advertised then atmosphere=free reaction mass until you are outside it.

So the bonus comes a lot down to drag vs thrust, ie how fast you can go and still have enough atmosphere. The idea is to accelerate to as high a speed as possible before starting to use on-board reaction mass. As drag coefficients are difficult and quite complex it is hard to say how much such an air-breathing engine will add and how much thrust it has at different atmospheric pressures, so the "realistic" maximum speed before having to use own reaction mass is kind of hard to say..

But in the extremely simplified system in Spacehips you would need just over 2G to add 1 MPS "free" delta-v, but the spacehips there seem to have only good streamlining, not the extremes you would strive for in a space plane like that.