No, drag is not insignificant at 40,000 ft.
Imagine you could eliminate all of the air above 40,000 ft and fly
ballistically from there to 100 km. You would need to be going 1325 m/s
straight up at 40,000 ft to coast to 100 km.
No sweat, you say, I've got a rocket with Isp = 200 and MR = 10, so delta-v
is about 4500 m/s. I should have no problem paying gravity 1325 m/s with
plenty to spare.
Now add the air back in. If you can pack your rocket into a 75mm airframe
and get a Cd of 0.75 your drag is about 875 Newtons. If your rocket weighs
5 kg then drag is decelerating you at 175 m/s^2, or 17.5 G's!
Now, if your balloon can get to 100,000 ft then drag at that same velocity
drops to around 50 Newtons, equal to gravity for a 5kg rocket, and drops
off further as you go higher so you'll need less than twice the
ballistic delta-v.
On Sun, Feb 28, 2021 at 8:55 AM Dr Edward Jones <RocketPioneer@xxxxxxxxx>
wrote:
Thank you. Now suppose the single-stage balloon launch has an overall
mass fraction of 0.90 ... Air launched at 40,000-ft ... drag goes from
"nil" to "nada," correct?