Rocket Science for Earthlings
a continuing series for the gravitationally impaired. Rocket Science for Earthlings 6

REENTRY, GETTING BACK DOWN

A couple of months ago space junk started showing up in farmers fields around South Texas. It turned out to be a tank and a pressure vessel off of a Delta rocket. This stuff is supposed to burn up in the Earth's atmosphere. What are the requirements of a successful reentry?

When an object with high velocity passes through the atmosphere, it experiences drag due to friction with the air, and lift if the aerodynamic shape is correct. First drag, the general drag equation is;

DRAG = (1/2*d*V(sq))*Cd*A

Cd = drag Coefficient

d = air density, typically (0.002265) at sea level

V = Velocity (ft/sec)

A = Area (in sq ft)

The drag coefficient is a number related to the shape of the nose. For a flat plate use 1.5, for a blunt nose use 0.7, for a typical rocket nose cone 0.5 will do.

Drag is fairly predictable until you get close to the speed of sound, then you get a bump in the curve. Near sonic velocities the drag will increase to a peak value 50% greater than the equation would indicate. Above Mach 1 the drag will be about 10% below the expected value. Since rockets and reentry vehicles transit the atmosphere and the sonic velocities only briefly, the general drag equation will give a good average of expected drag.

If the reentry vehicle is tilted at an angle to it's flight path, the resulting forces will create lift. For high velocities, the angle of attack produces the lift, not aerodynamic wing shape. This simplifies things such that a vector diagram will show the resulting lift.

Once the drag is calculated, the lift can be determined. Note that the lift is much greater than the drag for this low angle of attack situation. With a high reentry velocity, the vehicle can be made to skip across the top of the atmosphere, losing only a little velocity at each dip. This is the idea which Eugene Saenger figured out during WW II. While never actually built, the antipodal bomber would have circled half way around the world at under orbital velocities. The original plan for the X-20 Dyna-soar winged reentry vehicle also called for atmosphere skipping, but this was changed to an orbital mission profile just before the X-20 was canceled.

If anyone would like to play with reentry mission profiles, I have written a Basic program called rentry1.bas which models the flight of a vehicle reentering the earth's atmosphere. The program demonstrates the skip principal quiet nicely. Contact me if you need a copy. markgoll@wt.net.

Next issue I will discuss the mysteries of reentry heating. Very strange.