Here is way to estimate or "guesstimate" descent impact loads. First, get an estimate of the descent rate. One way to do this is to go into Rocksim and choose a tube. Just use the "mass over-ride" option to assign the mass of your rocket. Then attach a parachute to the tube and look at your descent rate. This is how fast your rocket will fall with your chosen parachute diameter. Now, you can estimate the equivalent drop height (H) for your rocket without a parachute by using the following formula:
H = 0.5 x ( (velocity)^2 ) / 32.2
"H" is the drop height in feet
"velocity" has units ft/sec.
32.2 ft/sec^2 is the acceleration of gravity.
As an example, if your rocket has a descent rate of 20 feet per second, the impact will be the same as if the rocket was dropped from 6.2 feet. Imagine dropping your rocket from 6.2 feet on the salt flats or the Pony Express without a parachute! This will be the same impact load. If you can make your parachute large enough to slow the descent rate to 10 feet per second, your equivalent drop height will be only 1.55 feet.