Funny, but I've been looking at the physics of impacts a lot recently. F=ma is a good start point, but its a bit more subtle than that. An equivalent form defines force as F=m(dv/dt) because acceleration is the rate of change of velocity.
The problem with calculating the force from impact is that we have no idea how long the impact lasts. We know the change in velocity dv because the rocket comes to rest from its descent speed. It's important to know how long that deceleration takes. As dt becomes shorter, the term dv/dt increases so the force increases. In the extreme case of the rocket coming instantly to rest, dt becomes zero and the force becomes infinite, which is clearly nonsense!
The only real way to measure the force on a rocket is to mount accelerators onboard and measure the deceleration over the time of the impact, a bit like crash test dummies. This would give a profile of how long the deceleration lasts and the profile of the acceleration, from which the average dv/dt can be estimated.