# How to work with rocksim simulations

#### Tamir Friedman

##### Active Member
Hello,

Rececntly we have launched our first J class rocket. Now we are working on analyzing the telemetry of the flight but we are having trouble understanding the rocksim simulation. I have plotted the "Y-acceleration" as function of time and the graph I got is confusing to me. Somehow post deploying the parachute the simulation shows the acceleration jumping to 1 gee and staying there. This is confusing because at 1 gee I expect the rocket to be free falling (aka, the rocket didn't deploy the parachute), but upon checking the graph for velocity I do see the parachute deployment and the landing velocity does make sense. How does the velocity graph derived out of the acceleration one? Because I can't see a way that an integral does that.

#### wonderboy

##### Well-Known Member
The accelerometer sitting still on a table will sense 1 g. The rocket, once stabilized (no longer accelerating, moving at constant velocity) under a parachute will also sense 1g. If you are going to integrate to find velocity, you have to subtract out the 1g.

#### ihbarddx

##### Well-Known Member
The accelerometer sitting still on a table will sense 1 g. The rocket, once stabilized (no longer accelerating, moving at constant velocity) under a parachute will also sense 1g. If you are going to integrate to find velocity, you have to subtract out the 1g.
It's many years later, but I have to say this.

What if I told you that, by subtracting 1g, you are not REMOVING the effect of gravity, you are ADDING IT IN! That is, you add -1g, which is the acceleration of gravity. The unmodified reading represents EVERYTHING BUT GRAVITY!

The 1g you get on the table is the effect FROM THE FORCE OF THE TABLE. It doesn't register the force from gravity (pushing in the OTHER direction). If you drop the accelerometer, it accelerates at -1g, but it registers NOThING. It doesn't register gravity. When you subtract 1g, you get the correct reading - but you have ADDED IN the effect of gravity (-1g). You have not taken it out.

The accelerometer reading is the sum of thrust and drag accelerations along the long axis of the rocket. It doesn't matter if the rocket is going up, down, or sideways. Gravity affects the rocket's motion, but not the accelerometer reading.

Replies
7
Views
578
Replies
8
Views
436
Replies
13
Views
1K