# Desent rates

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#### bill2654

##### Well-Known Member
Are descent rates calculated for a fully loaded rocket with motor hardware or without? I'm assuming without but I think it needs to be added to the weight of the rocket. How else can you get an accurate rate? I have a 3.5 lb rocket and I plan on flying everything from H's to K's. My K motor weights 4 lbs. so how do I calculate it's weight? Average it out? It's going to come down a lot quicker with the K hardware than the H.

Is the motor hardware inside the rocket coming down under parachute? (Yes)

Therefore, shouldn't the motor hardware be added to the weight of the rocket?

(Short answer, yes. Weight of entire rocket plus motor case)

Therefore, shouldn't the motor hardware be added to the weight of the rocket?

No. Open rocket utilizes the weight of hardware with whatever motor you select unless you are using hardware with spacers. That said, the descent rate is based on weight you enter plus hardware less propelant.

When I said my K motor was 4 lbs, I exaggerated, that's with propellant. Empty 54/1706 hardware is 20 oz, and 38/360 hardware is 5.2 oz. Wouldn't you take an average of say 12.6 oz. and add that to final weight?

No. Open rocket utilizes the weight of hardware with whatever motor you select unless you are using hardware with spacers. That said, the descent rate is based on weight you enter plus hardware less propelant.
Hey,
Just curious, where did he mention anything about openrocket? I think his post was talking about calculating parachute size using a size calculator.

The engine has specs in OR for total weight and propellant weight. The total weight is hardware + propellant. For descent rate, the propellant weight is subtracted. Hardware weight is still there. You can see this effect by selecting a smaller engine that has lighter hardware. It will have a slower descent rate.

Hey,
Just curious, where did he mention anything about openrocket? I think his post was talking about calculating parachute size using a size calculator.

How else do you calculate your descent rate? Me, I use OR! Seems that mccordmw does too!

I use OR. It allows me to add an additional parachute to account for the airframe tumble plus drag. Also, takes the drogue drag into account when the main is opened. I can get within a couple percent accuracy comparing to my flight data.

Whether you use an online calculator, open rocket, or a table that shows descent velocity for a weight range for a particular parachute, descent weight must include the weight of everything descending, including the spent motor weight.
If you use OpenRocket or Rocksim that's automatically done based on values in the rocket motor file. If you use a descent rate calculator or a descent rate table it's up to you to add the extra weight.

Steve Shannon

I guess my question is should I figure an average weight in the range of motors I plan to fly? Looking at Top Flights chart for example, most chute sizes cover a 2 lb range difference. I fly at farm fields and sod farms so it's soft landings for the most part. I'm thinking a 36" will cover most of my needs.

If you use a descent rate calculator or a descent rate table it's up to you to add the extra weight.

Steve Shannon
Thats what I figured. I looked at most of the chute manufacture sites and none mention hardware as final weight. I know common sense says to add the hardware but...

I guess my question is should I figure an average weight in the range of motors I plan to fly? Looking at Top Flights chart for example, most chute sizes cover a 2 lb range difference. I fly at farm fields and sod farms so it's soft landings for the most part. I'm thinking a 36" will cover most of my needs.

If you just want to get 1 chute, then size it for the heaviest motor hardware you will use.

I guess my question is should I figure an average weight in the range of motors I plan to fly? Looking at Top Flights chart for example, most chute sizes cover a 2 lb range difference. I fly at farm fields and sod farms so it's soft landings for the most part. I'm thinking a 36" will cover most of my needs.

I would agree with BigBlueDart; pick a chute that encompasses the weight of the biggest motor you're likely to fly.
I know people who change out chutes depending on where they fly. Our range has soft and hard spots, so I tend to use a size larger, but I know I can always reef the chute, which means shortening the shroud lines to pull the canopy closed more, which decreases the effective size of the parachute. I would do that if it were windy and I wanted to reduce drifting.

Steve Shannon

If you just want to get 1 chute, then size it for the heaviest motor hardware you will use.

... and reef it a bit for the lighter weight flights.

You need the burnout weight of the motor, not just the empty case.

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