OR deployment velocity question

[dhkaiser]

In the attached OR file I have a drogue less delayed deployment. The F15 motor results in a deployment velocity of 174 ft/s with deploy at 450ft. A F26 has 59 ft/s and a G40 has 61 ft/s both at the same deployment altitude of 450ft.

I don't understand why a lower altitude motor would have a higher deployment velocity. Can I have confidence in any of these deployment velocities?

View attachment Mammoth-TRS-Cable Cutter-nosecone-5-10-15.ork

 

[markkoelsch]

The F delay is likely too long and the rocket is arcing over and coming back down hence accelerating.

 

[Rex R]

the delay is fine...OR doesn't handle drogueless descent well, think it assumes that the rocket is streamlining down nose first. give the rocket something to deploy at apogee(1" chute would work). that should improve the simulations.
Rex

 

[dhkaiser]

Mark thanks for the quick reply.

The motor delay will separate the nosecone with Cable Cutter bound parachute near apogee, in all three cases. The plan is to have the separated nosecone\LBT free fall to 450 ft where the Cable Cutter will release the main. I don't see how the delay on the motor matters much as I am concerned about main parachute deployment speed at 450 ft altitude. Am I missing something?

 

[dhkaiser]

Thanks Rex, I will give that a try.

 

[SammyD]

I've run into that too with OR, and I do what Rex suggests.

If you're using a logging altimeter that can graph flight data, you can pull up the flight profile plotted on an X-Y axis. A quick calculation will yield the decent rate in whatever units you wish to convert to, mph, fps, etc...

 

[dhkaiser]

I did the 1" chute at eject charge. Now I am getting scary deployment velocities of 250 - 300 ft/s. Not sure I want to deploy at those speeds.

 

[dhkaiser]

SammD, yes we have a logging altimeter. However this is first flight and I would like to have some assurance it will survive.

 

[SammyD]

I've downloaded your ork file, and I'm seeing a single deployment rocket, but what you're describing above appears to be a dual-deployment rocket. Am I correct?

 

[dhkaiser]

Sammy, Yes we plan a single deploy at 450 ft with a 24" chute. I added a 1" drogue as suggested by rex after uploading that file and got the ~300 ft/s figures.

 

[mikec]

I did the 1" chute at eject charge. Now I am getting scary deployment velocities of 250 - 300 ft/s.

Your drogueless descent rate will likely be around 50-100 fps. I put in a small chute and tweak the diameter until I get a rate of 75 fps or so, and then tweak after the first flight to match reality.

 

[SammyD]

That's a bit confusing in the Motor tab under Deployment. You seem to be trying to force a deployment at 450', but you're reaching altitudes between 631' and as high as 2800'. If there is no motor ejection (or any separation) at apogee, the rocket will accelerate HEAVILY on the decent to 450'. That would explain your high deployment speeds.

 

[SammyD]

There is a check box in the Deployment section of the Motors tab that allows you to set the same deployment configuration for every motor selected. That doesn't appear to have been done in your file.

 

[SammyD]

In a single deployment configuration, you need to deploy at APOGEE; otherwise, you're going to have to slow the bird down on decent to deploy a main later (dual-deployment). I think you need to concentrate on observing the optimal delay for the motor selected in the Simulations, then set your deployments to Apogee in the Motors tab.

 

[dhkaiser]

Our plan is to separate at apogee with motor eject and then free fall to 450 ft where we deploy the main/only parachute with the cable cutter. The main will be out of the body yet bundled with the cable cutter during free fall. We were hoping to reduce drift by delaying full deployment until 450 ft.

 

[SammyD]

What you're describing is dual-deployment. Deployment 1: apogee with motor ejection, Deployment 2: main at 450' with the cable cutter.

Where does the rocket separate? I don't see a separation point on the main airframe, and I don't see any rear deployment apparatus either.

 

[dhkaiser]

My terminology seems to be at fault. when I said separation at apogee I mean the nosecone separates from the LBT. I did not say deployment at apogee since we are not deploying a chute at apogee. Drogueless with delayed deployment.

 

[dhkaiser]

Your drogueless descent rate will likely be around 50-100 fps. I put in a small chute and tweak the diameter until I get a rate of 75 fps or so, and then tweak after the first flight to match reality.

Thanks Mike. I'm hoping those figures are correct! First flight can be a bit scary.

 

[SammyD]

Ok. How are you containing the parachute once the nosecone is kicked off by the motor ejection charge?

 

[SammyD]

I reread the above scenario, and see that you're tying the chute to flutter on descent. I've edited the file to show what I understand you're trying to do, removing many of the motors to clean it up, removing the overrides in the deployment configurations, and running the simulations. Sending the file in a few seconds...

 

[SammyD]

View attachment Mammoth-TRS-Cable Cutter-nosecone-5-10-15.ork EDITED.ork

Drogueless recovery looks scary on a simulation file, but I've flown a 3" Patriot that weighed 4lbs without the motor to 4000' on at least 5 dual deploy and drogueless flights with no recovery issues (I use 4lb since its freefall will be with a SPENT motor, negating the propellant mass weight). I use a 38" main that is set to deploy at 400'. Again, no issues, though the OR file says it's deploying that 38" chute at 305mph.

 

[dhkaiser]

OK, looks like drogueless in OR is an ignore the numbers. Thanks everyone.

 

[cerving]

I've gotten reasonable drogueless numbers out of OR by using a drogue chute equivalent to 1/2 the surface area of the rocket. For example, for a typical 38mm 42" long rocket a 10" chute works out fine.