Deployment bag advice/consultation for Bare Necessities

[CarVac]

How much force can you expect a small deployment bag to require? I've been mulling over using a small deployment bag (https://fruitychutes.com/buyachute/deployment-bags-c-3/3-deployment-bag-p-44.html, a 9-inch long one) in Bare Necessities instead of a cable-cutter-bound burrito, because it will offer more reliable sequencing and reduced risk of premature deployment from buffeting during the high speed descent.

However, there are two concerns: one is that we would probably use the Rattworks ARRD to retain the drogue and that might require more length (which for our design is bad for stability, it so happens). The other is that the drogue chute (a Rocketman "24-inch" (really 12, maybe?)) might not have enough pull.

Can anyone offer advice?


My plan for deployment is here:
[video=youtube;bWWxnTwjmk0]https://www.youtube.com/watch?v=bWWxnTwjmk0[/video]


If there isn't a clear consensus that this will work, we'll probably just use the cable cutter because we are pretty sure we can get it to sequence right. It's just that the deployment bag thread has given me second thoughts, and I realized that we probably can indeed fit a deployment bag in the rocket. We're going to do a recovery test next weekend, and time is running short for purchasing materials before LDRS.

 

[troj]

It's easy to test -- get your hands on a fishing scale, load up the recovery system, and attach the scale in place of the drogue. Have someone hold the airframe steady, while you extract the recovery system fully by pulling on the scale. Watch the measurement and you'll know how much force is required.

You can then use your simulation data to tell you the velocity the rocket will be falling at with the drogue deployed. Hook the drogue to the same scale, hang it out a car window and drive at the appropriate velocity. Check the scale to see how much force the drogue is generating.

We extract bagged C9s with pilot chutes for reserve chutes from skydiver rigs, and they're not very large. I would estimate 30", but I don't have one handy to measure.

-Kevin

 

[CarVac]

The appropriate velocity, huh? We've been assuming it's about equivalent to a 12" normal chute. RASAero says right before main deployment it's going 160 ft/s = 110 mph..... We'll have to go back to Black Rock just to go that fast.

 

[TheAviator]

The appropriate velocity, huh? We've been assuming it's about equivalent to a 12" normal chute. RASAero says right before main deployment it's going 160 ft/s = 110 mph..... We'll have to go back to Black Rock just to go that fast.

Assuming that the rocket is traveling at terminal velocity when deployment occurs, deployment force should be roughly equivalent to the weight of the rocket.

 

[bobkrech]

descent

Assuming that the rocket is traveling at terminal velocity when deployment occurs, deployment force should be roughly equivalent to the weight of the rocket.

Not quite. In this case the rocket is supposed to have a terminal descent velocity under drogue of 160 fps. If the terminal velocity under main is 15 fps, then the average deployment force is F = m a = m x delta V/delta t. Delta v is 160 fps -15 fps = 145 fps. The unknown is delta T. If the parachute opens in 0.1 seconds, the a = 145 fps / 0.1 s = 1450 fpsps / 32 fpsps = 45 g average. If the rocket weighed 20 pound, the average force would be 20 pounds x 45 = 900 pounds. Large parachutes have one or more reefing lines to slow the opening time and reduce the peak shock loads to prevent stripping the chute. The simplest method to extend the deployment to reduce the shock loading is to use a slider on the shroud lines.

Bob

 

[CarVac]

descent
Not quite. In this case the rocket is supposed to have a terminal descent velocity under drogue of 160 fps. If the terminal velocity under main is 15 fps, then the average deployment force is F = m a = m x delta V/delta t. Delta v is 160 fps -15 fps = 145 fps. The unknown is delta T. If the parachute opens in 0.1 seconds, the a = 145 fps / 0.1 s = 1450 fpsps / 32 fpsps = 45 g average. If the rocket weighed 20 pound, the average force would be 20 pounds x 45 = 900 pounds. Large parachutes have one or more reefing lines to slow the opening time and reduce the peak shock loads to prevent stripping the chute. The simplest method to extend the deployment to reduce the shock loading is to use a slider on the shroud lines.

Bob

TheAviator probably means that the force applied on the deployment bag by the drogue is 20 pounds, which is probably about right, and that was what I was asking about. Of course, moving that quickly, the drag of the rocket body will contribute more than usual, so it would be slightly reduced compared to that.

I do intend to use a slider; I was playing around with a metal D-ring simply slipped over the lines and with me freelining (skate type things) really quickly and then tossing out the parachute, it dramatically slowed the opening of the parachute.

 

[bandman444]

TheAviator probably means that the force applied on the deployment bag by the drogue is 20 pounds, which is probably about right, and that was what I was asking about. Of course, moving that quickly, the drag of the rocket body will contribute more than usual, so it would be slightly reduced compared to that.

I do intend to use a slider; I was playing around with a metal D-ring simply slipped over the lines and with me freelining (skate type things) really quickly and then tossing out the parachute, it dramatically slowed the opening of the parachute.

Why aren't you guys planning a test flight to check over all of the systems? Just seems like because neither you nor Chris have any experience flying a rocket like this (although things have changed since you started the project) and having a test flight would have saved your fantastic design from the embarrassing failure at XPRS last year. Your two are extremely smart and if you knew the problems ahead of time you can totally fix them.

I would have made it a Number One priority to test fly our rocket had we not had the pressing deadline. But me and Jared had flown a nearly identical rocket twice beforehand and had a list of the things we had learned and corrected.

[Douchbag mode off/]

Have fun. Be safe. Test before the flight, not after.

 

[CarVac]

Why aren't you guys planning a test flight to check over all of the systems? Just seems like because neither you nor Chris have any experience flying a rocket like this (although things have changed since you started the project) and having a test flight would have saved your fantastic design from the embarrassing failure at XPRS last year. Your two are extremely smart and if you knew the problems ahead of time you can totally fix them.

I would have made it a Number One priority to test fly our rocket had we not had the pressing deadline. But me and Jared had flown a nearly identical rocket twice beforehand and had a list of the things we had learned and corrected.

[Douchbag mode off/]

Have fun. Be safe. Test before the flight, not after.

Uhh, we are planning a test flight. Next week.

Interestingly, this is actually more similar to last year's design than the cable cutter strategy, except better than last year in every way since we will now have a deployment bag instead of just cramming the chute into a C-shaped bay. Funny how things like that happen.

Well, that's what learning's all about. Last year I didn't know anything about deployment bags, and we didn't want to use BP (which we are this time), so we wasted space on a bulky CD3 which made EVERYTHING awkward. Last year we had no time for a flight test, and this year we worked it out so that we can flight test. Last year we used a female mold and hand layups, and this year we're using a male plug and vacuum infusion.

I think we've learned more than we would have by this time had we flown and succeeded last year. Or flown and shredded. Or flown and lake-staked (!).

 

[troj]

The appropriate velocity, huh? We've been assuming it's about equivalent to a 12" normal chute. RASAero says right before main deployment it's going 160 ft/s = 110 mph..... We'll have to go back to Black Rock just to go that fast.

So try the drogue at 25mph. If it's generating sufficient force at that speed then it'll do just fine at 110mph. If it's not, try 35, then 45....you get the point.

BTW, unless your bag is really tight (which is a problem in and of itself), I'll be surprised if your drogue is insufficient.

-Kevin

 

[CCotner]

So, actually, we won't be making the deployment test next week, because when push comes to shove and it comes time for people who have promised to help you out to actually step forward, it all comes apart. I don't have the hours in the day to finish the rocket before friday, although that won't stop me from trying, i don't even have the finished design of the pseudobooster from Carlo yet (I've been busy finalizing the rest of the design, and started a big part of it over when we decided to change recovery systems yesterday, after finalizing the system Tuesday... etc.).

But yes, the point is we will do a test flight, if not next week, at FAR on the 6th of July (I'm out of town for the most awkward 2 weeks in existence in between, and while I will be having people work for me that week (they'd better get their **** done, they owe me), I don't trust them to know what to look for during the test flight.

 

[kramer714]

been thinking, could the top of the d-bag be put inline with the shock cord so the chute gets deployed when the shock cord becomes taught? No drogue for the D-Bag?

Mike K

 

[CarVac]

been thinking, could the top of the d-bag be put inline with the shock cord so the chute gets deployed when the shock cord becomes taught? No drogue for the D-Bag?

Mike K

Theoretically, yes, but:

We are flying this dual-deploy, so we can't trust the two sections of the rocket to pull apart strongly enough without a drogue after the initial deployment shock. Also, the drogue is needed to keep it from falling WAY TOO FAST. As it is, it'll be falling ~160 ft/s with the drogue. Any more and I'm afraid we'll shred the main upon deployment.

 

[CarVac]

Thanks, everyone, for the advice.

The Fruity Chutes deployment bag for 3" airframes was significantly smaller than CCotner expected; it's more like 2" across. Which means that the resulting bundle is smaller than the burrito we had been using.

He tried pulling it out, and it slipped out of the bag smoothly and easily...his words were "it practically explodes out when you pull on it".

Not bad.

 

[EddieJ710]

I've hammered down the force generated by a pilot chute by using Kevin's fish scale idea in a car (we used a load cell, actually), but then used the force data to back out the Cd, then you can mathematically calculate the force generated at any speed, making the loose assumption that the Cd is constant. Even though the Cd does change with airspeed, using it as a constant will get you in the right ballpark when calculating the force generated.