Descent Rates

Hi all,

Last Sunday, as my Sudden Rush fell under its drogue chute, someone said "I don't think it even needs the main chute." While that comment was probably about the soft mud on the ground more than my choice of chutes, it got me thinking... My drogue (18" Spherachute+ from Giant Leap) descent rate was 67 or 68 feet per second, and the main (54" PML hexagonal chute) produced a 17 or 18 ft/sec fall. I was thinking the drogue descent could be a bit faster, and was considering an 18" x-form chute. What do you think?

Many people don't even use a drogue. So it should be fine.

How did you figure out that it would fall at 67-68FPS. If you used any sort of simulation that is not correct. The sims take the speed and weight and do the math. The one thing the simulations cant do is drag from the rocket... while under drogue the nose and aft sections are horizantal and cause a little drag. If the sim says 67 I would guess it is acually going 55-60FPS. Also if the size of your aft and forward sections are similar and their weight is similar then you dont need a drogue, especialy with a small light rocket.

Originally posted by jraice
How did you figure out that it would fall at 67-68FPS.

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Maybe he has a recording altimeter. That would be one way to accurately tell.

That's it, Luke. My AltAcc2C saves some great data, and no "sim" is involved. So should I try to increase the descent rate of the drogue? What is a normal drogue descent rate?

50FPS is good for small rockets or kraft tubed rockets

you can go up to 60FPS safely with a glassed rocket and if you need to you can go to 70FPS with a very strong composite like CF. Don't sim it to find the rate, use real data. Simulators can't simulate the drag given from the body tubes.

I dropped my L2 rocket (54mm LOC paper tube) from 6800 feet to 1000 at 100 FPS. I had a longer cord between my upper attachment point and my parachute for a nice slow opening and recovery. I didn't have any problems.

Edward

how long were the cords? I still think, unless you are flying that high (54mm rocket on a J, K or L) you should play it safe and use a drogue that will take you to 50FPS. My last DD rocket was calculated at 50FPS for the drogue but the drag from the rocket made it more like 30-40FPS!!! Slow but safe!

25 feet of 3/8" tubular nylon. Also, it is sometimes preferable to get the rocket down quickly from apogee. The rocket was light (5 lbs with motor) and I didn't have any problem letting it fall that fast with dual deploy. Between the fincan and payload was 15 feet of 3/8 tubular nylon. Between the chute and payload was 25 feet. It was a very safe flight as everything was recovered intact with no damage.

Edward

well that is a very light rocket with a LOT of shock cord up top, I dont have enough room for that. My rocket is 15 pounds and even tho it may be able to survive the main chute ejection my main concern is while falling under drogue. When a heavy rocket is falling to fast the parts go wherever and bang HARD if the rocket is heavy. I that was my rocket I would atleast use 25' of cord in the drogue section as well, so they dont bang. I use 21' for the drogue and 16' for the main, works nice but only when falling slow.

For the Delta II project our rocket weighed nearly 100 lbs and we used 25 feet of tubular nylon between the two sections for drogueless dual deploy. That came out at the top and then at 1000 and 900 (two altimeters) we popped the main chutes. We only had a small zipper on the main section due to the shear pins not holding the main chutes in when it drogueless deployed. I'm a fan of drogueless because it brings the rocket down fast and you don't have the chance to tangle everything up spinning under a drogue. A guy at our last launch just about lost his fincan because the drogue was spinning his retention in the top of his motor undone while coming down.

Edward

Originally posted by rocketgroupmike
That's it, Luke. My AltAcc2C saves some great data, and no "sim" is involved. So should I try to increase the descent rate of the drogue? What is a normal drogue descent rate?

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As mentioned elsewhere, drogues aren't about low descent rate. They're about stability, bringing it down low enough for main deployment is an attitude that prevents it from breaking. Any descent rate the drogue should have would be dictated by the rate your main should come out at. Too slow, and your rocket will start to tumble and deployment won't be smooth.

This probably sounds REALLY dumb, but how can you attain 60FPS falling down? Unless your rocket goes unstable and becomes stable pointing down, I compute the speed of gravity (9.8m/s, right?) to be 32fps. Nothing can fall faster than this unless it has power behind it, right?

My rocket spun and swayed a bit under drogue, but the booster never banged into the forward section... So I guess I'll keep it as is. (would an x-form chute result in more sway than a Spherachute+ ?)

Ps. gravity is 32 feet per second Per Second!

I skydiver can fall at 180MPH even with the drag of his stomach, when he turns head first he can reach almost 300MPH, I would have to say that is more then 32FPS and he has no power either.

and when a rocket lawn darts to you really think it is going 32FPS... come on, lawn darts can break mach (over 1000FPS)

Originally posted by rocketgroupmike
My rocket spun and swayed a bit under drogue, but the booster never banged into the forward section... So I guess I'll keep it as is. (would an x-form chute result in more sway than a Spherachute+ ?)

Ps. gravity is 32 feet per second Per Second!

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No, an X-form should sway less if at all. Since it has slits well up the sides, it takes less tilt to spill, empty out a bit, speed up, and refill.

I've seen a well balanced X-form pulsing down like a jelly fish, and seen them spin if unbalanced, but rarely ever seen one sway significantly. I always let the shrouds run through a quick link freely (ie. not tied) so they balance themselves under load.

Quick way to figure free fall: 10 meters per second for every second, cumulative. 1 sec = 10 m/sec, 2 sec = 20 m/sec, 3 = 30, etc.

Quick way to do conversions: Google search window.
Put in "30 meters per second in miles per hour" and get 67.1080888 miles per hour.

Put in "186282 miles per second in furlongs per fortnight" and get 1,802,613,657,600 furlongs per fortnight. At that rate you'd only have to ride your horse for 112.181532 fortnights to reach Proxima Centauri. They don't have the calculations for parsecs in yet, so if you're doing the Kessel run, you're on your own.

Thanks for the clarification Dyna. It was some science thing (maybe my school science teacher) that said /second.