Minimum parachute size for small rocket

[Rev Lovejoy]

In the techniques forum, there was a thread on using mylar balloons to create parachutes. I had a couple around, and my kids each made a fliskits whatchamacalit. I am upgrading the streamer to a parachute, but the whole piece of mylar would not fit in the tube, so I've been trimming it down to fit. I'm left with a much less impressive looking parachute, but it's still more substantial than a streamer. Is there a formula for rocket weight: parachute diameter?

 

[powderburner]

I hope you know about one more option: skip the parachute (and streamer) completely.

This is called "nose-blow" recovery (yeah, there are many many disgusting jokes to be made on this one, but it's a legit technique). The nose cone is still tethered by cord or shock cord to the rest of the rocket, but no other recovery device is fitted. At ejection, the NC is separated and the rocket is destabilized, and the whole thing comes tumbling down. It will descend a little more quickly than if you used a streamer or chute, but it will usually be OK.

When to use this approach: if it is a windy or gusty day, and you want to get your rocket down quickly without drifting away. Or, if your launch field is relatively small and you want to improve your chances of drifting outside the boundaries. Or, if you screwed up and there isn't room in your rocket for a recovery system.

When not to use this approach: if your launch/recovery area has rocky soil or short grass, and if you are afraid the landing impact might be too much. Or if your model rocket has a lot of delicate external detailed features that you don't want banged up.

 

[Rev Lovejoy]

I do realize that is an option, but the factor I failed to mention is that this rocket is for my 2 (almost 3) year old, and he loves parachutes. We went to LDRS to watch on July 4, and while the takeoffs were awesome, his question after each one was "where's the parachute daddy?" and when they deployed, he was thrilled. I'm confident of recovery with the field size where we will be shooting, and this model goes up on an A3-4T max, so drifting far isn't as big a concern.

I did a streamer recovery for an edmunds elcie G booster (as the kit was made) and that is about as small a rocket to come back down aside from a micro-maxx - after 8-10 flights, and really quick descents into a field with thick tall grass, the impact was still enough to show serious wear on the body. I know it's overkill, but I think I may chute everything I build.

 

[powderburner]

If you are having trouble getting a chute to fit in there, try a smaller chute. A six or eight inch diam chute will still be surprisingly effective.

Another way to reduce chute packed volume: cut a big spill hole out of the center. Your rocket will still be visible during descent, but will have more of a "donut" shape than a solid disc.

Other ideas: I can't imagine that your mylar is thick enough to cause big problems, but you can also make parachutes from drycleaner-bag-type material. They are much more delicate and stretchy, but that plastic is THIN and lets you put a (relatively) lot of parachute into a small space. (Find a drycleaner bag with a dark pattern or label printed on it.)

Or, fold the chute so you have eight layers of flat plastic (paper-doll fashion, if that makes sense) and punch a whole bunch of holes through all the layers with the sharpened end of a metal tube. Or, fold the chute so you have eight layers again and trim off a width of the middle of one edge so you have eight slots in the opened canopy (don't forget to leave the center connected, as well as a continuous rim). Or, start with the slotted chute and remove every other gore to leave an "iron cross" shape.

Gotta keep those kids happy!

 

[hardinlw]

Here are the relationships you need. The drag is given by :

D = CD * q * S

where D is the drag in pounds which must be equal to the weight of the rocket, q is the dynamic pressure in pounds per square foot, and S is the parachute area in square feet.

CD is the drag coefficient. For a flat sheet parachute, use CD = 0.75 and for an elliptic or spherical chute (one assembled with individual gores) use CD = 1.5 which is the theoretical maximum for a parachute.

The dynamic pressure can be calculated by :

q = 0.5 * Rho * V * V

where Rho is the air density (at sea level it is .00238) and V is the desired sink speed in feet per second. A good target is 15. At sea level, for a 15 fps descent, q is .268 pounds per square foot.

If you know your rocket's weight (in pounds) you can calculate the necessary area using :

S = W / CD / q

Remember to convert W to pounds. 16 ounces = 1 pound.

If you know the parachute area, you can figure how much weight it will support using :

W = CD * q * S

Finally, the area of the parachute in square feet is given by :

S = Pi * D * D / 4

where Pi is 3.1416 and D is the diameter in feet (inches / 12)

If you know the area and want to find the diameter use :

D = Square Root (4 * S / Pi)

D as calculated by this formula is in feet which you multiply by 12 to get inches

 

[Handeman]

In the techniques forum, there was a thread on using mylar balloons to create parachutes. I had a couple around, and my kids each made a fliskits whatchamacalit. I am upgrading the streamer to a parachute, but the whole piece of mylar would not fit in the tube, so I've been trimming it down to fit. I'm left with a much less impressive looking parachute, but it's still more substantial than a streamer. Is there a formula for rocket weight: parachute diameter?

Since your son love parachutes so much, you might want to consider getting one of the short fat rockets so you can put large chutes in it and the large size will limit the altitude. An Apogee Invader might be a good kit. It comes with a 24" nylon chute but you could swap out that for your home made ones. It gets just over 300 ft on a D12-5 and should be closer to 150 - 200 ft on a C11-3.
Also with the 3" OD, you could fill it with small toy solders, each with their own chute and then he would have lots of them to chase.

 

[BobH48]

You can use the parachute size calculator on EMRR.

https://www.rocketreviews.com/cgi-bin/resources/recoverybox.cgi

I can't imagine a Whatchamacallit needing anything bigger than a 8" chute.

You can get a plastic tablecloth at the dollar store that will be big enough to make dozens of small chutes and will pack smaller than the mylar chute.

 

[bobkrech]

I agree with BobH48. Spend $5 dollars at the dollar store and get 5 different colored plastic table cloths. You can make something like 100 8-10" chutes for a whatchamacalit from each tablecloth so you can make a bunch of square, hexagonal, octagonal and round chutes to keep your son happy.

Bob

 

[KerryQuinn]

One more example where nose blow doesn't work... Perhaps you more experienced guys already have a rule of thumb about this but it was news to me....

I attempted a scratch built flight this weekend of an 18mm, 3FNC rocket 24" overall length with nose-blow recovery. The small nose cone had nowhere near enough drag to overcome the rocket wanting to lawndart/coresample straight down from about 500'. It was ugly to watch (fortunately was far out into the field from the flight line.)

-Kerry

 

[The EGE]

One more example where nose blow doesn't work... Perhaps you more experienced guys already have a rule of thumb about this but it was news to me....

I attempted a scratch built flight this weekend of an 18mm, 3FNC rocket 24" overall length with nose-blow recovery. The small nose cone had nowhere near enough drag to overcome the rocket wanting to lawndart/coresample straight down from about 500'. It was ugly to watch (fortunately was far out into the field from the flight line.)

-Kerry

A 24" long 18mm rocket is over 30:1 length/width ratio and is very stable, even without the nose cone on. I only use nose-blow recovery on shorter rockets, where the body is not overly stable on its own, and where the nsoe cone is heavy so that it and the body tumble roughly equally.

 

[MarkII]

Another example where nose-blow recovery is not appropriate: when the RSO says that your rocket must have a recovery device.

You really ought to be able to fit a Mylar parachute into the Whatever's BT-20 sized tube. An 8" or 12" diameter one should fit in without any difficulty.

MarkII