My first thought was to use a PIRM2 system, but I wanted easier redundancy. With an internal body tube diameter
of 4.75" I'll have more room to play, so I started thinking. What about a mortar tube system, using two separate
drogues and a main.
In the sketch above is a scale drawing of a top view of the body tube. I'd have room for 2) 2" diameter tubes to hold a 36" drogue chute in each tube. The large open area next to the two tubes could be used to hold an oblong mortar tube for the main chute. Each of the tubes would have a cover to prevent ejection gases from entering the tube. Each tube would need a small vent to prevent the cover from popping off as the ambient pressure decreases, a weak glue, wax or even tape could help hold the covers in place.
I added the oblong mortar tube for the main chute and 2 ejection charge holders for apogee ejection.
Here's how I think it should work. At apogee, two separate altimeters/timers would fire one ejection charge each. The nose cone would be displaced in the normal fashion and one of the drogue chutes would be pulled from a mortar and be deployed. This gives me two shots at getting the nose off and deploying the first drogue.
The next drogue will use the flight computer with three channel deployment to deploy the second drogue several seconds after apogee, in addition, a second charge in the mortar tube would be hooked up to a timer, again with a several second delay after apogee. The timer would also be connected to a charge in the first mortar tube to give the first drogue a third deployment charge. Now I have three more shots at getting a drogue out of the rocket.
Then of course there is the main chute, with two charges, one to each altimeter for main deployment. In this scenario, if both of the drogues failed to deploy the main is still has a Hail Mary chance to slow down the rocket. With five chances at getting a drogue out, it's unlikely the main will be alone.
So what could possibly go wrong with this configuration? My initial thought was that the second drogue could tangle in the first. But if I connect the shock cord to the outside edge in the body tube, the rocket should be held under the chute at a slight angle, preventing a tangle. Could the two drogues do a dance and tangle after they deploy? It's been my experience they won't, as the spill of air from under the chutes tends to hold them apart.
Here are the descent rates under the chutes assuming a weight of 42.23 pounds:
One 36" drogue: 82 fps
Under 2) 36" drogues: 58 fps
Under both drogues and a 72" main: 33.5 fps
Under one drogue and main: 36.7 fps
This design eliminates some potential problems. The early deployment on the A2MD shouldn't be a big problem here. If a drogue charge goes off prematurely and shreds one drogue, the second drogue is on totally separate electronics and in its own mortar tube, and should still deploy in a normal fashion at apogee. I'll use three separate power supplies and electronics, the likelihood of three systems all failing has to be close to zero. Three sets of electronics, three power sources, seven deployment charges and three chutes all capable of deploying on their own. That should do the trick, but to be sure...
I think I'll flight test this rocket on a small motor first. An "L" class APCP motor should give me adequate thrust off the pad, and a nice little pop up flight to 2,500' to test the electronics and chute deployment. Now I'm really starting to get excited, there's a lot of construction ahead of me and I love working out all the details.
5 July, 2006:
Work has started on the recovery system. Above is an aluminum disk I turned down to fit inside the upper body tube. The disk will form a bulkhead, above the bulkhead will be the parachute mortars, below the bulkhead will be the electronics bay. Notice I turned an o-ring groove in the bulkhead, no deployment charge gases in the electronics bay for me!
Here's the bottom, or electronics bay side of the bulkhead. The bulkhead is 4.74" wide and 1" thick, the bottom was bored out so the bulkhead is .45" thick to save weight.
Next I chucked up the bulkhead in my lathe to hold it while I cut the threads for deployment charge holders. 5 charge holders will screw in from the electronics bay side, and two long holders will screw in from the top side.
Looking at the electronics bay side, you can see the 5 short charge holders in place.
Here is the top side with the 2 long charge holders screwed in place, as well as the 2 aluminum drogue mortars and fiberglass main parachute mortar set in place.
Here is a view from the top end of the system. The fiberglass tube needs trimming and all the tubes need small retaining brackets installed.
1/4" x 2" NPT pipe holds about 3.7 grams black powder.
Here you can see the small brackets holding the mortars in place. I drilled and tapped blind holes in the bulkhead for #6-32 stainless steel machine screws.
Here's another top view, the 9/16" tubular nylon shock cords have been tied to the forged eye bolts and are ready to connect to drogue chutes.
I used electrical tape to put some tension on the tubes and help retain them, the electrical tape was then covered with some foil tape for protection and added strength. You can see the white silicone caulk at the base of the tubes to seal them to the bulkhead.
In this photo I have three charges installed, I used whatever fittings I had to close off the other open ports. All the various deployment charge ports are labeled as well.
Here are the covers over the mortar tube openings. Bottom left tube has the shock cord pass through a hole and attaches to the nose cone. The other two covers could fall free of the rocket, so I attached them with some o-ring cord. I used two 3/8" wide strips of foil tape to secure the covers.
7 July, 2006:
To assure venting, I drilled a 3/32" hole a inch from the top in each of the mortar tubes, and another in the upper body tube 6" from the top of the tube. 2 holes were drilled and tapped for #4 nylon shear screws to retain the nose cone.
I did my first deployment test today. I only loaded the primary charges for each parachute, three charges in all.
Charge #1: Apogee Nose Cone Charge; 4.8 grams
Charge #2: Redundant Drogue Charge; 1.8 grams
Charge #3: Main Chute Charge: 2.4 grams
The apogee/nose cone charge was loaded into a 24" x 1/4" aluminum tube, then gently packed in with a wad of tissue paper. I sealed the paper in place with 9 drops of wax.
Redundant drogue charge was loaded into the short 2" x 1/4" pipe, packed with tissue paper wadding and sealed with about 1/4" of wax.
The main chute charge was loaded into another short 2" x 1/4" pipe, packed with tissue paper wadding and sealed with about 1" of wax.
Click Here for a 30 second video of the deployment test. 2.72 MB.
The nose cone charge was way too light, the nose cone did shear the nylon screws and eject, but just barely. The nose cone needs to come out with enough force to pull the first drogue out, which it didn't. Of course, in a real flight I would have had a back up charge in case the drogue wasn't pulled out.
The redundant drogue deployed perfectly, no changes look to be needed.
The main parachute charge failed to deploy the main chute. I was a little afraid I'd overdone it with the wax, and I was concerned the charge was so well sealed the aluminum pipe charge holder may explode. While the pipe held the pressure, the charge went off like a shotgun and blew right through the cellulose wadding and the parachute. A little less confinement is definitely in order.
I think what I'll do is confine the charges in surgical tubing. I already ordered several sizes of the tubing, that should solve the black powder not burning at altitude problem, as well as relieving the over-confinement problem in the aluminum pipe charge holders. I'll still use the charge holders, but I'll simply extend the charges out of them. The 3/8" wide strips of foil tape holding the mortar tube covers weren't enough, I'll increase the tape width for the next test.
Early next week the surgical tubing should be in, so I'll do another test then.
I decided to do another test of the nose cone/primary drogue deployment to get an idea of how much black powder I would need. For this test I used a double layer of latex glove fingers to hold a 10 gram black powder charge. Judging by the first test, even 10 grams might be a little light.
Click Here for a short video of the test.
This test went much better, the nose cone shot up a good 10' fully deploying the drogue chute. I need to keep in mind this temporary nose cone is much heavier than the working nose cone will be, so the working nose cone will travel much farther. Even with that, I think I'll go with a little more charge still for the next test. I think about 12.5 grams should do the trick.
For the third ground test of the recovery system, I used latex tubing for deployment charge holders. The nose cone apogee charge holder was large 5/8" OD x 1/2" ID latex tubing, the charge holders for the back up drogue and main chute were smaller 3/8" OD x 1/4" ID.
Here is the nose cone with bulkhead installed, short it's U-bolt to hold the shock cord. You can see the deployment module assembled with the 10 gram BP charge in the latex tubing. I used 3/4" wide strips of foil tape to retain the tube caps.
Again for this test I only used the primary charge for each parachute. After setting up in my yard, I fired each charge, first the nose cone apogee charge, followed by the back up drogue and finally the main charge.
Click Here for a short video of the test. 1.64 MB, 17 seconds
It turns out I made a stupid mistake, I didn't tie the shock cord to the nose cone! The nose cone flew about 50'
in the air, bounced once and stuck nose first in the ground. The nice new nose cone didn't mind a bit, as it looked
as good as it did before the test. It really is very strong! I'm not sure what made a bigger difference; the BP
latex tube holder, the lighter nose cone or maybe a better seal in the body tube, but the 10 grams really shot
the nose out this time. Of course, it should have pulled out the drogue with it, which would have slowed it down
a bit too.
Next was the back up drogue with a slightly larger charge than last time of 2.0 grams. As in the last test the back up drogue deployed perfectly.
Last was the main chute with a 2.6 gram BP charge. The main barely made it out of the upper body tube. Yes, it would have deployed but just barely. Most of the shock cord stayed in the mortar tube. The main chute is going to be a problem. It's such a tight fit in the mortar tube. More BP will help of course, but somehow opening up some more room in that tube would really help. I really only have two options, I could go with kevlar shock cord, because of its added strength I could use smaller diameter cord. Or I could make a lighter parachute. I'll try it again with a larger BP charge, and decide after that test what to do.
Here are the charge holders after the test. You can see each one burst right about in the middle of the tube.
I performed a second recovery test today. This time I only loaded charges for the nose cone apogee charge and the main chute charge, as the back up drogue has deployed well in each of the previous three tests.
Click Here for the video. 15 seconds 1.5 MB
The apogee nose cone charge was again 10 grams. The deployment was very crisp, with the nose cone reaching the end of the shock cord before slowing down. I could either lighten the BP charge or lengthen the shock cord. I think I'll end up reducing the BP charge to 9 grams, and extend the shock cord another 5 feet or so.
I increased the main chute charge to 3.6 grams and it seemed to be just about perfect.
Assuming a burn out weight of 52.4 pounds, using the parachutes I have now would result in a descent rate of about 35 fps or 23.8 mph. That is a little fast, I'm sure the rocket can handle it though, as lesser built rockets have taken even higher landing speeds with no problems. Now, I wouldn't want the rocket landing at this speed on concrete or frozen ground... But our soft soil is rather forgiving. The next test will be a low altitude flight to test the system in the air.
Part of the redundancy of the recovery scheme is to use one of my little timers on the drogue chutes. I wanted to use a pull wire, but being a minimum diameter rocket there was no body tube to run the wire pull wires to the base of the rocket. I didn't really want a hatch either, so I decided to run a couple of small bolts as penatrators through the body tube. I needed these bolts to be accessible, so I drilled the two holes below the electronics board and above the mid body coupler. I had just about 1.5" of free space so this wasn't a problem. In the picture above you can see the holes with nylon bushings inside to prevent the bolts from making electrical contact with the body tube.
I used fiber washers on each side of the body tube to keep the bolt heads and nuts isolated. Inside the body tube the bolts connect to two wires with plug connectors at the ends. In this picture I have a test wire clipped to the bolts. At the launch, the wire will be retained, (tied or taped) to the launch pad. When the rocket moves up the rail the clips will pull free of the bolts leaving the wire behind and starting the timer.
Here is the altimeter side of the electronics board. You can see the back side of two double pole single throw switches used to turn on all the electronics. Two of the four batteries are also retained on this side of the board. Deployment charges screw into the bottom of the bulkhead and into the mortar tubes. At the very upper left of the picture you can see the two plug connectors that will mate to the plugs on the body tube for the pull wire.
Here is the other side of the board with the last two batteries and the timer. The switches are activated through one of 3) 1/4" ambient pressure vent holes in the body tube surrounding the electronics. It's really important to drill the hole in the exact location of the center of the switch toggles. I've missed before, so now I measure about 4 times, making sure the rotation of the tube is aligned, then drill the hole. I practice a few times flipping the switches to make sure I've got the feel for where they are.
The first flight of the Defiance has been performed, the flight was done to test the deployment system in Launch Test 132. While the flight was successful, the main chute tangled in the nose cone causing a somewhat harder landing than intended. To remedy this, I've reconfigured the deployment system to use a PIRM2 to pull the main chute out of a deployment bag. The dual drogue system is still in place, so that much redundancy remains.
Here is the deployment module with the PIRM2 installed. Laying beside is the new deployment bag and a new 10' diameter parachute. The two 9 gram apogee charges are next to the deployment bag.
This is the switch/timer side of the electronics module. You can see all the hardware connecting to the PIRM2 as well. Things are pretty well packed into the upper tube area. Virtually no space is left unused. It's always a challenge with a minimum diameter rocket to find room for the recovery system. Especially with a system as complex as this. Some day I'll have to do one of those big dumb rockets that are 10 or 12 inches in diameter with a little "O" motor in it, boy would it be easy to build a deployment system in a rocket that size!
Here is the altimeter side. You can see all the wires coming and going from the two altimeters to the 7 different charges. Well, five charges now and 2 to the PIRM2.
The Defiance was launched a second time in Launch Test 133, deployment and recovery went just as expected other than a dead battery failed to deploy the back up drogue. Testing of the battery later indicated the battery was only sourcing about 1.4 amps, odd, because pre flight it had tested at over 4 amps. I started a page on making homemade flat sheet parachutes, this page shows both the new drogue and 10' main chute used on the Defiance.
A couple of little items I wanted to take care of before the full load launch of the Defiance. First, I received my Missile Works RRC2X 40K altimeter so I replaced the Translove P6K with the new 40K unit. The Transolve has proven a dependable workhorse altimeter, but isn't designed for a flight of this nature. I also added one more bracket to hold the electronics board in the electronics module (see the picture and arrow). I wanted a little more strength holding the board in place, now I have two brackets on one side and one on the other side.
Let me explain how I see the recovery.
In a perfect deployment the rocket reaches apogee, and is moving at a very low forward speed, the nose cone blows and the drogue chute is pulled out of the upper body tube. What can happen, is the nose cone reaches the end of the shock cord with excessive velocity, and slingshots back at the upper body tube. This slingshot can cause nose cone and/or upper body tube damage, but probably isn't going to affect the deployment. To eliminate this damage, it is generally advisable to use long lengths of shock cord so the nose cone can slow down due to air friction before it reaches the end of the shock cord.
In a more typical situation, the rocket makes an arcing apogee and has fairly high horizontal speed. In this case the nose cone is blown off pulling the drogue out, but rather than snapping back at the upper body tube, the nose cone and drogue are immediately subjected to the fast moving air flow. In this fast moving air stream the nose cone and drogue trail behind the rocket, so the drogue opens behind the rocket, that means the shock cord goes out the upper body tube, then makes a 180 degree turn over the lip of the air frame and goes back behind the rocket. This is what causes a zipper, or a cut down the length of the upper body tube from pressure on the tube from the shock cord. This is also the location of the broken shock cords, right where the shock cord meets the body tube lip.
To prevent the shock cord from being cut over the lip of the upper body tube, you can sand down the sharp edges on the lip, use a Kevlar protective sleeve over the shock cord, or use a Kevlar shock cord for the entire length.
Back to how this applies to the Defiance:
In this picture you can see the recovery components laid out. I installed everything as it would be in a flight, then pulled out the primary drogue as the nose cone would. Then I pulled the PIRM2 release pin and pulled out the main chute. The back up drogue is still in it's mortar tube.
I had a decision to make, either use the 9/16" tubular nylon as I had been, or to switch to 5/8" Kevlar shock cord. Of course, that's really a no brainer, the Kevlar is rated at over 10,000 pounds breaking strength, isn't nearly as likely to get cut on the body tube lip and isn't susceptible to burn through from a deployment charge like nylon is. But the Kevlar is bulkier, and will take up more precious space. With no room to spare I'll have to use less length of the Kevlar. So I weighed the disadvantages of less length versus the advantages of using Kevlar. I decided to go with the Kevlar on the primary drogue, and also from the PIRM2 to the main anchor point on the rocket.
So I assembled everything using the Kevlar shock cord, to make sure it fit without binding and was long enough. It all seemed to work well, even though there is only 22' of shock cord now on the primary drogue chute. The other Kevlar cord going from the PIRM2 to the main anchor isn't so fussy about length, it just has to be long enough to allow the main chute to be fully pulled out of the upper body tube. Which at 20' it is long enough.
The back up drogue is still on 9/16" tubular nylon. Because I have a deployment charge and wadding in that mortar tube, there wasn't any way to get the 5/8" tubular Kevlar cord in there.
I'm a little more comfortable now using the Kevlar shock cord, that should eliminate one more potential failure point.