The Intrepid rocket is intended as a test bed for 2 stage rockets. I wanted a rocket large enough to give me practical data for future, larger rockets, but I had to keep the altitude inside my waiver of about 23,000' AGL. I'd like to stage a 3.5" diameter rocket off the Callisto 6" motor. So I proceeded with a design with those parameters in mind.
The big debate on 2 stage rockets always seems to be whether to drag separate or to have a positive release mechanism and/or blow the stages apart. Both designs probably have their merits, but I looked at a sub orbital 2 stage NASA rocket from years past that used simple drag separation. That sort of sold me on the viability of the concept, and since simpler is almost always better, that's the approach I'll take on this first design. I do however, reserve the right to change my mind down the road!
With drag separation on a rocket this size, the interstage coupler will have to withstand fairly large bending forces with little play and no binding, yet smoothly release the sustainer at booster burn out. In reality, the booster can stay with the rocket until sustainer ignition, at which point the sustainer motor would create separation force. Since this is a 6" to 3.5" the booster should be pulled away from the sustainer by drag force.
The easiest interstage coupler would be to make it out of aluminum, the problem with that is the sustainer is also aluminum, aluminum on aluminum can gall, making smooth separation problematic. But I've worked with aluminum on aluminum before, and found a spray on graphite paint that all but insures the metal can't gall. In fact the graphite paint is so effective, I've found once the outside diameter of an aluminum part is painted, it's impossible to chuck that part in a lathe, it simply falls out.
Here's an Aerolab screen capture of both stages.
A screen cap of the sustainer.
A screen cap of the 6" booster motor.
You may have noticed the booster is a bit of an odd design. The burn is very regressive. That's a result of wanting to use 6" diameter grains but keep the total impulse down. It's hard to design a 6" motor that's only a "N" class. The good thing is that this is going to be a KNSU motor, and KNSU burns well even at very low Kn ratios. This certainly isn't a design for optimum performance. It will however, give me a lot of thrust off the pad to quickly get the rocket stable.
A screen cap of the sustainer motor design.
The sustainer motor is a little more practical. A four grain "L" class KNSU motor. Again, I had too keep the total impulse down to keep the rocket inside my waiver.
This graphic shows the interstage coupler.
Trying to describe the interstage coupler and what goes on in there isn't easy, so take a look at the graphic above. Under the coupler you see a green tube labeled fiberglass parachute tube, which of course will house the parachute for the booster. The problem is, the sustainer motor could potentially ignite while the stages are still together, which would burn the parachute and probably blow it out of the tube. The plan is to enclose the parachute in another fiberglass tube, that tube will protect the parachute and can be secured inside the booster with shear pins. The space between the 6" booster tube and the 3.5" parachute tube will house the electronics. While there isn't a lot of room in there, I think I can make a hatch with the electronics mounted to the hatch door itself.
Here's the start of the interstage coupler. I'm working on the bottom (part that goes into the 6" booster). The inside was bored out to fit the parachute tube, the outside diameter was turned down to fit inside the 6" booster tube. The holes that you can see drilled in the bottom are just to lighten the coupler. In my early design notes I had estimated the coupler weight at 6 pounds. Now that it's done, the weight came in at 5.8 pounds, so I'm right where I expected.
Here's the coupler pretty much done. The holes for weight reduction have been enlarged to 1/2".
The interstage coupler was the most challenging part to machine, and close to the most expensive part on the rocket. I paid about $120 for the aluminum. But I can imagine this part would cost $600 or more to have custom machined at a machine shop. This part also cost a fair bit of blood, when drilling out the weight relief holes a strand of cut metal really gashed my right index finger. Probably should have had stitches, but I'm getting pretty good at patching myself up. Not sure if that's a good thing though... Sad part of this story is that I was really making an attempt to prevent getting cut by flailing metal strands. I used a wood block to deflect the strands as they came out of the bore hole, but one just happened to get by me when my hand was on the feed lever at it's lowest level.
4 April 2011:
Spring being what it is, the weather today was very cold and windy so I didn't work in the shop today. I did want to get something done on the rocket though, as I'd like to fly the rocket at our next launch on April 23. That may be wishful thinking, but we'll see how it goes, if the weather stays reasonably warm I don't mind running some heat in the shop, and if I can get the shop hours in, I should be able to get the rocket completed. So today I took a shot a casting a grain for the 6" booster. I've never tried, nor have I ever heard of anyone doing a 6" KNSU motor. I knew I'd never get a 6 pound cast in one pour, so I'd have to use multiple melting pots. I've been trying to get the local guys to come over and help cast the grains, but no one ever seems to have the time... So, I decided to try casting the grain in two, 3 pound pours. It takes a good 30 minutes to melt a 3 pound batch, so I did one batch, poured the first half of the grain, then did a second melt and finished the grain. I wasn't thrilled with doing 2 pours half an hour apart, but I think the second pour should bond pretty well to the first.
After the second pour I let the grain cool for a good 80 to 90 minutes, then removed the base and pulled the coring rod. The entire process all seemed to work pretty well. Although my hands and arms were about spent after stirring the thick propellant for an hour, one grain a day would be plenty...
I needed a new desiccant container large enough to hold these 6" grains. I ended up using a large cooler, I made a wire shelf in the bottom from an old refrigerator shelf, suspending the grains above a layer of desiccant (calcium chloride).
6 April 2011:
I made some good progress yesterday and today.
Here you're looking through the nozzle end of the booster case. The forward bulkhead retaining ring is actually about in the middle of the tube, while the interstage coupler is attached to forward end.
Here's the interstage coupler with the sustainer slid on.
Here's a view from the front giving an idea of what the rocket will look like.
And here's a side view.
If you look about mid way down the booster you'll see a bolt ring. I turned down a piece of heavy 6061 T-6 tube to fit inside the case, then drilled and tapped the case/ring for 18) 1/4" bolts. That bolt pattern and ring will retain the forward bulkhead in the motor. I'm planning on leaving the ring retainer in the case, it's now permanent. That does lead to some issues though. The forward bulkhead will have to go in from the back of the motor, not a huge problem but I'll have to make some sort of mechanism to drive the bulkhead in squarely, as it would bind if it wobbled when inserted or removed. Then there's the issue of getting the nozzle out of the case after firing. The easiest way to get a nozzle out is to remove the forward bulkhead first, then drive the nozzle out. Since I can't do that, I'll have to make a tool to insert into the nozzle through the throat, then pull it out.
I spent way too much time making the internal retaining ring for the bulkhead. I did it on my mill using the rotary table, I should have just turned it then parted it on the lathe. Next time I'll know better. The bolts on the retaining ring for both the nozzle and the forward closure are 140,000 psi UTS steel, more than adequate for the job at hand. The case has a (room temp) burst pressure of about 1,800 psi, the motor should run under 700 psi.
I laid up the recovery system tube tonight as well. With luck it'll be cured up and ready for installation tomorrow. A big job ahead is cutting the hatch in the side of the 6" tube. I still haven't decided how best to do that. I might just rough it out with a jig saw, then finish it to size on the milling machine. The big job that remains is the 6" fin can. It's going to be a really big fin can, lots and lots of glass work to do...
8 April 2011:
Pictures should make it easier to explain the deployment system of the booster. It really is pretty simple, just hard to explain.
On the left is the motor forward bulkhead, I machined a 1/4" NPT nipple to accept a 3/8" eye bolt, then welded a washer on the shoulder of the eye bolt. The cup the left is the base of a 3.5" tube that will hold the parachute ejection system. The eye bolt goes through a hole in the cup base and attaches it to the bulkhead.
Here's the bulkhead installed in the motor. You can see the threaded hole the eye bolt screws in to. Notice also the picture was taken through a hatch that has now been cut in the side of the tube.
In this picture you can see the cup attached to the bulkhead by the eye bolt.
The cup was simply a base to mount a 3.5" glass tube that holds the parachute. This parachute tube slides into the base cup.
Now the interstage coupler is installed, it has the inside bored out to a length of 2" to allow the parachute tube to slide into the coupler. So the parachute tube is now captured between the cup at the bottom and the coupler at the top.
Here you see the parachute tube and cup through the side hatch.
When the interstage coupler is off the rocket, the parachute tube simply slides out of the rocket. I needed that so I could easily install the deployment charges, and get at the eye bolt for changing out the shock cord or other unforeseen jobs. Now you understand where the parachute goes, and how it comes out of the rocket. While you could just stuff a chute in there like any single deploy rocket. I had other concerns that needed to be addressed. First of all, the rocket may not drag separate and the sustainer ignition would destroy the parachute or blow it out of the tube. So I needed to retain the parachute in the tube and protect it. To accomplish that, I'm going to make a fiberglass shell that the parachute goes into. This shell will be like a clam, a tube closed at both ends, then split down the middle the long way. So you stuff a chute in the clam shell, then slide it into the parachute tube. I'll use a couple of nylon shear screws to keep the clam shell in the rocket until the apogee charge goes off. When the apogee charge goes off, the tube exits the top and splits open, allowing the chute to deploy.
The hatch in the side of the rocket didn't go exactly as planned. I wanted to drill out the four corners with a 1/2" radius, so I'd have rounded corners. A hatch would be made that would drop into the opening flush with the surface. First, I tried using my milling machine to cut out the opening. I couldn't get the tube strapped on the mill table well enough to mill the opening. Next I started drilling one of the corners, but I got greedy and rather than start out with a smaller pilot hole, I tried drilling the 1/2" hole right away. The material chattered and I pretty much ruined that corner. So I ended up just cutting the hatch with a jig saw and an aluminum blade. It went fairly well, of course as with most cutting blades I had to stop periodically and clean out the blade teeth. I'm not too concerned with the drag on this booster, so I'll just make a hatch that covers over the opening and screws right into the case.
Here is the nozzle end of the sustainer motor after painting the end with graphite paint. 4" of the sustainer will slide into the interstage coupler.
I started working on the sustainer today too. I've got the holes drilled and tapped to attach the fin can to the motor tube. I also laid up the fiberglass hatch cover, it's curing now.
12 April 2011:
I've been working on a lot of little odds and ends, one thing I wanted to get done this week was some ground testing of the booster deployment system. Yesterday I did quick test using 1.8 grams of black powder, the parachute tube just about made it out of the deployment tube, but not quite. Considering I didn't have a shear screw in there yet, that wasn't going to do the trick. So today I added the shear screw and tested again with 4.2 grams of black powder. This test was pretty much a "flight configuration" test. The following is a series of video captures of the test:
Just after the charge has gone off you can see the parachute tube leaving the booster.
Here the parachute tube is just starting the "clam shell" opening.
The clam shell is further open at this point.
Now the parachute itself has cleared the clam shell.
The parachute extends the shock cord.
The parachute is fully opened.
Click here for a short video of the test.
Needless to say I was very pleased with the test. It was just what I was hoping for. I'll probably add just a little more shock cord and do a couple of more tests to make sure it's repeatable.
In other work I made a battery holder for the booster electronics. I want the batteries attached to the deployment tube inside the booster body. I was going to make the holder out of fiberglass, but instead decided to give my vacuum forming table a try. I made the vacuum table this past winter, but haven't had the opportunity to make anything with it yet.
This shot was taken just after pulling a vacuum on the parts.
I used a very thick (.1") plastic to form the parts. I did two right away, good thing too because the top part didn't turn out very well. The bottom part came out great. For the mold I used 3.5" PVC cut in half and old 9 volt batteries hot glued to the PVC. I'll cut and trim the holder and cable tie it to the deployment tube.
18 April 2011:
It's been almost a week since the last update, not that I haven't been doing anything. On the contrary I've been busy, even though I may not have a whole lot of hardware changes to show for it.
I've been working on the electronics, primarily sustainer ignition. I had in mind the code I wanted to use in one of my HLA altimeters, but after spending numerous hours writing and testing code, I wasn't getting any closer to what I wanted. I'm still not sure what the problem is, it may be the new editor has a glitch in it, but I had to change directions and go about it in a totally different way. I won't have as many safety protocols in the software, but for this first try that may be better any way. After all that time, what I came up with is a simple minimum altitude the rocket has to reach before activating the ignition sequence of the sustainer. That at least "safes" the sustainer for a grossly "out of the window flight", such as a cato or horizontal flight path (god forbid!).
I've got an HLA altimeter and a Missile Works altimeter in the sustainer. The HLA will light the sustainer after a pre-set altitude and time, after a mach delay the HLA will then sample as a normal altimeter and fire the second pyro channel at apogee. The Missile Works altimeter is set with 15 seconds of mach delay and to fire both pyro channels at apogee. I tested a standard, homemade igniter using the HLA to fire the igniter. I was disappointed on my first attempt when the HLA failed to light the igniter. There just wasn't enough amps available to get it going. For my second attempt I used one of my low current PCB e-match heads with a hot primer coat on it. I'm also using a 50 caliber Pyrodex pellet slid on the igniter wire to aid ignition, the whole igniter head and Pyrodex pellet is then dipped in my standard black powder green meal/magnesium shavings/nitrocellulose lacquer coating.
Here you can the second test is successful, this first stage of burning is the hot tip of the igniter lighting the BP/Mg coating.
Now the Pyrodex pellet burns, now that should light any motor!
Click here for a short video of the test.
The sustainer will have forward/head end ignition. This is the same device I used to light the Callisto Q class motor. It threads into the forward bulkhead via 1/4" NPT nipple. The entire inside is filled with epoxy. To make sure there aren't any air pockets inside, I fill each section with epoxy then assemble the next section.
The booster section electronics are pretty straight forward. A single HLA altimeter set for redundant apogee is mounted on the fiberglass hatch cover on the 6" case. I made a new parachute for the booster section, it's made from super tough 4 ounce coated rip stop. My dog an I have been testing a scrap of the material in tug of war contests, he's about 90 pounds with teeth like a wolf, and we haven't torn the material yet. I'll get a picture of the chute outside in the wind if it ever stops raining...
Here's the start of the fin can. This was a couple of days ago. I'm on the last set of tip to glassing now. I'll have the fin can done it time for launch, but chances are I won't have it all nice and painted.
20 April 2011:
Here's a picture of the new chute for the booster.
This is the sustainer electronics module. The 2 red wires going out the lower bulkhead go to the sustainer forward end igniter.
Here's the back side with the batteries. I solder the power wires to the batteries, you can also see how I mount the batteries between 2 pieces of angle aluminum and wire tie them in place. The end of the batteries with the power contacts are insulated with a layer of EPDM between the contacts and the aluminum. Not that the batteries would short out regardless, as the positive contacts are above the aluminum.
The battery pack has been mounted to the deployment tube in the booster.
You can just see the battery pack inside the body tube, the red and black power leads from the battery plug into mating ends from the altimeter. I'll admit, I'm not a huge fan of crimp on wire connectors, but in a case like this I had to use something to separate the parts. I think the real problem with crimp on connectors comes with time and corrosion (as seen in outdoor or auto use).
Both sets of electronics have been checked and rechecked, I did numerous vacuum chamber tests of both sets and they are working exactly as expected. One last minute change I did make was to the program of the sustainer HLA altimeter. The apogee channel is being used to ignite the sustainer motor, with the second (main) channel being used as a back up for apogee deployment. I decided I should have some mach delay after sustainer ignition, so I added 7 seconds of mach delay.
Here's break down of the flights main events:
| Time | Event | Altitude | Speed (fps) |
| 0 | Booster ignition | 0 | 0 |
| 2.9 | Booster burn out | 1,533' | 1,020 |
| 5.9 | Sustainer ignition | 4,196' | 759 |
| 7.5 | Sustainer burn out | 6,035' | 1,546 |
| 9.6 | Sub sonic | 8,788' | 1,138 |
| 34.7 | Apogee | 20,413' | 0 |
| 453 | Landing | 0 | 0 |
Notice it's going to take a long time for the rocket to land. I made a last second decision to use bigger, much tougher parachute in the sustainer. It's a 60" X form chute I made as a drogue for really big rockets. It's reinforced with 9/16" tubular nylon and 1" nylon webbing over the top, with a double layer of 1.9 ounce rip stop over the apex of the chute. It's a pretty big chute for this rocket, to make it fit without forcing it in there, I wrapped the suspension lines around the chute. I know a lot of people do that all the time, but I've never been a fan of doing it that way. So I did take extra care to think about how the lines will unwrap, to prevent tangling. The idea for this fight is to make apogee deployment as reliable as possible, and I felt the strength of this chute out-weighed the disadvantage of the wrapping technique.
The booster chute should be plenty strong, if a bit undersized. At 60 inches in diameter the dry weight of the booster at 46 pounds, it should land at about 52 feet per second. The landing speed is fast enough that I'm expecting fin damage, that's another reason I'm going to spend a great deal of time finishing the booster fins. They'll probably need repair work after the flight.
A couple of other thoughts. The apogee prediction was getting a little close to my waiver. So I backed off a little on the sustainer propellant. I also had a nozzle with a .85" throat, and rather than opening it up, the backing off on the propellant load killed 2 birds with one stone. The booster fin can is done, I'll probably give it one more good sanding and call it good. I finished casting propellant for the sustainer today, so for all practical purposes the sustainer is done and ready to fly. The booster needs one more grain cast, then a thermal liner installed. That should be done tomorrow.
22 April 2011:
Two days to launch, today was a busy day doing a lot of little odds and ends. John came in early afternoon and we cast the last propellant grain for the booster motor. An EPDM thermal liner was cut to fit, then later after John left I set about trimming the 6" grains to length. Unfortunately I managed to start my last grain on fire when trimming it to length. No real damage but lots of smoke! So John came back and we cast another grain to complete the set for the booster motor.
Here's the new parachute in the clam shell tube.
Here's a first look at the full rocket with the booster fin can in place.
The booster fin can is butt ugly, I sanded down any major high spots but that's it. I warned you earlier I wasn't going to finish the fin can and I meant it! I installed launch lugs as well. One other noteworthy detail is that I overstated the inside diameter of the casting tube in my calculations, I had it at 5.375" and it's only 5.2". So that cuts down some on my expected propellant load, but it's not a significant change.
The waiver is in for a Sunday launch, I called in the NOTAM today and the weather is looking good for Sunday. I decided to cancel the Saturday portion of the waiver since the weather didn't look good, and being Easter weekend it looks like John and I will be the only people flying.
26 April 2011:
The Intrepid flew on April 24, details of the flight are here. Overall it was a good flight, the rocket was nice and stable with the booster supplying a nice kick off the pad. Drag separation appeared to work fine and the flight and recovery system worked perfectly on the sustainer. The only problem with the flight was the booster recovery system activated too soon.
Expected timing of flight events.
| Time | Event | Altitude | Speed (fps) |
| 0 | Booster ignition | 0 | 0 |
| 2.9 | Booster burn out | 1,533' | 1,020 |
| 3.0 | Drag separation | 1,664' | 1,005 |
| 5.9 | Sustainer ignition | 4,196' | 759 |
| 7.5 | Sustainer burn out | 6,035' | 1,546 |
| 9.6 | Sub sonic | 8,788' | 1,138 |
| 34.7 | Apogee | 20,413' | 0 |
| 453 | Landing | 0 | 0 |
Actual timing of flight events.
| Time | Event | Altitude | Speed (fps) |
| 0 | Booster ignition | 0 | 0 |
| 2.9 | Booster burn out | 1,533' | 1,020 |
| 3.0 | Drag Separation | 1,664' | 1,005 |
| 4.5 | Booster Deployment | 3,062' | 865 |
| 7.5 | Sustainer ignition | 5,574' | 740 |
| 9.1 | Sustainer burn out | 7,395' | 1,517 |
| 11.4 | Sub sonic | 10,451' | 1,138 |
| 37.3 | Apogee | 21,200' | 0 |
| 453 | Landing | 0 | 0 |
The two things of note are the early booster deployment and the sustainer was about 1.6 seconds late. I had allowed .5 seconds for the sustainer motor to come up to pressure, but that may have been wishful thinking. So the only real issue I have to deal with is the early booster deployment. I really only have two scenarios in mind. The first, which I doubt is the problem. The booster may have gone unstable and tumbled after separation. I doubt that simply because the booster would have been very over stable after separation. Not being able to see the event was just bad luck. About the only cloud in the area that could have obscured our vision did. The more likely cause was air pressure entered the recovery section of the rocket through the deployment tube around the gap in the clam shell tube. I'm still considering viable options to prevent this from happening again. More on that once I decide what to do.
Here's the sustainer back on the work bench.
This is the bulkhead with the forward end igniter removed from the sustainer motor.
Here's the booster sans fins.
The booster thermal liner and casting tubes.
The booster nozzle came out easier than I expected, and was in excellent condition. Providing I don't break it some how, it looks like it should last for quite a few flights.
Here's the interstage coupler on the booster. Odd where the shock cord broke. The clam shell tube attaches to the shock cord, so it may have slid down the shock cord some and caused it break at that point.
More info coming as I prepare the Intrepid for a June launch.