ESTES new kit: MIRV
I am currently building the new kit introduced by Estes recently, known as the MIRV (Multiple, Independant Reentry Vehicle). I chose this kit because it is unique, not just in appearance but it represents a whole different method of multi-staging than is commonly done. The first stage boosts as usual, but it then ignites - simultaneously - three independent second stages! That's the concept anyway, we'll see how practical it is.
What follows is a sort of review of the kit thus far. Estes was not very comunicative with this kit about how it works in their catalog or online, so here's the scoop. The ejection from the 18mm ("standard size") booster stage goes through a pre-molded plastic manifold that is designed to split the ejection charge into thirds, with each 1/3 leading to the nozzle of a 13mm mini-motor. Each of the three upper stages will then launch from the booster which then acts as a moving launch base. The booster has a wooden-dowel version of a launch rod, and the upper stages all have lugs. They fly ahead independentaly while the booster then tumbles the ground. Each of the upper stages then fires its own ejection that pops off the nose cone, and the three stages fall / tumble to the ground with the theory that the added drag of the shock cords and seperated nose cone will let the model fall slow enough. I might want to add a streamer to the shock cords, but I'll try this model stock for the first test flight.
I'm skeptical, as I can see trouble getting one motor to reliably ignite three mini-motors with their small nozzles, but it is worth a try. I am hoping to be the first one at the club's launch to fly one of these, so I can get the praise that comes along with such a unique concept. Apparently this concept is not catching on just yet, as even after a half of a year or so I still don't see any reviews here of this rocket, or even any flight logs on here.
The booster motor is mounted with a traditional metal clip. Estes recommends a B6-0 or C6-0. The upper stages have to be friction-fit. Estes only recommends A10-3T motors, I assume because they have a fast-starting, high impulse to get the stages moving faster off of the lower stage so the fins will have enough air to be effective.
The good and bad of this kit is that it uses pre-molded styrofoam pieces, one for the booster and one each for the upper stages. On the booster, it is really helpful to "automatically" align the fins because they are inserted into slots in the foam block. The purpose of the upper stage foam is, I suppose, to make the model appear as a single, six-sided upper stage instead of three discrete tubes. The top-view outline of the upper stages appears to look like a flattened diamond shape. The three nosecones match this pattern, so when all three are placed together on the booster they form a single, hexagonal shaped nosecone. What's good is that the Styrofoam material is light and produces a shape that would be very difficult to reproduce with paper or cardboard, and is much lighter than that or if it was made of balsa. What is bad about the foam I found out from building: 1) In shipping, it is very prone to surface dents, scratches and such from the other parts in the bag, and even when painted it is a somewhat fragile surface. 2) You must be careful what primer, paints, glue and fillercoat etc. chemicals you use, because some finishing materials can literally disolve the foam. Most distressing to me is that the finished surface will still be prone to knicks and dents, I expect this rocket to look pretty beat-up after a few flights. I had to add wood filler and sand a lot of the dents in the foam just from shipping. I have seen plenty of Estes kits shipped in boxes, so why wouldn't Estes ship this kit in a box is beyond me.
For the upper stages, the core body tube consists of a BT-5 tube which is surrounded on three sides with a molded Styrofoam piece. This piece also provides the advantage of acting as a permanent fin alignment jig, as the fins are glued to each side and one is inserted into a slot in the foam. The foam on both the lower and upper stages makes it difficult to sand the balsa withouth gouging the foam, so I recommend doing all of the sanding - even the balsa filler - before attaching the fins. I should have used a paper covering layer on the fins, which I do to all my rockets now to avoid the filler-sand-filler-sand-filler-sand routine. Paper makes a nice, clean, flat surface free from wood grain, adds strength, and is much easier and cheaper than balsa sanding sealer or fillercoat. (Use photo-mount ahesive sold in spray cans at craft and office stores).
One aspect of this kit that seemed a bit strange to me was how the fins are built up from two pieces of laser-cut balsa sheets each. While I totally get this for larger fins, or fins with unique shapes or strakes, it didn't make much sense to me because these shapes are basically parallelograms that are swept forward. A single sheet of balsa would have been easier to build for sure. I can only speculate that the offsets of the grain of the two pieces add strength to the fins. After all, it is a tumble recovery rocket and the fins may take a hard hit occasionally.
Most of the work on this kit is fin construction, sanding and painting. Each upper stage has three fins, and the lower stage has three, for a total of twelve fins.
So far in building this kit I made two known mistakes which shouldn't be too serious. One mistake was with the engine mount for the booster. The motor tube is glued and inserted into the plastic manifold so the ejection is ducted to three upper stage nozzles. I guess I was asleep at the wheel, because after placing the glue where needed, I didn't manage to get the motor mount tube fully inserted into the manafold. It sticks out about an 1/8 or 1/16 inch - no problem, but I hope that added fraction of an inch of length in the ejection manifold will not cause second-stage ignition issues. I pushed and pushed as hard as I could without buckling the motor tube / plastic and foam pieces, but the glue just stuck like glue before the motor tube was in position.
The other mistake is superficial really. On the upper stage's left and right fins (there are 3 each on three upper stages), I forgot to sand the trailing-edge taper before attaching them to the rockets. Not a big deal.
Another change I made which I consider an improvement was to substitute Estes silly shock mount method with loops of Kevlar, from which I will attach the shock cords. Just looking at the tiny diameter of the upper stage's BT-5 tubes, I couldn't even imagine fitting my little finger in there much less a glue-soggy paper shock cord mount.
As I was building this model, I was looking for a place where I could add a small payload for an Altimeter One, but I couldn't find a good spot without some major modifications in either the booster or the upper stage MIRVs. I tend to think that this design might be a bit trickier though, and less forgiving of untested changes, so I'm fine just following the directions on this model.
That's all for now, the model is in the paint shop right now. I'm painting the booster gloss black, and the MIRVs three different colors: Aqua, Magenta, Yellow. That means the supplied decals will not match, but I'll figure something out.
- Post a Comment -
Getting near completion, I think I have problems to solve... (2012-06-13 19:17:05)
This MIRV is just about done. There are three items that concern me about this particular design. I still don't know how I will address them just yet or if I even can.
First, when I test fit the second stage rockets onto the booster, I basically slide them down on the wooden dowel/launch rod of the booster and push the motor into the manifold. For starters I don't know how the upper stage motors will stay in the manifold long enough to ignite since they slip in rather loosely. It seems to me the initial pressure of the booster will pop them off before they stick around long enough to ignite. Maybe I should use a little masking tape to snug them up? If so, will this prevent them from releasing? Something to consider. The Estes instructions do not mention this at all. All I know from experience is that 2nd stage motors are usually held tight with celophane tape which they melt through to release. Maybe I'll use that tape and hope for the melting.
Second, when the second stages are on all together, they do not stay tight together at the nose. Will this air gap between these rockets cause them to seperate or even rip out when 50-100 mph winds blow down between them during boost? Even if they remain undamaged, it doesn't look very aerodynamic. I was thinking of using some sort of hooks, but I don't know which of the three stages will launch first, so I considered thread holding them together that breaks at launch. It would be hard to determine the thread strength needed with out a lot of trial and error and ensuing disasters. So then I figured small magnets on the nose or just below it, but I'm not sure they would have enough strength to stick with those hurricane-force winds. What I finally decided upon was a little bit of tape or adhesive round sticker on the tip of the nose cone. I don't know if that would be strong enough but at least the wind force will keep it pressed against the three noses while going up the ground-based launch rail. After tha, who knows? The first rocket off of the booster will pull the tape off with it. It's worth a try anyway. Looking at how this thing was designed, I don't understand why they didn't extend the "launch rail" up closer to the tip of the second stages where a second lug could keep the noses tight together. IF the individual rockets have a very big variation in ignition/launch times I could see how they would interfere with each other. It's too late for me to do anything about it now, but in hindsight I would use three much longer launch lugs on a longer wood dowel, so that they would stay together on the booster stage better.
The third item that worries me is the soft styrofoam body tubes. I hoped that the paint would provide a stronger shell to these soft pieces, but as I add spray paint layers to it, it appears to not have much of an effect. I am fairly certian with the short rubber shock cords supplied that those heavy plastic nosecones (yes, they are quite heavy), with sharp angles (they are not very rounded) will punch some deep gashes into the body tubes when they eject and then recoil back to the rocket(s). I could use longer rubber but it would be difficult to put much shock cord in those tiny BT-5 tubes, and it would sure add a lot of weight. Even after that while tumbling down, they'll probably be banging into each other.
Should I trust Estes to have tested and worked out all the problems before shipping this product? I just don't know about that with the way corporations are run these days. Maybe some higher-up just started yelling "Shut up and ship it!" to meet his quarter projections and not look bad.
In my experience as an engineer (but not as a rocket scientist) this is what good engineering is all about. Looking for potential problems and solving them with open-minded brainstorming before it's too late. Well I don't have any plans or solutions to the above problems. I'll just have to go for that first test flight and see what disaster develops if any (that could be fun for all the spectators). Certianly there will be a lot of anticipation just prior to the launch. I must remember to breathe during the countdown so I dont' faint and miss the launch.
It's tough being on the bleeding edge, I wish somebody else here would have done this MIRV thing and wrote a reveiw already!
- Post a Comment -
First test flight (2012-07-05 18:17:08)
I used the lower-power engines as recommended by Estes: the B6-0 booster, and three A10-3T sustainers for this first ever flight of this new and very different rocket. I did not apply the decals, simply because I ran out of time to get them on and dry and I wanted to test fly this at the club launch the next day. I wasn’t too worried of the breezes, since the B6 is a strong booster even for a rocket that is 124 grams, and the A10’s on a 33 gram rocket would do well. (Note that these weights are a bit lighter than the overall weight published by Estes, I presume because I didn't use a seperate primer.)
It did take quite a bit of time to prepare four motors. Also those three wiggly shock cords each had to be stuffed into the tiny BT-5 tubes. The boost phase was as good as can be expected and all parts stayed together. Almost too fast to be seen, the three next stages with higher impulse and less mass left the booster behind rapidly and sailed pretty far up almost out of sight. Fortunately all of them lit. I only heard and saw two of the three upper stages ignite and thought the other one didn’t light or got stuck, but sure enough all three stages were flying high. So it turns out Estes got that right.
The boost was fairly straight and the second stages all continued up and didn’t wander far from each other either. I wondered if the small bit of celophane tape on the nosecone was enough to hold them together, so for the next flight I will not use it. All three successfully popped their nose cones, and they all seemed to come down together in a tight group. The tumble/drag recovery worked fine and all three independent rockets landed within 20 or 30 feet of each other. Luckly, the soft-ish foam bodies did not appear to suffer any recoil dings, but with closer inspection afterwards I found that sustainer #3 has a deep triangular puncture in the foam from the nosecone. The swaying, swinging dance of the three sisters tumbling down was an interesting sight. The heavy nose cones appear to lead, with the lighter rocket body fluttering around behind them. I'm glad I painted my nose cones silver, the glinting in the sun made them easier to see both in the air and on video.
There was no altimeter on board so no detailed data could be obtained. Booster stage seperation appeared to be at about 150 feet, while the upper rockets may have reached about 350-400 feet. Post-flight inspection revealed some interesting anomalies though. First, the booster landed on the sun-tent top of the launch control officer. It was broken (re-kitted actually) in two pieces. The two plastic parts of the ejection ducting manifold had separated at the glue joint – so much for Estes strongly worded recommendation to not use too much glue here! I recommend you use a lot of glue. I will look into the glue type I used, which was plastic cement. It may not have been the right kind of plastic for that glue. Plastic cement is designed to basically "melt" polystyrene and let it re-harden together (sort of like welding). If this is (and it appears to be) a different type of thermo-setting plastic, then it would probably work better with CA glue, epoxy or another kind of cement. I think the plastic manifold broke when it struck the canvas tent top of the launch control officer, and not in flight, since both parts were found together. The booster stage was all quite sooty, and will require a good cleaning after each flight. There is the soot of three motors on it.
As for the upper stages, #3 had ejected its engine casing, although the motor did eject the recovery device just fine before it went away by itself. The #1 (upper) stage appeared quite sooty in the back. It also had a burn-through to some of the masking tape which Estes recommends to be used to hold the motor in. The rocket’s finish was burned a bit. I think I’ll paint the inside back ends with high-temperature paint in hopes to prevent future burning. I actually can't imagine how the backs of the upper rockets would NOT get burned.
My substitution of the Estes shock cord mount with simple Kevlar loops glued to the inside of the tube worked well again. I've tried this before without trouble, not only is it simpler, lighter, but it also allows for untying and changing out the rubber for a new piece or a longer or shorter one. I didn't bother attaching the Kevlar to a loop on the engine mount, I simply glued it to the inside of the tube where you would normally put the folded paper shock cord mount, fraying both ends of a 2-inch piece of Kevlar. I use Titebond III glue which I have learned to trust, it produces a really strong bond that has never failed me yet on wood/paper.
I was forced to ground this rocket with the plastic manifold broken, but I will need to fly this again but using the other recommended booster, the C6-0 motor. Should be a great flight, but the upper stages will probably fly beyond visual range if I fly on a hot hazy day again. They didn't appear to drift very far at all even though it was a windy day, so I wouldn't worry about loosing them, they should all end up near the launch pad. As for the upper motors, Estes recommends A10's only. It appeared the booster was going quite fast while staging occurred, so I imagine that A3 motors might work also.
So in summary, it appears this multi-stage odyssey does work, it flies well and was a bit of a curiosity to even the old-timers. But this "new technology" has some bugs to be worked out. The cons for me would be the considerable prep time (4 motors), the cleaning required afterwards (not only the booster fins etc., but the manifold interior needs to be swabbed out to prevent soot and burned-chunks buildup.). The soft foam body needs to be delt with. Also, the burning of the stages needs to be addressed. If doing this again I would paper-cover all the fins before attaching, and I would prefer to fill in and coat the styrofoam with a thinned layer of white glue for a harder finish. I would also consider using a longer dowel for the launch lug, but these options will certianly add to the weight.
Stay tuned some day for the MIRV tweeks and the next test flight with decals and a C6-0 booster...
- Post a Comment -
All three top stages blow their nosecones and return with a fluttery dance back to earth
As the booster falls away, the three upper stages seem to want to contue flying together.
By now, MIRV has rotated 2/3, showing upper stage #1 (Aqua)
Lower view of the MIRV booster stage
MIRV Clears the launch rod, showing upper stage #3 (Yellow)
MIRV has rotated 1/3 clockwise, showing upper stage #2 (Magenta)
MIRV is now finished with decals
MIRV Nosecones P6240363 small.JPG
MIRV starts it first test flight - Nothing to do now but sit and watch.
Mirv's Aqua tail section
MIRV's Magenta tail section
MIRV's Yellow tail section
Nose Cone Damage to Soft Styrofoam
Note tape on upper #1 is burned away, also part of the right fin's balsa is burned.
Rear view of a single upper stage
Showing the #1 (Aqua) and #2 (Magenta) sustainers
Showing the #2 (Magenta) and #3 (Yellow) sustainers
Showing the #3 (Yellow) and #1 (Aqua) sustainers
Tape holding upper stage motor, this side is OK
Tape on #3 upper stage could not hold onto the motor. The motor ejected the nosecone and itself.
The booster is spent, now the three upper stages take over and still fly in a tight cluster
The combined soot of three A10's left a mess on the booster stage - it was painted GLOSS black
The glue joint for the two pieces of the manifold failed when it struck the tent's tarp. Estes warns don't use too much glue. I say don't use too little!
The MIRV's first/booster stage
The MIRV: Just a hint of smoke appears after ignition.
The tape holding the #2 upper stage motor in place, and also snug on the ignition manifold - all OK
This shows how the upper stage sits on the booster before launch
Top view of the manifold, where the three A10 motors sit before igniting
Well after booster seperation, the three upper stages still seem to want to fly together.
What You Can Do