Mean Machine Mk. II

In an earlier post, I review the Estes Mean Machine (001295) and discuss the benefits and shortcomings of this really great rocket. With this build, I’m going to try and address some of the shortcomings of the “out of the box” build of this model.

Planned mods:

  • Replace 3/16 launch lugs for 1/4″ lugs + rail guides
  • Ejection charge baffle
  • Mid-body separation
  • Piston ejection with dual chute independent recovery
  • Payload bay

My construction techniques will tend toward “over built” for several reasons. First, I usually fly on smaller fields, so quicker descents are desirable. “Hard and fast” is far more preferable than waiting for the 40′ tree I’m hung up in to fall down so I can get my rocket back. Second, when I do have the opportunity to fly in larger spaces, it’ll be fun to put some higher thrust composite motors in this rocket and try to make it look small. I’ve simmed up through “F” RMS motors in OpenRocket and all the results look good. Third, this rocket is just plain fun to fly. It looks great on the pad, at launch and in the air. It’s going to see a lot of launches, and I want to minimize turnaround time and repairs.

Design Choices

Launch lug changes
The kit provides 3/16″ launch lugs, which require a Maxi Launch Rod. Personally, I already own a pad that has 18″ rod, and a pad with a 1/4″ rod for mid-power rockets. I don’t really want to have a 3rd rod for the “crossover” class of rockets that the 3/16″ rod is intended for. Also, this rocket “stock” flies on D or E engines, and a 1/4″ inch rod is recommended for anything over a D impulse engine. Finally, this tall rocket really needs all the support it can get. On a thinner rod, this thing really sways around in the slightest breeze. All you need is an unexpected gust to come up at motor ignition, and suddenly you’re dealing with a land shark.

I really prefer to launch this rocket from a rail when one is available, so adding rail guides is a natural choice. I don’t own a rail, and will likely want to launch this rocket when I’m not at a club launch, so in the end, I’ll have 1/4″ lugs on one side, and rail guides on the other. This provides the additional support this rocket needs in all but the calmest conditions and allows me to launch when and where I want.

Ejection Baffle:
Anything over a BT-50 requires quite a lot of wadding to protect your chutes. I spend enough on “expendables” buying motors, so the less I spend on other one-time-use items the better. Yes, I could use “dog barf”, or cellulose insulation from the local Home Depot, but that’s one more thing I have to haul to the launch field and store at home. Also, there have definitely been times where ejection wadding has been forgotten for any of a few different reasons (having a 10 y.o. First Officer is definitely one of them…). Eliminating sources of human error is solid engineering, and one less thing to do at launch prep reduces turnaround time.

Singed parachutes are no fun. So, as a rule I try to use an ejection baffle in any rocket in which one can be reasonably installed. Beyond parachute protection, baffles provide a convenient and robust anchor point for your shock cord … another mark in their favor.

Mid-body Separation:
The stock build of this rocket deploys a single 24″ parachute in traditional fashion behind the nose cone. This is simple and effective, but a 24″ parachute is pretty floaty, and provides ample opportunity for the rocket to drift far beyond the desired LZ.

This recovery choice is fine on larger fields, and provides a dainty enough landing for the relatively delicate airframe. If you want to come down faster, the thin body tubes are liable to kink and the fins are going to take a beating.

One option to ensure in-field recovery would be to use a Jolly Logic Chute Release, but unless you already own one or want to invest in one for future use, that’s a pretty extravagant expense for this model. Another option is to reduce the mass that’s going to be hitting the ground. You don’t really want to build this model any lighter than it already is, so the next option is to break up the mass.

Splitting the rocket in the middle at ejection and having the two parts descend independently turns it into two 3′ rockets coming back to Earth, rather than a big 6′ long rocket. The body tubes will be spared the long bending forces, and the fins won’t be subjected to the full weight of the whole airframe.

Piston Ejection / Dual chute independant recovery:
Splitting the airframe in half at ejection presents some interesting challenges. One is that you don’t want the 3′ long upper section smashing into the lower section. There are certainly many ways to mitigate this problem, but I thought it’d be fun to have the upper and lower sections come down independently under their own ‘chutes.

Parachute deployment of the upper half isn’t a concern. When it falls away from the lower section after separation it’ll pull the chute out on its own. However, this does leave the question of how ensure the chute for the lower section will deploy.

In “traditional” ejection, the nose cone will drag the chute out. It’s possible, maybe even likely, that the ejection charge will push the lower chute out, but that leaves us with no redundancy and a failed deploy will likely result in the lower section coming home ballistic and taking a nice core sample of your flying field.

To resolve this, I’ve opted to go with a piston ejection system. A piston is made from a piece of coupler with a bulkhead and a hole through the middle. The shock cord passes through the bulkhead with a knot on each side to keep it in place and is anchored to the ejection baffle below.

During launch prep, you slide the piston into the body tube, and then you follow it in with the lower chute, and then the upper chute. When the ejection charge fires, the piston will act like a sabot. The pressure wave from the ejection charge will drive the piston forward, forcing the upper body to separate and push the parachutes out ahead of it.

Payload bay:
Now that there’s a bulkhead at the mid-body coupler, this creates a large sealed space in the forward section of the rocket. Since I’m not going to be using the twist-lock connector at the mid-body point, I now have this part to spare.

This provides the opportunity to move the twist-lock coupler to the first body tube junction providing access to a nearly 3′ long payload bay. Of course, this volume is not likely to be necessary, so I’ll add a ‘floor’ with a bulkhead at the bottom of the lower twist-lock connector.

I’ll now be able to give random grasshoppers from the local flying field the ride of their lives in a roomy cabin. Of course jelly beans, action figures, and even boring things like altimeters are also a possibility.

Another advantage of moving the twist-lock connector up to the front of the rocket is that it now breaks down into three pieces, making transport and storage that much easier.

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