PERSHING MISSILE
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Page Index
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BACKGROUND
One of my first and favorite mid-power kits was the Pershing MGM-31a from The Launch Pad. It's cool because of the extended nose cone/transition area and the nearly non-existent aft fins - the design is remarkably stable despite its look, and for reasons altogether different than the traditional 3FNC rockets. The large profile of the 'booster' airframe and the tapered 'warhead' airframe both serve to move the center of pressure aft, reducing the need for substantial fin area.
True to form for Rocket Team Vatsaas, I felt it was necessary to modify the Launch Pad kit to fit a 29mm motor. It always flew great on an F12 and even better on a G35. It was retired after a recovery system failure on October 14, 2000.
Having this busted rocket on my shelf was the inspiration for building a rocket double the size of the Launch Pad kit. The main airframe of the mid-size version was 2.75" - multiply the dimension times two and it equals 7.5", a perfect application for an 8" QuikTube from Home Depot. (The tube is marketed as being 8" in diameter, but don't let that fool you.)
I found an awesome photo archive at the Redstone Arsenal Website. I used measurements from photographs on this site and from the busted Launch Pad kit to design my scratch-built high power Pershing.
I've actually built two high-power Pershing 1a rockets. As the components for the version 1 came together and the total actual weight grew to exceed the RockSim projections, it became obvious that in order to fly properly it would have to be powered by a J motor or higher. So this became my Level 2 certification rocket. A bit gutsy since it is fairly non-conventional for a certification rocket, but I liked the challenge. v.1 was launched twice - once on an Aerotech I211 (yup - way underpowered) and once on a Kosdon J450, an ill fated flight where the motor suffered a failure of the aft thrust ring, resulting in a CATO.
I opted to start over from scratch rather than repair the CATO'ed v.1. I'll describe their design and build simultaneously since they are virtually identical, and highlight any differences as I go.
You may notice that the overall length of v.2 is longer than v.1. I had used the dimensions from my Launch Pad model and a photo of a similar service missile, then later chose the paint scheme from a photo of an early test flight. It wasn't until I was painting the stripes that I realized the second stage (the section between the trailing edge of the forward fins and the transition) of the test missile was several feet longer than the service round. I incorporated this extension in v.2 since the added length will make it less difficult to stabilize.
These two photos show the difference. Note the width of the uppermost white horizontal stripe - it is almost nonexistent on the missile on the left.
PERSHING DESIGN
Simulation
A design was created in RockSim. Here is the file:
I decided that the best way to create this rocket was to basically build a 54mm inner core and hang the external airframe around it. The base of the nose cone is 3", so the 54mm core expands to that diameter in the transition section. This also serves as the parachute tube.
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Fin Templates
I'm quite pleased with the interlocking design of the aft fins and the aft centering ring. The slot in the tab is for the centering ring.
The forward fins are wedges constructed out of five pieces. The 'thru-the-wall and thru-the-fin' tabs are made from 1/4" birch plywood. The rest of the pieces are made of 1/8" birch plywood.
Details on how all the pieces fit together are on the Pershing Construction page.
Transition Section
I made the mistake of making the length of the parachute tube too short in v.1 (I needed to share some 3" tube with another project), so in v.2 it runs the entire 30" length of the transition.
The transition is a paper cone filled with foam. Brother Rick provided a nifty transition calculator to reckon the arcs. I used the same calculator to reckon the nose cone.
Transition Calculator
Details on how all the pieces fit together are on the Pershing Construction page.
Flexible Flight Configurations
I'm not sure how many ways I'll be able to use this, but as I planned this rocket I decided to make it possible to have the rocket separate at the nose cone or at the joint between the aft tube and the forward transition. If I ever use an altimeter and multiple ejections, I would mount the avionics in a can in the parachute compartment. I can eject one parachute out of the transition joint and one out the nose. The two sections are screwed together for a single ejection.
I had to get creative when deciding on the best way to mount the shock cord when the rocket will have to come apart in a variety of ways. I designed a removable Shock Cord Mount and an Interstage Baffle Assembly that bridges the gap between the forward transition section and the aft airframe. This way I can remove the shock cord mount from the forward section to install the altimeter and ejection charges
Details on how all the pieces fit together are on the Pershing Construction page.
I'm also playing with the idea of building a third Pershing and cutting the aft section apart just behind the forward fins to make this into a two-stage rocket. Not yet, though. I'll let this one fly a few times as a single stage first.
Click HERE to go the Pershing Construction Page for a detailed description of how the 2/5 scale Pershing was made!
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