As part of a project to build a liquid bipropellant rocket for a Northeastern University aerospace club, I am working to develop a Marman clamp to rigidly hold the two halves of the rocket together during ascent and allow them to separate at apogee to release a parachute.
The requirements for the system are as follows:
- Retain the top half of the rocket during ascent.
- Separate the top half of the rocket at apogee and allow for parachute deployment.
- The mechanism must be mounted at the transition from a 10.75″ body tube to a 6″ body tube.
- The mechanism should minimize weight and volume.
Common ways to address this problem involve pyrotechnics, but we do not have access to reliable pyrotechnic fasteners, and we do not have the resources to develop our own. We do often pressurize rocket bays to blow them apart, but this technique is has been found to be unreliable and difficult to test safely. It also entails certain other inconvenient design constraints. The project leadership determined that an attempt should be made to find an alternative separation mechanism.
I have addressed the problem by designing a Marman clamp. A Marman clamp is a strap tensioned around the interface of two flanges to hold them together. Marman clamps are a standard method for holding two rocket sections together or for holding a payload inside a rocket.
However, pyrotechnic fasteners are generally used to tension the spring steel strap, and we do not have access to these. Instead, I am developing an alternative tensioning and release mechanism that is loosely inspired by a design presented in a US Navy whitepaper for use in small payload deployment (see my whitepaper below for more information).
Just as with a traditional tensioner, the strap tension is introduced by tightening a screw. However, in this case, that screw has very high thread pitch (high lead) and is threaded into a rotary apparatus mounted inside the casing. Since the screw has high thread pitch, the rotary body on the inside of the casing will naturally spin to release the tension in the screw. However, pins are inserted through the casing and into cutouts in the rotary body to prevent it from spinning. The pins are held in place by a wire wrapped around the casing and a short piece of nichrome (neither shown here).
When a voltage is applied to the nichrome wire that tensions the wire wrap, the pins spring free, and the rotary body rotates to allow the clamp to loosen. The video below shows a test of this process with a prototype. In this case, the lead screw is tightened down onto the casing itself since the mechanism is not mounted to an actual Marman clamp. When the nichrome melts, the wire wrap loosens, and the pins spring free.
For this design to be useful, I needed to understand the physics governing the behavior of the clamp and the tensioner. I spent considerable time adapting solution techniques presented in a NASA marman clamp design guide document to determine the appropriate geometry for the v-clamps and the flanges and the necessary preload. Once I got to the tensioning apparatus, I derived a series of formulas from scratch to determine the best geometry for pin cutouts, the loads the mechanism would experience, and the necessary preload in the wire wrap. To facilitate future design modifications and help myself keep track all the calculations, I wrote a brief whitepaper detailing the process.