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- May 9, 2018
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Been working on a prototype 2-stage BP powered rocket for our WTSV advanced summer camp project. Hadn't done gap staging before so wanted to run a test before I ask kids to build a bunch of rockets and have a bunch of failed flights. I wanted to avoid the zero gap staging with scotch taped motors because it requires friction fitting both the booster and sustainer motor and I find that difficult to get correct. So I wanted both stages to be able to use engine hooks to hold the motors in place. This was the motivation for gap staging. The basic design is 18 mm motor tubes in BT-50 airframe tube. The booster motor tube is 3.5 in long and buts up to the sustainer motor tube, which is a standard 2.75 in length. Figure 1 shows an image of the design from OpenRocket. The coupler tube has been removed to show detail of the motor tube connection. Figure 2 is a photo of the as built design. You can see the motor clips in both booster and sustainer, the coupler, and one of the vent holes in the booster.
Figure 1: Motor tube detail
Figure 2: Photograph of booster and motor mount of sustainer
There are three 1/4 in diameter vent holes in the booster. This design was based on Harry Stine's guidance in "Handbook of Model Rocketry" 7 Ed, pp 164 - 168. Harry used two 1/4" vent holes, 180 degrees apart. I used three hole because I'd made a three fin rocket and the the holes had to fit between the fins. The design makes it easy to install the motors and assemble the booster on the sustainer. The motor clip on the sustainer, just slides inside the top of the booster motor tube. I did not use any tape. The friction of the coupler is all that holds the booster to the sustainer.
To validate the design, I performed one static test using Estes motors, B6-0 to B6-6. My test stand consisted of simply duct taping the sustainer to a metal pole in my side yard. The test work just fine validating the design. I took video of the test. Figure 4 shows four frames from the video of the test. Frame 1 is before booster burn through. Frame 2 is after booster burn through but before sustainer ignition. In this frame you can see the venting and, very interestingly (to me at least) fire balls at the leading edges of the vent plumes. Frame 3 is after sustainer ignition showing the booster successfully separated from the booster. Frame 4 is just after ejection charge ignition, showing the nose cone separating from the sustainer tube.
Figure 3: Images from video of test 1) before booster burn through, 2) after booster burn through but before sustainer ignition, 3) after sustainer ignition, 4) after ejection charge ignition.
So what did I learn?
1) You can design an easy to assemble 2-stage rocket using motor clips for motor retention and avoid the taping/friction fit issue.
2) Harry Stine's 1960s gap staging design works fine.
3) You get burning exhaust out of the vent holes. This was the coolest discovery, at least to me.
I'm looking forward to using this basic design for our advanced rocketry camp this summer. It's the first week of June. I'll report back afterwards.
Figure 1: Motor tube detail
Figure 2: Photograph of booster and motor mount of sustainer
There are three 1/4 in diameter vent holes in the booster. This design was based on Harry Stine's guidance in "Handbook of Model Rocketry" 7 Ed, pp 164 - 168. Harry used two 1/4" vent holes, 180 degrees apart. I used three hole because I'd made a three fin rocket and the the holes had to fit between the fins. The design makes it easy to install the motors and assemble the booster on the sustainer. The motor clip on the sustainer, just slides inside the top of the booster motor tube. I did not use any tape. The friction of the coupler is all that holds the booster to the sustainer.
To validate the design, I performed one static test using Estes motors, B6-0 to B6-6. My test stand consisted of simply duct taping the sustainer to a metal pole in my side yard. The test work just fine validating the design. I took video of the test. Figure 4 shows four frames from the video of the test. Frame 1 is before booster burn through. Frame 2 is after booster burn through but before sustainer ignition. In this frame you can see the venting and, very interestingly (to me at least) fire balls at the leading edges of the vent plumes. Frame 3 is after sustainer ignition showing the booster successfully separated from the booster. Frame 4 is just after ejection charge ignition, showing the nose cone separating from the sustainer tube.
Figure 3: Images from video of test 1) before booster burn through, 2) after booster burn through but before sustainer ignition, 3) after sustainer ignition, 4) after ejection charge ignition.
So what did I learn?
1) You can design an easy to assemble 2-stage rocket using motor clips for motor retention and avoid the taping/friction fit issue.
2) Harry Stine's 1960s gap staging design works fine.
3) You get burning exhaust out of the vent holes. This was the coolest discovery, at least to me.
I'm looking forward to using this basic design for our advanced rocketry camp this summer. It's the first week of June. I'll report back afterwards.
Pointing straight down, in a couple of cases and once in a cherokee-d modded for an e engine, which was a bad batch, and exploded about head level. 
