Engine explosion protection

Added by Damien Hartmann over 9 years ago

In a discussion of the test stand design at my hackerspace yesterday, one idea to increase safety was to add a protection shroud around the sides and top of the engine made of 2-3 milimeters of kevlar and the same width of aluminium. Although that would remove a part of the visual entertainment of an engine test, it would make an appropriate protection for testing hybrid rocket motors.
Any thought on the interest of this?

Replies (7)

RE: Engine explosion protection - Added by Aaron Harper over 9 years ago

This multilayer approach is a good one, but the outer layer of Aluminium should be considered carefully. Aluminium is a frangible material, and the material used in a blast shield should be ductile to prevent the shield itself from becoming shrapnel.

RE: Engine explosion protection - Added by Jeremy Wright over 9 years ago

Aaron - The way you've worded that, it sounds like you would feel a little better about it if the aluminum was the inner layer and the Kevlar the outer. Is that correct?

Damien - Thanks for contributing. I have a few thoughts on the idea.

What size hybrid motor are you going to start out with? What diameter and length are you looking at for it? What will it's peak thrust (or at least the desired peak thrust) be?

For hybrid firings, especially larger ones, I would probably prefer the distance-barrier method. You put the appropriate distance between you and the motor under test (and then some), and you may additionally get behind a barrier. With your hybrid motor I'm assuming there will be at least some metal parts and housings (equals metal shrapnel), and the shroud makes me nervous for two main reasons. First, you can't see what the motor under test is doing. If you think you had a misfire, your options for safe visual inspection are limited (no binoculars, etc). Second, it could give people a false sense of security. In my experience, people tend to stand too close and let down their guard more when there's a shield or shroud, and I'm not sure that's what you want with something like this. As you mentioned there's also the decrease in entertainment value, although safety always trumps entertainment. What I've been doing with Shepard firings so far is placing cameras in the danger zones to capture the action. I can get very close-up high speed video, which is especially good when there's a motor failure (although you might lose a camera in the process). It wouldn't take much to stream that video real-time to somewhere well outside the danger zone. That gives you an up-close view while you could be standing 1000 ft away.


RE: Engine explosion protection - Added by Damien Hartmann over 9 years ago

Jeremy - in the end the Alpha rocket should be in the 200mm diameter, but I expect testing to be somewhere in the 50-100mm diameter range (and closer to 50 I'd say). We still have some calculation left to do to set the length. Regarding thrust, we expect the final rocket to be in the 5-10kN range. Once again, for testing, we will begin far below that (100-300N?).

Your obejctions both makes sense. I indeed feel that it would better to find a safe site with a long distance security barrier, rather than give the false sense of security in a not-so-safe site (with, say, residential buildings not far). And you're right, the entertainment value IS definitely important in an amateur project.

Do you have general guidelines or rules of thumb to set security distances based on the thrust / size of the engine you plan to test?

RE: Engine explosion protection - Added by Jeremy Wright over 9 years ago

Thanks for the information Damien. For our Shepard class test stands, our safe distances are based on the NAR (National Association of Rocketry) guidelines for launched rockets. So, we ended up with a 15 foot (~4.6 m) safety radius for A through D motors, and a 30 foot (~9.2 m) radius for E motors. There's been some discussion that we can stay with 15 feet for all motors since we're holding the motors fixed, but for now we're sticking with the 15/30 setup. The NAR Standards and Testing Committee has a testing manual that gives some general rules on motor testing procedures including safety.

The whole manual is well worth a read, but have a look at sections 4.1, 8.3.5, and 8.3.11.


Below are the safety codes that are referenced in the NAR S&T manual.

Model Rocket Motors (gives our safety distances)

High Power Motors (see "MINIMUM DISTANCE TABLE")

Is that what you're looking for?

RE: Engine explosion protection - Added by Christopher Sigman over 9 years ago

I think there might be a good compromise here, which is blast shielding on one side. This would allow a decreased radius on that side, and the capability of engine inspection from the other. Thoughts?

RE: Engine explosion protection - Added by Jeremy Wright over 9 years ago

Chris - What scenario(s) would you a single sided shield being an advantage in?

RE: Engine explosion protection - Added by Aaron Harper over 9 years ago

Aluminum is not a good material for this device because it is considered a "frangible" metal. "A material is said to be frangible if through deformation it tends to break up into fragments, rather than deforming plastically and retaining its cohesion as a single object" (wikipedia). A better choice would be a ductile, though not necessarily malleable material. This would allow the material to retain it's shape to a degree during an explosion event, and if the deformation became extreme, the material could shear but would do so as one or more large sections, possibly even tearing, but not coming free of the larger assembly. A good example of such a material is mild steel.

If the rocket motor were connected with nylon zip ties to a 22ga steel cradle formed as a 90 degree vee, a blast would be deflected by the angle in an upward vector with most if not all energy moving in a 120 degree arc. In this configuration, the motor's forward travel can be arrested with two tabs on either side of the vee in the front. These tabs should be large enough to cover the sides motor casing, but not so large as to block the ejection charge. Holes should be cut or drilled to accommodate the nylon zip ties and the field of view for the IR sensors.

This configuration is easy to build since it only involves cutting thin steel sheets and bending them. It is also is able to fit any motors up to a radius the same size as the width of the vee cradle, the limiting factor being that the largest motor to still engage the stop tabs when secured would have the ability to scatter debris in a much larger arc than a small diameter motor positioned deep in the vee. As time permits, I will produce some pictures to illustrate this design.