A new approach to the delay plug, first pass

The current concept for Sugar Shot to Space uses a two burn solid rocket, a novel approach to getting the most altitude out of a single stage. The rocket is two chambers of solid propellant, separated by a delay; the first fires for a few seconds, the rocket coasts for as much as 15 seconds, and then the second firing occurs.

This reduces drag losses compared to a single longer burn, both because the maximum velocity is lower and because more of the burn happens at higher altitude. It also slightly increases the specific impulse (similar to the fuel efficiency of a rocket) and thrust of the second burn because of lower atmospheric pressure.

This is, however, very odd in solid rockets. In most tests the rocket fails during the second burn. It presents a number of challenges, one of which is having something that separates the two charges of propellant but that doesn’t impede the second burn when the time comes for it to do its thing.

The prior approach was a “delay plug” made of slow burning solid propellant. It would burn away sufficiently in the interval between firings that the second could run unimpeded.

Richard Nakka wanted to look at a new approach to it, using a material that is burned away by the second firing. It would have a lower mass, as well as not putting heat into the first burn’s motor casing during the coast phase.

This is the concept specification Richard sent me: DSS Chamber Separator Disc concept. In short, a plastic disk would separate the two motor casings, and would be melted or burned away by the firing of the second burn

It’s a very interesting idea, but I have concerns about the concept. While it would melt and burn away quickly, until it does the hole in the disk would act as a throat. Supersonic exhaust would shoot through the first motor casing until the casing pressurized enough for the real rocket nozzle to act as the sonic choke. It’d be interesting to see it tested, though.

I’ve started on it, and wanted to share the first very rough drafts before I get to a final concept that the modeling says should work.

The material in these analyses is PMMA, a kind of acrylic. It vaporizes well, and has a relatively low melting point, so it would burn away quickly. It’s of average strength for a plastic.

The outer part is a steel sleeve that couples the two motor casings together. It is mild steel. It is sized for the “DSS Boilerplate” engine, it has an outer diameter of 6.35 inches. I modeled a pressure difference of 1000 psi between the lower casing and the top, which creates a fairly huge force on the disk.

First, this is a scenario where a 0.5 inch disk of plastic is bonded into the coupler, both on the edge of the face and the sides of the cylinder. It isn’t strong enough, I’d like to see at least a 1.5x factor of safety (FOS), and ideally 2x.

I wanted to see how it would change if the disk was only bonded on the edge of the face, and not around the sides of the disk. It makes it weaker, but loads the coupler more realistically. Sorry for the color scheme change, but this style is better for a number of reasons. I also removed the screw holes, as they aren’t relevant to the issue being examined.

This is the same study as the last FOS plot, but this is showing how the material will move. It shows it bending in by nearly a half inch at the center. In reality, it would have broken.

This is a different approach, with the disk turned into a dome. Domes hold pressure better, which is why the ends on your air compressor tanks aren’t just flat plates. The situation is significantly improved, with nothing under a FOS of 1, so in theory this would actually hold the pressure.

From here I’ll be doing further work on the dome concept, as well as one with a flat plate reinforced with steel gussets as described in the specification document. I’ll also be looking at other materials, as Richard asked in a subsequent email. I’ll include dimensioned drawings in the next update on this, as well as more FEA images.


One response to “A new approach to the delay plug, first pass

  1. Timothy Young

    If you invert the dome, with the convex facing the pressure of the 1st burn, would you obtain a more advantageous distribution of forces? Somewhat in compression toward the coupler ring?

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