Category Archives: Midbulkhead

Boilerplate midbulkead

After the previous work on the midbulkhead, I was told that the specifications I’d been working from had changed. So I dumped all that and designed a boilerplate version instead.

boilerplate bulkhead-Study 1-Results-Factor of Safety1.analysis

PDF of print. And yes, it now has four holes, but I haven’t renamed it. It’s made out of a 0.75″ plate of plain carbon steel, and weighs 5.11 lb, compared to 1.31 lb for the 7075/PMMA laminate proposed in the last post.

Rick Maschek asks, You say “I remain not a fan of this overall approach.” Reasons and or suggestions for something different?

There’s a saying that “An engineer is someone who can do for a dollar what any fool could do for two.” At this point we’ve definitely spent a metaphorical three dollars in time and effort on the midbulkhead concept. The best approach to it, since its function is just a check valve, would be to make a check valve. It’d be a bit more complex, but would be easily reusable and easy to test.

More generally, a two burn sugar motor is never going to go to space, the stated goal of the project. A two stage rocket is the way to go and is superior in almost every way. It would require relatively little R&D compared to the four dollars that’s been spent on the two burn concept so far.

If it turns out that it isn’t possible to build a sugar motor large enough to be a first stage for a two stage rocket due to the propellant’s physical characteristics, then it similarly wouldn’t have been possible to do a two burn rocket of the same size. Time spent trying to figure out the two burn architecture is time wasted until it’s been demonstrated that it’s possible to even get one burn of sufficient impulse.

Sandwich bulkhead thinned down

Regarding the latest design, Richard asks, “can the edges be modified to fit with the space shown in the attached sketch?”

richards question
(original PDF: MBH CSD interface detail)

Short answer: no. I reduced the thicknesses as low as I could go and still get reasonable factors of safety. It is thinner, but it’s still not half an inch thick at the edge.

three hole laminate
(as a PDF: three hole laminate)

Further, the retainer ring will have to be machined after a couple complete sandwiches have been made, to find out how thick the final part is including adhesive.

There’s also the question of how it’s sealed into the midbulkhead. I didn’t add an o-ring gland. I expect silicone would work, as long as the parts are kept at a relatively constant temperature.

I remain not a fan of this overall approach.

Three hole bulkhead: revision 2 of a stressed skin approach

Rev 2 of the stressed skin approach to the bulkhead. Holes are added to the PMMA disk, and covers are added over the holes. Each cover is 1.495″ in diameter, to fit through the 1.6″ diameter throat.

Material is changed to 7075-T6 for the aluminum parts to increase the FOS. Contours are simplified to make machining simpler. Top and bottom of sandwich are now identical parts.

three hole laminate-Study 1-Results-Factor of Safety1.analysis

three hole laminate-Study 1-Results-Displacement1.analysis

Cross section view in midbulkhead:
three hole laminate in bulkhead

The holes through the PMMA are 1.3″ in diameter. This give a full area just under 4 square inches, compared to 2 square inches for the nozzle throat. This should ensure that there is no sonic lock in the flow as it goes through the delay disk. Having sonic flow into the aft chamber would result in a large increase in heat flux into the walls of the chamber, and should be avoided if at all possible.

Sandwich construction melting bulkhead

PMMA was shown to be the fastest to erode in engine tests, and is one of the weaker materials tested. My original mass-effective solutions to the problem were discarded when the midbulkhead was machined, so I started over with a stressed skin approach, similar to the concept of honecomb or foamcore composites, where faces of a part take tensile and compressive load while a weaker interior takes shear.

Here the faces are made of 2024-T3, though it could be a steel of similar strength. The upside of the aluminum is that it’s much lighter, the downside is that the firing will likely anneal it and require new parts.

The aluminum faces are bonded to a simple disk of PMMA. Ideally with a perfect layer of cyanoacrylate, as it bonds extremely well to PMMA, but a high strength epoxy would probably suffice as well and give more working time.

The parts assembled in the midbulkhead, the retainer and screws are omitted:
bulkehad in midbulkhead

The analysis of the sandwich with 1000psi on the bottom surface; displacement:
laminate sandwich-Study 1-Results-Displacement1.analysis

laminate sandwich-Study 1-Results-Factor of Safety1.analysis

It could be bumped up to over 2 for the entire part by increasing the 2024 thickness beyond the current quarter inch. The coaxial groove on the top surface isn’t needed for a boilerplate motor, it is a weight optimization. The lower surface does need some thickness removed from near the edge so as to fit in the space provided in the as-machined part.

The port through the center is not shown for simplicity in simulation. A plurality of ports would reduce the odds of the destruction of the lower casing.

This approach is generally inelegant, and would be best replaced with a flapper or retained pop-off valve mechanism, which would eliminate the supersonic jet of gas into the lower casing that this concept is bound to create.

Richard’s chamber separator disk geometry, using polycarbonate, 20% glass filled

An analysis of a CSD based on geometry specified by Richard Nakka. In this run glass filled polycarbonate was used as the material, with the following properties:

Elastic modulus in X: 1200000 psi
Poisson’s Ration in XY: 0.37
Tensile Strength in X: 17100 psi
Compressive Strength in X: 18000 psi
Yield Strength: 17400 psi

Loaded from the bottom with 1000psi, displacement plot:

Loaded from the bottom with 1000psi, FOS plot:

Loaded from the top with 1000psi, displacement plot:

Loaded from the top with 1000psi, FOS plot:

In short, it is unsurprisingly better than the weaker delrin, but retains areas with factors of safety lower than desired.

Richard’s chamber separator disk geometry, using delrin

An analysis of a CSD based on geometry specified by Richard Nakka. In this run delrin was used as the material, with the following properties:

Elastic modulus in X: 448803 psi
Poisson’s Ration in XY: 0.35
Tensile Strength in X: 10805 psi
Compressive Strength in X: 16824 psi
Yield Strength: 9355 psi

Loaded from the bottom with 1000psi, displacement plot:

Loaded from the bottom with 1000psi, FOS plot:

Loaded from the top with 1000psi, displacement plot:

Loaded from the top with 1000psi, FOS plot:

In short, it is insufficiently strong.

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.