Designing the All-PVC Amateur Hybrid Rocket Engine

Evan Daniel

Last Updated March 13, 2006

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Goals

There were several goals that drove the basic design. The motor needed to be

• Safe
• Reliable
• Easy and low cost to operate
• Easy and low cost to build
• Capable of flying the intended rocket

There are actually two different vehicles I want to be able to fly -- the Blue Moon, a minimum diameter 54mm rocket (28" motor mount, 4 lb weight without the motor), and the Orange Bird 2, a 4" rocket with a 3" x 44" motor mount and a dry weight of about 13 lb. Desired altitudes are modest (2000-6000 ft), so a J or K class motor would be about the right size. Required thrust is dictated by a 5:1 thrust to weight ratio on liftoff.

Basic design decisions

In order to minimize per-launch labor costs, a hybrid motor was chosen. I know of no way to avoid spending a rather large amount of time casting solid propellant grains, whereas hybrids can burn plastic pipe without too much work beyond cutting it to size. This made for an easy win for hybrids instead of solids (assuming the hybrid could meet the other design constraints).

PVC pipe for both tank and chamber was an easy enough decision. Ease of working with it without a machine shop, precision tools, or welding ability was the main deciding factor. PVC up to 1-1/2" size should be able to hold nitrous pressures to most reasonable launch temperatures, and 2" should work on cooler days. However, a requirement to fit the motor in a 54mm mount makes 1-1/4" PVC the obvious choice -- larger than that, and the fittings don't fit. PVC fuel grains were chosen for simplicity at a modest cost in performance (about 10% vs. a higher performance fuel like polyethylene or polypropylene).

For simplicity of operation, a UC valve ignition scheme was chosen. This uses a nylon fill tube connected to a compression fitting at the bottom of the tank, and a vent hole at the top of the tank. The tank fills through the tube thanks to the pressure differential from the vent. When the tank is full, liquid nitrous can be seen leaving the vent (it forms a highly visible white cloud). At that point, a solid propellant grain at the top of the chamber is lit, which cuts the tube and lights the chamber. The remaining fill tube stub becomes the injector.

It should be noted that PVC fittings vary in dimensions slightly, so you should measure yours to determine exact lengths to cut the parts to so that they fit properly.

Modeling the engine

Stephen Daniel (my father, and the flier of the Orange Bird 2) helped create a computer model of the burn of a hybrid engine. The model was originally based on Todd Moore's Hybrid Design Assistant (HDAS) spreadsheet, version 1.5. Under my influence and with my assistance the model has evolved substantially. The model now more accurately describes the variation of C*, Isp, and Cf as the O:F ratio changes through the burn. We also added correct handling of over- or under-expanded nozzles. The program retains Todd's fuel regression model and a propensity to work in English units of measure. The program also outputs thrust curves in Rocksim 8's RSE format.

Since the program is still not terribly user-friendly, and since we plan a complete re-write, we've chosen not to make the program generally available yet. Once we've completed version 2 and validated it against test firings and test flights we intend to make the program generally available.

Despite the simulator's shortcomings, we made extensive use of it to predict performance, guide the decisions about dimensions of the chamber and fuel grain, and establish minimum safe launch conditions.

Basic engine parameters

Two different designs were decided upon, one with a larger tank and injector for the Orange Bird 2, and a smaller motor for the Blue Moon. For both tanks, 1-1/4" PVC was selected for simplicity of construction.

OB2 motor:

C* is the ideal C* (from cpropep), reduced by a factor of 0.9 to account for incomplete combustion. The value for Cf used is (1 + 0.85 * (ideal Cf - 1)). This is an approximate model for nozzle inefficiency, which keeps Cf > 1. Pe/Pt was kept the same as given by cpropep (ie ideal).

Testing

For static testing, I used a fairly simple method -- I attached some long steel angle brackets to a 2x4 and pounded it about 16" into the ground. A steel plate that once served as a lawnmower mulching insert became a blast deflector, the bottom 2' or so were painted with epoxy to improve survivability near hot exhaust, and the same cylinder used for hydro testing became a load cell when placed at the top.

A line filled with hydraulic fluid and a pressure gauge provided for thrust measurement. For the 3/4" bore cylinder, every 1psi on the gauge corresponded to 0.441 lbf or 1.97 N thrust.

The motor was then attached to the test stand via hose clamps (not too tightly; it needs to move a little to register thrust properly), and connected to the nitrous fill system.

A Volt-Ohm meter made an excellent continuity checker before filling with nitrous -- it should read between 5 and 6 ohms resistance (long igniter wires add a bit).

Finally, fill and light the motor and videotape the load cell pressure gauge to get a thrust curve.

Unfortunately, when I static tested the motor, the pressure gauge failed to read more than 25 lbf thrust -- well below the design goal of 140 lbf. However, the load cell was bent back in the dirt substantially (Whoops! It needs to be better secured next time.), which testing showed required about 100+ lbf thrust, enough to launch the rocket.

The other problem encountered was that the vent clogged during filling, resulting in an underfilled tank and a very short burn. On disassembling the motor, I found small bits of PVC, presumably from drilling and tapping the hole for the vent. (This tank was built slightly differently, to take a 1/8" NPT brass fitting as the vent, so that vent exhaust could be routed to the bottom of the rocket, aleviating the need for a vent hole in the airframe.) These were removed, and the motor prepared for another test. Unfortunately, in this test the fill tube failed to cut for as-yet-undiagnosed reasons.

At this point, investigations were conducted to explain the discrepancy between low measured thrust and the movement of the test stand. I decided (and my father and observers concurred) that the slop in the hose clamp attachment mechanism was sufficient to allow the motor thrust to be directed off-axis, and that a small amount of off-axis motion was sufficient to cause the load cell to read drastically low.

Having decided that the test failures were well understood, we made the (probably unwise) decision to launch the rocket despite bad test data, largely because there were a bunch of people who were hoping to see a launch, not a series of failed tests.

Click to enlarge	Full size image

The Orange Bird Two took flight successfully, however it only reached an altitude of 570 feet, instead of a predicted 3000. It was somewhat wobbly off the pad, but even in relatively stiff wind remained stable. Work is still underway to determine exactly what happened, but there are several theories.

First, there was a larger than expected delay between the end of the tank fill and ignition. When the motor was disassembled to unclog the vent, I ended up enlarging the vent hole by an unknown amount. The combination means it is possible that a substantial amount of nitrous boiled off, leaving a lower pressure and underfilled tank. Testing to determine the effective size of the enlarged vent is planned, along with the construction of a numerical model of the tank fill process, to determine how large a role this played. Early, back-of-the-envelope calculations suggest as much as 25% of the nitrous may have boiled off in this period.

Second, it is possible that the ratio of core area to throat area was too small, resulting in rapid flow of nitrous and combustion products through the chamber. This could cause poor mixing and combustion, resulting in lowered C* efficiency. I plan to test a motor of equivalent injector and nozzle design, but larger core diameter, to compare the results and attempt to shed light on this theory.

Third, it is possible (though I believe improbable) that the nozzle did an exceedingly poor job, resulting in an Isp on par with a sonic choked nozzle (or worse, due to friction after the throat). Comparisons of nozzles are also scheduled.

(Some of the lower performance comes from a colder nitrous supply than the 70F model above; however, the model used was for colder nitrous temperatures.)

Blue Moon motor data

The motor intended to fly the Blue Moon is not as well documented or tested, but details will be forthcoming once more data has been gathered. (A scheduled flight in February was scrubbed due to electronics failures; the static test was scrubbed because of time pressure.) In short, it is a similar motor, with a shorter tank, and a 3/16" tube injector (and correspondingly smaller nozzle). A slightly older design (with an overly large nozzle throat) lofted the Blue Moon into the air in November. Unfortunately, there is no data from this flight, since the altimeter switch was bumped into the off position on landing, resulting in loss of the altitude reached. Ballpark estimates are in the 3000-4000 foot range, which is consistent with a motor performing reasonably well but the rocket tipping over substantially.