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Design, Analysis, and Prototyping of 3-D

Printed Hybrid Rocket Engines

By

3D Printer Team

Thaddeus S. Berger: [email protected] Clyde D. Brown: [email protected]

Abstract

The problem presented by Dr. Brenner in conjunction with a group at Aerojet Rocketdyne was to

develop a three dimensional printer capable of constructing a hybrid rocket engine’s solid fuel

component in any shape necessary to produce optimal thrust and efficiency throughout the main rocket

burn stages. Theoretically, by 3-D printing a porous solid fuel component, the oxidizer flux rate is

exponentially increased, which can provide better performance and lower fabrication, transportation,

and operation costs.

Design, Analysis, and Prototyping of 3-D Printed Hybrid Rocket Engines

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Fundamentals of Hybrid Rocket Engines In a hybrid rocket engine, the burn process is initiated by a pressurized gas forcing a liquid oxidizer

through a valve and into the solid fuel component, which has a tubular cross section. The oxidizer mixes

with the solid fuel, which is then accelerated out a nozzle. Thus, the oxidizer flux rate across the surface

of the solid fuel is what drives engine performance. The diagrams below illustrate the operation of a

hybrid engine and the abilities of various cross sections of the solid fuel components to produce power

over time.

By 3-D printing the fuel component, the engine’s performance can be specifically tailored to provide

high power when quick acceleration is needed (as in takeoff or when reaching escape velocity), or to

achieve excellent fuel efficiency when long, sustained propulsion may be desired. All of this can be

attained while decreasing the cost of producing, shipping, and managing the fuel as well as operating

the engine.

Figures 2.1–2.6

The Burn Process: The diagram in Figure 1 (top) shows the processes by which a hybrid

engine operates, while Figures 2.1–2.6 (bottom) show the performance of six commonly

used cross sections of the solid fuel component in terms of thrust over time.

Figure 1


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Advantages of Hybrid Engines and 3-D Printing the Solid Fuel Component

The performance of a rocket using solid fuel is described using the regression rate, or the rate at which

the fuel is burned. This rate is modeled as shown in Figure C, perpendicular to the cross section of a

given fuel grain.

In solid-propelled rockets, the burn rate of the solid fuel grain is proportional to the pressure inside the

chamber. As a result, these solid fuel grains are potentially highly explosive since, as the fuel ignites,

there will be an expansion and therefore a pressure increase, resulting in higher temperatures and a

greater burn rate. This means that solid propellant fuel grains are dangerous to handle and also have an

uncontrollable fuel regression rate.

The main advantage of hybrid rockets over solid-propelled rockets is derived from their burn process. In

hybrid rockets, as shown in the diagram in Figure 1, the regression rate is driven almost entirely by the

oxidizer flux rate, which can be independently controlled both by changing the pressure in the gas

chamber or by using the valve to change the mass flow rate of the oxidizer into the fuel chamber. The

result of this control is that hybrid rockets are very unlikely to exhibit grain flaws. Also, there is rarely

any pressure coupling. Furthermore, imperfections in the solid fuel component only increase surface

area, which may cause some error but has no real negative effects on ballistics. What may be the most

important of these advantages, though, is that the solid fuel component cannot be ignited without the

oxidizer and are therefore safe to transport and store. This will drastically reduce shipping and handling

costs and could allow for a significant reduction in overall system operating cost. One matter that must

Burning surface at time t3



Inhibited surface

Propellant

grain

Sectioned view of

burning propellant

grain segment

Figure 3

Fuel Regression Rates: The performance of a rocket using solid fuel is described using the

regression rate, or the rate at which the fuel is burned. This rate is modeled as shown in

Figure 3 (above), perpendicular to the cross section of a given fuel grain. A hybrid rocket’s

regression rate can be independently controlled, unlike those of solid-propelled rockets.


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be carefully considered is ensuring a deflagration instead of conflagration of the fuel during all stages of

production and operation. The best way to do this is to very carefully control the flow of the oxidizer

into the fuel chamber, ensuring that there is never too much oxidizer or an excessively high or low

pressure in the fuel chamber. Another way to ensure deflagration instead of conflagration is for the

rocket engine’s interior and exterior shell to be as resistant to temperature change as possible, since it is

possible that an error caused by temperature changes in the rocket’s surroundings could cause

conflagration of the fuel. What may be possible is that different fuel blends in separate layers of the

solid fuel component may have different combustion temperatures which could help prevent

conflagration. Further research into this matter will increase the safety and reliability of these hybrid

rockets, further increasing their advantage over traditional solid-propelled rockets.

Innovations to Current Printer Design

The following innovations and revisions must be made to the current three dimensional printers.

Vacuum chamber: A sealed vacuum chamber surrounding the entire printer and contributing

system outside of computer-based components to minimize toxic or explosive gas buildups as

well as maintain a stable environment during the printing process.

An extrusion pump: Since the material, hydroxyl-terminated polybutadiene, or (HTPB) is

originally mixed in component form as a liquid material before casting the liquid would need to

be held in a separate well from which the material would be pumped through the tubes to the

extrusion nozzle.

The nozzle(s): The three space nozzles are built to heat solid materials and melt and extrude

them as a pliable solid in the shape of the specified object. The nozzle required for this task

would involve as previous stated, first pumping liquid through the extrusion tube into the

nozzle. From that stage, the HTPB goes into a casting stage in which it would be heated to a

temperature theoretically right before the solid form (or still pliable solid) and then extruded

through the nozzle to form the specific conical shape required for the hybrid engine.

Jamming Issues: When dealing with liquid to solid three dimensional systems, it is expected that

there will be an elevated amount of jamming issues due to residue buildups over long periods of

printing within the tubes running to the nozzle. The original three space printers only have solid

polymer rods passing through the tubes. Therefore, an innovation must be made to solve or

reduce the amount of jamming or a method which involves no jamming at all must be produced.

Avenues for solutions could stem from the addition of an accurate and powerful pump as

previously mentioned or a tube specially lubricated to resist the clogging.

Cooling system: Finally in the future of the project, after further testing and research is

conducted, it may be determined that the system may also need a cooling system before or

around the extrusion nozzle to prevent damage to parts or the process.


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Conclusion

Some of the largest issues associated with the use of hybrid rockets are their relatively high

development cost and the aerospace industry’s current inability to produce these on an industrial scale.

3-D printing of the solid fuel component could be the first step in eliminating these issues. The proposed

innovations listed on page 3 to the basic 3-D printer design may be adapted for use in an industrial

setting, and may be scaled up as needed. A further potential tweak for industrial applications is to use a

large mixing chamber and pump the mixture to multiple nozzles so that one large printer can extrude

several solid fuel components simultaneously. Yet another possibility is the use of a system which can

print both the fuel and exterior shell components, effectively producing entire completed rockets in one

smooth process.


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Reference Material and Works Cited

The following reference materials were used extensively throughout the compilation of this report:

1) Common cross sections of the solid fuel component and their respective thrust vs. time graphs:

Space Exploration Stack Exchange, 2014.

http://space.stackexchange.com/questions/4153/could-3d-printing-be-used-to-achieve-perfect-

grain-geometry-of-solid-and-hybrid

2) Conceptual knowledge of the hybrid rocket system, fuel regression rate concepts, and the figure

on page 2 illustrating the effects of the burn process on the cross section was drawn from the

master’s thesis of Johnathan M. McCulley, written in 2012 at Utah State University, entitled

Design and Testing of Digitally Manufactured Paraffin Acrylonitrile-Butadiene-Styrene Hybrid

Rocket Motors.

http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2451&context=etd

3) Issues with the current process which partially led to our proposed innovations:

Stratasys Ltd., 2014.

http://www.stratasys.com/resources/case-studies/aerospace/rocket-crafters

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