There isn’t anything much cooler than watching a rocket launch, so just imagine the excitement for MIT AeroAstro students who are part of the MIT Rocket Team and have dozens of their very own rocket launches under their belts. This team is a student engineering group dedicated to flying high powered experimental sounding rockets and are all certified members of the National Association of Rocketry.
So how did Markforged get involved with this group? First, some background…
Rocket motors commonly consist of long, sleek, metal tubes surrounding dark rubbery propellant – this design is commonplace in rocket motors, and for good reason. The solid rocket fuel inside these motors can burn at over 5,000 degrees Fahrenheit and generate pressures of over 1,400 pounds per square inch. It’s quite possibly the last place you’d expect to find a plastic part. That sounded like an interesting challenge, so three students it up and ran with it. As a result of that project, they created the first rocket motor 3D printed from plastic. Many groups, such as Space X and NASA, have printed rocket motors from metal, but metal printers carry a six or seven figure price tag. This motor is the first to be made on an accessible, high strength 3D printer: the Markforged Mark Two.
Design and Material Selection
The design consisted of a long cylinder with a cavity to hold the propellant, a threaded section to allow for the installation of the propellant and the addition of instrumentation, and a nozzle to direct the flow of hot gasses and produce thrust. This entire assembly needs to remain intact as the engine lights. First stop: CAD. The CFF (Continuous Fiber Fabrication) technology Markforged provides enables a plastic part to withstand the extreme environment of a rocket motor. Fiber infill enables the material to withstand both pressure and thermal loads. The internal pressure creates two directions of mechanical load in the motor case: a hoop stress (circumferential) and an axial stress. The Mark Two enabled the team to support the hoop stress by laying hoops of continuous fiberglass around the inner circumference of the motor case. The fiberglass increases the tensile strength in the circumferential direction by an order of magnitude compared to nylon alone, and Onyx helps support the axial stress on the motor case. The nylon-chopped carbon mixture offers improved strength and layer adhesion over other filament deposition plastics.
Fiber infill also enables the material to withstand the thermal loads of the rocket exhaust. Like most solid rocket nozzles, this nozzle is ablatively cooled. This means that as the material is heated part of the material ‘boils’ away. This process carries away heat, helping the remaining material to stay cool. Because part of the material is sacrificed to cool itself, ablative materials get used up and have a limited lifetime. With good engineering, they can make the lifetime long enough to survive a few firings of the motor. Ablative materials are typically a mixture of glass or carbon fibers with a plastic (usually phenolic resin). The nozzles of the Space Shuttle Solid Rocket Booster and the reentry heat shield of SpaceX’s Dragon are both protected from heat by fiber/phenolic ablatives. As the material heats up, the phenolic resin slowly ‘boils’ away. The fibers can resist much higher temperatures (2000-3000 degrees Celsius), and remain behind as a porous char. The char layer at the surface insulates the material behind it. This char insulation beneficially slows the rate of ablation.
Markforged’s fiberglass/Onyx material is a decent approximation of fiber/phenolic ablative. The inclusion of fibers is essential. If the team tried to make the nozzle out of PLA or ABS on a traditional filament-deposition printer, there would be nothing to form the insulating char layer. Without the fibrous char layer, the nozzle would be destroyed by heat and erosion. Nylon gasifies at a somewhat lower temperature than phenolic, and the fiber loading is somewhat less that typical ablatives. However, the printing materials have a huge manufacturing advantage. Traditional ablative nozzles require investing a lot of capital in specialized mandrels and filament winding machines. Design changes are expensive and incur a long lead time. By contrast, with fiberglass and Onyx they can print the nozzles on a relatively low-cost, multi-purpose machine and can create a new nozzle in days.
The team used Markforged’s Eiger slicer to lay out the infill and set up the fiber routing required for the design. The full prints ended up taking two days and seven hours to finish printing, much quicker than the several weeks lead time manufacturing a motor case and nozzle via traditional methods takes. Even better, it doesn’t require any tooling changes. Two hours in the machine shop finished the parts necessary to hold the motor to the test stand, and with a few O-rings, some grease, pressure measurement equipment, and some sheet aluminum completed, they were ready to assemble and test.
The assembly of the plastic rocket is quick and easy: apply grease, slide in place, screw on the end cap hand tight. An electronic match gets threaded up the nozzle to light the whole assembly. A cap seals the nozzle closed until the motor lights, helping the motor pressurize and light correctly. The blast chamber door closed. The team work through a brief checklist, one they’ve worked through many times before. A countdown echoes off the concreate walls. “3, 2, 1, Ignition!” A pop. A rising hiss as the chamber pressurizes. A low rumble as the motor finally catches and burns for five seconds before sputtering to a stop. The acrid smell of hydrochloric acid hangs in the air as the ventilation fan sucks the smoke out of the blast chamber. And sitting there in the hazy view on the cameras is…an intact plastic rocket case.
The team was ecstatic. It’s always a thrill to take a material and use it beyond its physical limits through clever engineering. It is not immediately obvious if a commercial application exists for plastic rocket motors: for now they are heavier and lower performing than commercially available alternatives. However, they are so excited by the first test that they’re already following up with better designs and more experiments.
The MIT Rocket Team is grateful to Markforged for their excellent collaboration on this project, and to the MIT Department of Aeronautics and Astronautics for the use of their rocket test facilities.
Click here to read the MIT Rocket Team’s original post.
To learn more about how Markforged 3D printers can help you and your team, click here.