LAZARUS: Hybrid Transonic Rocket

Most of the flight dynamics were calculated using the flight simulation application RockSim.  We used RockSim to characterize our rocket designs. Elements like fin and airframe shape, size, and length, as well as mass distribution, changed a lot early on during the development of the rocket.

RockSim helped us narrow down and eliminate designs by determining if our rocket was too heavy or too unstable. It also helped us determine the expected apogee, and range, and simulate those parameters changing based on weather effects. 

We started off with designs purely in RockSim, and, after about three months of development and research, we determined a design that we liked, ordered parts, and started to fabricate the rocket. 

After doing a lot of research, we determined that a stability caliber between 2 and 3 would work best. We also determined that the stability margin would change during flight quite significantly, as for a hybrid engine, the oxidizer weight would be ejected from the middle-end of the rocket, shifting the center of mass, and changing our stability margin. We made sure to keep this in mind during our simulation phase.

The motor we used for Lazarus was a HyperTEK hybrid rocket motor with a 1685 cubic centimeter tank, L550 fuel grain, and the RG L fixed orifice injector bell. This specific motor configuration had an impulse of 3095 Ns, and a burn time of 5.6 seconds.  The oxidizer was nitrous oxide, and the fuel was a thermoplastic.

This specific motor configuration was chosen after running multiple simulations with multiple motor configurations and rocket builds. 

A common trend with the research and design phase of this project was just to try an idea in the RockSim simulation and see if it performed well or not. Do that multiple times while adding new design considerations and refining current ones, and you will eventually get a refined design that performs well and meets your objective. 

While we did not build our rocket motor, we still needed to know how it would perform and change dynamically in flight. In our rocket design, we had to account for things like the changing center of gravity, as the oxidizer was transferred from the tank and ignited in the fuel grain (as previously mentioned a few paragraphs above).

The challenging part of building a transonic hybrid rocket is figuring out what kind of problems you need to solve, and if you are asking the right questions.

How straight do our fins need to be?  Will the airframe survive transonic velocities? How do we know if the avionics bay can survive the stresses during chute opening? 

The key to success in an unfamiliar engineering problem is to approach every element of the problem with a clean mindset and determine a solution, if a solution is even required, with a first-principles mindset. 

For example, I had a pretty eye-opening experience while initially designing the avionics bay. How should I secure the batteries for the flight electronics? Can I just tape them or zip-tie them? I initially thought, "Oh I'm sure tape would work fine", but you have to really make sure, it's a rocket. Approaching the question with a first principles mindset revealed the answer pretty quickly. 

What kind of forces do the batteries experience during launch? Well, from the flight profile and RockSim, about 7g's of acceleration is experienced during launch.  A 9v battery weighs about  40 grams, and using Newton's second law of motion we can easily calculate that the force exerted on the battery would be a small 0.7 lbf. So yes, you probably could tape the batteries in the avionics bay, especially if they were resting on a solid surface. Should you? Probably not (we didn't).

If that 40-gram battery was instead a 450-gram payload resting in the nosecone, above the parachute bay, then you would not be able to use tape to secure it, as it would experience ~7 pounds of force during launch, and would break through the tape and shoot down into the parachute bay, changing the stability of the rocket and making it unstable.  This actually happened on the first rocket that I built during my junior year of high school, and after that, I swore that I would approach every problem with a first-principles mindset. 

The avionics bay was probably the coolest part of the rocket in my opinion. I had a lot of fun building it, and I enjoyed making it robust and functional. Lazarus used two MissleWorks RRC3 flight computers, both with dual deployment capability. They were set up in a redundant dual-deployment configuration. This was the safest way to set up our avionics system, if one computer failed, or if one deployment charge was not enough to deploy the main or drogue parachute, then the second flight computer would handle the error.  We also had a digital camera that wrote to an SD card that recorded the entire launch. 

In addition, there was a steel threaded rod down the middle that was connected to two metal U-bolts on either end of the avionics bay. This steel rod was more than strong enough to take the load of the parachute deployment.

The avionics bay was one of the first main components of the rocket that was constructed, as we had finished the design and knew it would not change. (Unlike the fins, which were created last.) 

After all the parts arrived and we finished fabricating anything we needed to, bulkheads, centering rings, motor casings, and the avionics bay, we decided to fully "assemble" the rocket, without gluing anything, in order to determine the center of gravity to make sure it agreed with the simulation.  We waited to fabricate the fins until we were sure everything was correct. 

The process we used for determining the center of gravity was quite simple. We attempted to balance the rocket horizontally on a fulcrum point, adding any extra weight (in the form of lead weights attached to strings) we needed, such as the fuel mass, glue mass, and fin mass, and once that point was found, we measured the distance to that point from the nosecone. That distance was where the center of gravity was located on our rocket. Once we did this, we found out that our measured center of gravity did not agree with the center of gravity from the RockSim simulation. 

We spent two days trying to figure out the problem. We re-weighed everything, measured every component to make sure it was in the correct position, and even re-created the rocket in a new simulation software. It was only after we calculated the entire center of gravity by hand we realized the simulation was incorrect. There was a mass distribution setting we missed while configuring the body tubes in our simulation, which incorrectly calculated the center of gravity. 

This problem taught me to not blindly assume how a program functions, such as RockSim, and to fully read the documentation and understand what is going on behind the scenes. It also taught me that running hand calculations to check your simulations, in addition to using multiple simulation programs, is important. 

Once we verified our rocket composition with our simulation, we fabricated the fins, cut the fin slots in our outside airframe, and glued all parts together. 

We also tested any system that we could test, such as the redundant dual deployment setup for the altimeters, first using a crude self-made vacuum chamber, as well as simulation software provided by MissleWorks. 

Another system we tested was the parachute system. Below is a video of a parachute packing test my brother and I did. From the video, it does not look like much of a success, but if you look closely, the main parachute comes out of the bag quite easily when it hits the ground. The test we did before used a separate packing method and the entire parachute system became terribly tangled, which is why doing simple tests on these systems is important. 

Launch Day

After months of research, problem solving, development, and fabrication, Lazarus was complete, and launch day came. 

After having our rocket and flight readiness review checked by a range safety officer, we loaded our four parachute deployment charges, and our rocket was taken to the pad for liftoff. 

The launch could have not gone any better. A perfect flight occurred, reaching our objective of transonic velocity, and safely recovering the rocket (in one piece!).

This project taught me a lot about engineering, problem-solving, and leadership, and inspired me to continue to pursue my dreams of working in the aerospace industry as an engineer looking to help increase humanity's presence in space.