A student society of undergraduates and PhD students dedicated to pushing the limits of high power rocketry and high altitude ballooning. We are based out of the Cambridge University Engineering Department and hold regular meetings.
Below are tests we carried out (prior to the Coronavirus lockdown) of the subsystems that will be needed for our next rocket rocket project, the Martlet 4. Powered by our Pulsar hybrid rocket motor, Martlet 4 will be our largest rocket yet while its hybrid propsulsion provides some unique challenges.
Fill line disconnect system: ‘The Tower’
The oxidizer for our motor is nitrous oxide, which must be loaded onto the rocket’s tanks entirely by remote control for safety reasons. We thus need a mechanism to separate the high-pressure hose used to fill the tank prior to launch. Our disconnect system, nicknamed ‘The Tower’ involves an electric winch, a tower-mounted pulley and a quick disconnect. Well done to the team that put the tower together!
Drogue deployment test
Below is a test of the deployment of the drogue parachute. The drogue is the first parachute to deploy, which later pulls out the main parachute for landing. It is ejected by pressurizing the parachute bay with carbon dioxide. This test was carried out on the ‘Martlet 3C’ airframe, a prototype airframe of the same diameter of the eventual Martlet 4 rocket.
Other milestones included the test of the rocket’s main feed valve. This lightweight aluminium valve is driven by an electric motor and opens to a wide orifice to admit the oxidizer into the combustion chamber. It has so far been tested with a back-pressure of 70 bar.
Cambridge University Spaceflight (CUSF) have successfully completed the first static firing of their custom hybrid rocket engine. Over last 18 months the team have been working tirelessly on Project Pulsar, designing and building the Pulsar engine from scratch, culminating in the successful test this week at Airborne Engineering Ltd’s test facility.
The Pulsar engine burns Nitrous Oxide combined with High Density Polyethylene fuel to produce thrust for a total of 36 seconds. Over the course of the test, the engine produced a measured impulse of 53,855 Newton seconds giving it the largest impulse of any Nitrous Hybrid rocket ever fired in the UK.
A freak snow storm threatened to delay the test but the team powered through to fire the engine after dark resulting in some dramatic views, a melted plume of snow and novelty snowballs rolled from rocket exhaust gases. Extreme cold caused two of the cameras to shutdown, but footage from two of the remaining cameras is shown in the video below.
The Pulsar project was first conceived in 2017 when the society’s Martlet 3 rocket, designed to break the UK Amateur Rocketry Altitude Record, was destroyed in flight by the explosion of a commercial off-the-shelf rocket motor. While obviously a major setback for the Martlet 3 project, CUSF team members were inspired to see if they could build something better. The success of the Pulsar project, which has delivered more than double the designed impulse of Martlet 3 in this static firing, shows that CUSF are well on there way to doing just that. The test itself marks a key milestone in the next chapter of high power rocketry at CUSF with the Pulsar engine intended to power the Martlet 4 rocket project set to launch in 2020.
Cambridge University Spaceflight are extremely grateful to Airborne Engineering Ltd (and the many CUSF alumni who work there!) for letting us use their fantastic rocket test facilities and giving up their time to help us over the last few weeks. The test captured a huge amount of data which the team are hard at work trying to analyse in order to learn as much as possible about the Pulsar engine. Watch this space for a more detailed technical breakdown in the near future.
Notes to the editor:
Cambridge University Spaceflight (CUSF) are a student run engineering society made up of current undergraduates and PhD students at the University of Cambridge. Founded in 2006, CUSF have grown to become leaders in amateur rocketry and high altitude ballooning.
A week later, and the balloon is still (at least partially) inflated.
Tasks for this week involve working out the optimal pressure for the HAB – too low and it won’t get off the ground, too high and it will leak under its own pressure. As the altitude increases pressure decreases rapidly, and at an altitude of approximately 13km the external pressure will be less than one fifth of that on the ground. Without pre-stretching the balloon and the right internal pressure, our project will not get very far off the ground!
Today we created our first (partially) working prototype of the balloon! The initial design for 4 semi-circular sections was quickly replaced with a 2-circle approach, where we cut out two one-meter diameter circles from our roll of plastic. This was because the extra work involved in welding 4 sections, combined with the welds being the weakest part of the balloon meant that there was no purpose in the extra effort involved.
We welded them together in tangential sections, eventually ending up with a mylar balloon shape. The plastic we were using was slightly damaged, meaning we had to re-weld some parts of it a few times, but at the end of today’s session we were able to fill up the balloon with air and leave it in the DPO for inspection next week – only time will tell if it has leaked!
When designing the recovery system for a rocket the maximum forces must estimated. There are two main considerations, the deceleration of the separating parts of the rocket as the shock cord goes tight and the deceleration of the rocket as the main parachute deploys (in dual deployment systems). Firstly the separation velocity must be found. This can be calculated if the pressure causing separation and therefore force acting to accelerate the components of the rocket are known using newton’s laws (remembering there are two parts of the rocket being accelerated). Estimating the pressure for standard black powder charges is tricky, there are some approximations to a gas equation around (eg http://www.vernk.com/EjectionChargeSizing.htm). I’d really like to set up a controlled experiment with some BP charges and a pressure sensor.
Next consider the deceleration as the shock cord goes tight, this can be model as two mass connected by a spring of stiffness k, length L and extension delta. Using the velocities from the previous calculation and iterating the differential equations using this Matlab script listed below produces the below graph of force against time (should also work in Octave).
The graph is for a shock cord 3m long and made of 5mm diameter nylon cord.
The next consideration is when the main deploys, the rocket is descending at the terminal velocity of the drogue. The main then deploys, the velocity is higher than the new terminal velocity therefore the rocket decelerates. The force is simply given by the drag from the main at the terminal velocity of the drogue.
My final year project was on building a guided rocket using canards during which I was fortunate enough to get access to a large wind tunnel for a week at very short notice. This proved extremely enlightening as the aerodynamics were much more complicated than I had been assuming.Firstly the lift from the canards was much lower than anticipated based on 2D values (see graph). This was due to the extremely low aspect ratio (span/chord) of the canards, this meant the flow was dominated by the tip vortex. Due to the pressure differential between the top and bottom of the aerofoil a vortex is generated at the tip (see diagram) this vortex induces downwash which reduces the effective angle of attack delaying stall (the canards stalled above 30°) and reducing lift.
However the lift forces back calculated from the rolling moment are much lower again. To explain this requires consideration of how the vortices interact with the aft fins. As can be seen in the diagram below the vortices generated by the canards resulting in an opposing roll moment from the aft fins. Source: The Effect of Tail Fin Parameters on the Induced Roll of a Canard-Controlled Missile By Melissa McDaniel, Christine Evans and Dan Lesieutre.
It is important that vents are drilled in the rocket so that the barometer on the flight computer can sample the atmospheric pressure (and calculate the altitude). There are various rules of thumb for sizing such ports but few explanations from first principles. Intuitively (or based on dimensional analysis if you will) there must be a ratio between the volume to be vented and the area of the vents. One rule of thumb is one 1/4” diameter hole for every 100 cubic inches. An approach based on first principles goes like this.First decide on the maximum allowable error in the altitude, say 10m. This provides the pressure difference between the altimeter bay and the free stream. Next find the maximum flight speed, this gives the maximum rate of change of pressure. Modelling the altimeter bay as a volume V with an orifice area A and pressure drop delta p this gives all the information needed based on Bernoulli’s equation for the flow through the orifice and mass conservation. This Mathcad sheet calculates the size of the vent holes. (Mathcad Prime Express can be download for free).
Based on an error of 10m and a flight velocity of Mach 2 the calculation yields a 0.176″ hole for every 100 cubic inches, a fair bit less than the rule of thumb above.