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Featured Car // Swiss SMP Slide :: Matt Mia
Currently converted to 6S Lipos / Castle Creations Monster Mamba Conversion / 83mph 2011 SCR UK
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Black Knight Hybrid Rocket Power

The idea of a Hybrid Powered RC car was given to me by Ben Jarvis. He had contacted me and wanted to know if ROSSA (Radio Operated Scale Speed Association) had any rules for the running of rocket or jet cars as he had an idea of running a ¼ scale land speed record competition.

After lengthy conversations I thought it would not be too difficult to build and run a smaller version he had in mind and the idea of the Black Knight was formed.

The name Black Knight comes from the UK test vehicle that was built in 1959
to test the engines and launch equipment for the then planned Blue Streak rocket.
I named my car after this as a tribute to the UK  space efforts in the late 1950s and 60s.

If I was going to build a rocket I first need to know something about them so luckily I came across Carolynne Cambell who had built and ran a full size hybrid rocket powered dragster. She gave me the technical input I needed to design and build the rocket engine

The basic design of a hybrid rocket engine is as follows. A liquid oxidiser, in my case Nitrous Oxide (N2O), is held in a tank and at room temperature is at 750 Psi. The N2O passes into a combustion chamber through an injector or orifice. The combustion chamber is lined with the fuel grain. The fuel grain can be any combustible material and in my case I used polypropylene. In the combustion chamber a small igniter starts the reaction of the N2O by splitting the liquid and releasing the oxygen. The oxygen then oxidises the combustible material and exits the combustion chamber through the graphite nozzle and produces the thrust.

In a vertical rocket the N2O is fed into the combustion chamber under the vapour pressure of the liquid and as the rocket is vertical 100% liquid N20 is fed into the chamber. When the rocket is laid horizontally there is a possibility that gas N2O can enter the chamber and upset the rocket thrust and give low power. I had to find a solution to this problem and came up with the idea of an accumulator.

The accumulator is a long cylinder with a siding piston on the inside. On one side of the piston is the N2O oxidiser and on the other N2 used to pressure the fluid. At 0 Deg C the vapour pressure of N2O is around 500 Psi. The accumulator is cooled own with ice close to 0 and then N2 used to pressure the cylinder on the other side of the sliding piston to 950-1,000 psi. When the rocket is fired the N2O is released and the N2 expands to keep the N2O in liquid form. At the end of the run there should be no N2O let and the cylinder at 500 psi.

The rocket motor in Black Knight is 40mm diameter and 350mm long 6082 aluminium. The injector is interchangeable and I used 1.5mm and 2mm on runs. The graphite exit nozzle from a RATTWorks H70 commercial rocket and I drilled it out from 7 to 8, 8.5,9 and 9mm. The fuel grain is polypropylene 340mm long with a 20mm port diameter. Inside the polypropylene I inserted a cardboard tube 20mm dia with a 15mm port. This is to get the rocket going quickly. For ignition I use a small commercial firework and an electric match activated by a 12v battery.

The rocket is connected to the accumulator by ¼” stainless steel tube with a solenoid valve activated by a Futaba 9351 servo. There is also a safety valve in place to be opened just before firing to ensure interference doesn’t activate the rocket by mistake.

The accumulator and motor are fixed onto the chassis of the car that is the front and rear end of a Kyosho Evolva 1/8th RC car joined by 2 lengths of 40mm aluminium angle. The rear brake is activated by the solenoid valve servo. Steering is activated by a Futaba 9351 servo and is linked to a Futaba GY401 high speed gyro. The radio transmitter and receiver is the Futaba FASST system.

The shell is 1 layer of fibre glass filled and expertly painted by Ricks Bodyshop, Kinver.

I carried out a series of static tests on the rocket motors in my garden to see if the actually worked and what sort of thrust and burn time I could expect. The first test was a RATTWorks H70 motor fitted with an external N2O bottle and this woked OK but burnt through the casing. This was because the fuel grain was designed for 2-3 seconds of burn and I ran it for 12-15 seconds.

The next test was the 35mm rocket with external bottle and this worked OK but as the bottle was horizontal gas leaked into the motor and it was unstable.

Test 3 was with the accumulator and produced 20lbs for 12 seconds. I used 14 oz of N2O and the N2 pressure was 950 Psi.

After the static tests I tried a test in the car. I hadn’t fitted the gyro so the car was unstable and span out at speed. Also the injector came out and allowed too much N2O so the rocket had too much power

The car was prepared for the speed event with a gyro fitted and longer chassis to allow the rocket to sit as far forward as possible. For the 1st run the rocket was prepared with the following 

  • 1.5mm injector
  • 7mm nozzle
  • 6oz of N2O
  • N2 pressure of 900 psi
  • Run up to speed trap 700ft 

I carried the car down to the start line and checked the radio gear and then opened the safety valve. The body was installed and the wire ran off to the side to ignite the rocket. I then took up my position near the finish line. The rocket was ignited and I allowed it to burn for 5 seconds before opening the solenoid valve. The rocket wobbled a bit off the line but the gyro kicked in and the car sped off to the speed trap. After about 300ft the rocket cut as expected due to low fuel and the car coasted in a straight line to a controlled stop. The speed registered at 58mph. I was delighted! The system worked and the car went straight.

For the next run I decanted 12 oz of N2O into the accumulator and pressured it up to 950 psi. The rocket was rebuilt using new fuel grain and another nozzle drilled out to 8.5mm The rocket was ignited and the rocket took off as before with a slight wobble at the start before the gyro kicked in. This time the rocket burnt all the way to the speed trap and went dead straight. The speed was measured at 117mph and a new UK all time record!!

The car ran dead straight and came to stop almost 1/4 mile away around ¾ along the runway. Here is the spec of the run

2nd run

  • 1.5mm injector
  • 7mm nozzle
  • 12oz of N2O
  • N2 pressure of 900 psi
  • Run up to speed trap 700ft
  • Speed 117mph 

The 3rd run was with a rebuilt rocket and 12 oz of N2O and 8.5mm nozzle. I saw a small problem with the accumulator and thought nothing of it but later it was discovered there was a small piece of metal jammed in the piston stopping it working properly. The car took off as before and registered 107mph. Here is the spec

3rd run

  • 1.5mm injector
  • 8.5mm nozzle
  • 12oz of N2O
  • N2 pressure of 900 psi
  • Run up to speed trap 700ft
  • Speed 107mph 

I wasn’t planning to run the car for the 4th time as it was the main speed event and there were safety concerns with all the traffic. I asked the crowd if they didn’t mind standing up on the hill for the run and I would prepare the car. This time I loaded 18 oz of N2O and drilled out the injector to 2mm and the nozzle to 9.5mm. If it was the last run I might as well go for broke. The car was placed 150ft further back and set off. The car took off with its familiar wobble and then tracked straight. I had the throttle only slightly open until the rocket got going then planted it. I was amazed at the acceleration and it seemed to accelerate faster and faster during the run. The noise was crisp and very load and the rocket motor worked perfectly. Near the speed trap I noticed a slight left movement and tried a right correction. The car moved to the right and flipped sending the car on its back for about 450ft. Luckily this was just after the speed trap so no speed loss.

The guys ran over to the timing trap and read out the speed.. 163.51mph and a new worlds land speed record for an RC car. The confirmation was from the radar gun at 159mph. I could not believe it!

4th run

  • 2mm injector
  • 0.5mm nozzle
  • 18oz of N2O
  • N2 pressure of 1000 psi
  • Run up to speed trap 850ft 

Speed = 163.51 mph 

The car had taken me 1 year to design, build, test and run. I had a lot of help in my quest and would like to thank all the people who have given me help and support. These are…

My wife for putting up with my hermit like lifestyle locked away in the garage

Ben Jarvis from MARS amateur rocket society for technical input

Carollynne Cambell for technical help with design and support

H-Pack for pressure testing the accumulator

KPM engineering for the fuel grains

Ricks Body Shop for the moulds and the paint job on the body

Wacky Racers for the N2O supply

Aerocon rocket supplies for the nozzles and injector

Trackmate for the electronic timing gear

Swagelok for the pressure fittings

Jubilee fireworks for the ignition system

 The next step is trying to achieve a series of milestones with the car or an adaptation of the system. These are
300kph = 187mph
200mph
222 mph = fastest tethered car speed.

Watch this space...

(Auhtor) A.J Lovering
ROSSA Chairman


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