BANG!!

On Sunday 25th July, at the ‘Ladies That Launch’ event at Shakespeare County Raceway, ‘Laffin-Gas’ suffered its first start-line incident.

We were there in support of the very first event dedicated to the women who take part in Drag Racing. We were also using the weekend as an opportunity to train our newest team member, John London, who comes from the world of rocketry and was having his first experience of the world of Drag Racing. As training had gone well on Saturday, we decided to go for a two-rocket run on Sunday, which turned out to be a lot hotter than forecast.

There was a very loud bang, and a lot of gas was released from the car. We exploded one of the four main-throttle valves on opening the main throttle. The debris from the exploding valve was caught by the ballistic protection we had built in around the throttle assembly. While the bang was loud and, therefore, very energetic, the damage was limited to the valve itself and the hoses immediately upstream. No damage was caused either to the rocket or the pressure-vessel. The burst-disc protecting the pressure-vessel was not ruptured, indicating that the energy was dissipated further downstream. No-one, (including myself) was at serious physical risk from this event, but it does require an explanation.

It was the same sort of thing that occasionally happens in any big Nitrous burning car, often resulting in bits of carb and manifold being ejected through the air-scoop on the bonnet. In fact it was nothing like that big! When our car is on the line, everything is so quiet- the bang really stands out. In other cars it gets masked by the roar of the engine.

When we returned to the pits, we invited Paul Satchel (Senior Scrutineer and member of the MSA Drag Racing Committee) to inspect the car. Paul was happy that the safety systems had done their job and that we had acted responsibly.

The explosion was a result of something called ‘Compression Shock’, which happened when very warm gas (it was a hot day), which had built up in the very small space between the safety shut-off and the throttle valve, was exposed to back-pressure from the idling rocket just as the throttle was opened. We were firing two rockets. The one with a safety valve that was tightly sealed, did not have this problem. The volume of gas that did explode was less than 20cc. That is a tiny volume for such a big concussion!

We have built chillers into our race transporter to ensure our gasses and liquids are cold when we fuel up. We use insulating blankets to keep things cold after fueling. We pre-pressurise with Nitrogen Gas to ensure that Nitrous in the vessels and lines is always in its liquid state. But Sunday was a hot day, and a very small amount of N2O in a small section of the system, turned to gas, and was not purged (as the rest of the system is) when the start-up valves were opened.

We have now devised a new protocol to ensure that this section of the system is purged immediately prior to launch. We will also employ a cooling method directly on the throttles in future.

Photo courtesy of Missb Fotos

Something for the Techies

We're looking for testing partners who have nitrous oxide bikes and cars to help to develop a new application of our Accumulator technology. Below is a short paper on the subject.

NITROUS OXIDE AND ITS USE IN ROCKETRY AND HIGH-PERFORMANCE PISTON ENGINES

Nitrous oxide has long been employed both as an oxidizer in various forms of rocket propulsion, and as a performance enhancer for internal combustion engines. Many internet sites and published documents represent N2O as a harmless substance that can be handled safely with a minimum of expertise. As such it is very popular with amateur rocket enthusiasts, drag racers and others interested in getting more horsepower out of engines.

Nitrous oxide is, in fact, quite a unique substance with its own special physical and chemical properties, Properties that should be fully understood by anyone handling and using it in rockets or engines. Under certain circumstances, nitrous Oxide can explode in a very violent manner without any apparent source of ignition or oxidant to fuel such an explosion. These circumstances can easily occur during transfer and actual use and can occur in apparently normal conditions.

The attraction of N2O is that it is a good source of free oxygen, and that it is relatively easy and safe to handle and store. At lower temperatures it is a stable compound that is not very reactive, if a little corrosive. As it gets warmer it becomes less stable and predictable. High temperatures are needed to ‘split’ N2O into its two component elements, Nitrogen and Oxygen. Once split, a gas mixture of two parts Nitrogen to one part Oxygen is formed. One can see that this mixture is much richer in oxygen than is air. In air just under 80% of the mixture is Nitrogen and just over 20% Oxygen, four parts Nitrogen to one part Oxygen. It’s that extra oxygen in N2O that makes rockets burn so well and gives the horsepower boost to piston engines.

The other attraction of N2O arises from its quirky physical properties. Normally stored under pressure as a liquid, it has a very high vapour-pressure that is commonly used to push the N2O out of its containing vessel either into a rocket or into an engine. This ‘self propelling’ characteristic removes the need for pumps or other devices to move large volumes of liquid very quickly. At 20 deg C, the vapour-pressure is 58.5 bar or 850 psi (pounds per square inch). Nitrous oxide is very temperature sensitive. The higher the temperature, the higher the vapour-pressure, and the opposite applies to lower temperatures. A chart is included that shows the relationship of temperature and pressure. At 36.4 deg C, N2O reaches its CRITICAL POINT, when it starts behaving strangely. At and above this temperature all the N2O in a vessel becomes gas. It also becomes much more reactive as it approaches this temperature and can be made to explode by the application of a compression force. By ’explode’ one means an exothermic chemical reaction that is a true chemical explosion- not just simply a sudden release of high pressure gas. This stuff can go off like Semtex when it is too warm.

The boiling point of a liquid is determined by the gas pressure it is subjected to. If you go up Everest and want to make a cup of tea, the tea won’t be very hot, because, at that altitude, the air pressure is so low the water boils before it gets hot enough to make a decent cup of tea. Nitrous oxide behaves the same way. The lower the pressure, the lower its boiling point will be and vice-versa. At 20 deg C, the gas pressure needed to prevent boiling is 58.5 bar (850 psi). The instant that pressure is dropped, the nitrous oxide boils. The gas pressure that has this effect is, in this case, not air, but gaseous N2O, this gas is the nitrous version of the steam given off by water.

When a valve is opened to allow a rapid discharge of liquid N2O the gas pressure in the container will drop suddenly. As soon as this occurs the liquid N2O boils. Just as with water, the boiling will produce N2O gas and that gas will help to slow the pressure reduction. The gas can only be produced at a rate that is governed by the surface area of the liquid and the scale of the pressure drop. Once a valve is opened to release liquid, the pressure in the container will lower. This pressure is unlikely to be steady as all the N2O in the system is now boiling. Pockets of gas are forming randomly throughout the entire volume of the liquid, making the liquid itself compressible and, therefore, ‘springy’.
This applies even to the liquid in the line leading to the engine it’s supplying. This makes the rate of delivery to the engine much less predictable. It also means that gas bubbles are shot into the engine.

In the ideal set-up, for either a rocket or a conventional petrol motor, we want to deliver liquid N2O at a controllable rate. This makes the engine much easier to tune and to get a smooth performance. One often sees nitrous-aided cars venting the nitrous system to remove gas that may have formed in the lines. Initially this will deliver gasless liquid, but only for an instant as the liquid begins to boil.

Rocket motors that employ the ‘boil-off’ method to propel the N2O into the rocket always seem to ‘pulse’. The exit flame is never steady. One suspects that the same effect will occur in a petrol engine. In a rocket this effect is exacerbated by the resulting rapid changes of pressure in the combustion chamber. The back –pressure on the incoming line will cause the boiling point to fluctuate rapidly, making the pulsing effect worse.
In petrol engines employing a valve ‘overlap’ the same will apply. It’s quite common for my friend, Steve Woods, (who refills nitrous bottles at the track for his customers) to find that the bottle is so pumped full of air (with a little petrol thrown in, no doubt) that he has to let the air out before he can pump in the N2O. Racers beware – a pressurized container with air, petrol and N2O inside is a pretty unstable and dangerous bit of kit. The warmer it is, the more bomb-like it becomes.

For an engine tuner, or a rocket engineer, predictability is the key to ultimate performance. The ideal N2O supply system would never boil. It would deliver a predictable stream of non-compressible liquid, just like a petrol injector. This makes calculation of mixes and power output much easier and far more reliable. From both a safety and performance point of view, it would also be ideal to keep the N2O cold. Fluid densities go up and instability goes down. In a boil-off system, racers often employ bottle heaters to raise temperature in order to increase the delivery of N2O. The same effect could more safely be obtained by making the plumbing allow more N2O through.

In a boil-off system, as the bottle empties, gas will actually start to push the liquid aside and go directly down the line. If you watch water leaving your kitchen sink, as it empties a hole appears in the centre of the draining water as the liquid spins (coriolis effect). The same will happen inside a pressurized N2O container, with a resulting huge drop-off in performance. The emptier the container gets – the bigger the hole for gas to go down. Many racers counteract this by having more liquid than they need for the run, ensuring that the levels never get too low.

There is another complication with boil off-systems that is known as ‘Slosh’. In horizontal motion, the liquid will slosh towards the rear of the vehicle, climbing up the side of the container, again allowing gas to bypass the liquid.

In larger rocket motors, some of the downsides of the boil-off method are overcome by filling the gas-space with Nitrogen gas at a much higher pressure than the vapour pressure of the N2O. This prevents boiling. Often an external reservoir of Nitrogen is attached to lower the rate of pressure-drop as the container empties its liquid. This does combat many of the downsides of the boil-off system, but by no means all. Slosh can still happen as can gas blow-by. Back pressure can still make the system pulse quite heavily and the higher pressures mean that even more N2O must be left behind to minimise the blow-by as the levels get low.

So pushing with Nitrogen solves some of the problem but not all. The system is still somewhat unpredictable and the performance curve will be all over the place.
The ideal would be to keep the gas away from the liquid but still exerting enough pressure to prevent boiling. This is known as positive displacement. Gas/liquid shock-absorbers employ this technique. Either a bladder or a piston is used to keep the two separated. The gas acts as a spring damper, smoothing out pulses but the liquid remains free of bubbles and acts in a predictable manner. These shock- absorbers are more properly called ‘Accumulators’. We’ve tried bladders in our rocket-motor tanks, but they just weren’t up to the rigours of the high pressures. It is possible to buy ready-made bladder accumulators, but they cost a fortune, have materials compatibility problems and they are much too heavy for racing and rocketry.

So we tried putting free-floating pistons inside cylindrical vessels, Nitrous on one side, Nitrogen on the other. Now there can be no slosh, and no blow-by. The piston/gas arrangement also acts as a damper, minimizing the effect of back-pressure coming up the line. If the Nitrogen pressure is kept high enough there will be no boiling. The result was a very smooth and predictable rocket performance. We can stay well away from the Critical Point, vastly improving the safety aspects of the operation. In fact, the colder it is, the better it works.

Piston accumulators work very well for Rockets and we see no reason why the same shouldn’t turn out to be true for piston engines. All a tuner needs is predictability – then the rest is much easier.

Rained Off!!

Saturday didn't happen. Rain in the morning and a lousy forecast meant that we called off the trip to Bruntingthorpe. Just can't run in the wet!

Pending clearance from the track, we'll try again at Shakespeare County Raceway in two weeks' time. 12th & 13th June.

The Next Few Weeks

This coming Saturday, 29th May, we're puuting the car on display as part of the Rushden Power Fest, organised by Santa Pod Raceway. We're taking the opportunity to show our neighbours in the town waht has been built and engineered right here. Should be interesting...

On the 29th, we're off to Bruntingthorpe, a large active airstrip in Leicestershire, for a four-rocket running test. The runway is two miles long and very very wide, so it's a relatively safe environment for checking out handling and stopping. We'll be running bio-fuel rockets, not kerosene, as we really don't want all of that power for this test.

We have a few new team members, our fuelling supremo, David Rose can't make it, so we'll be going slow and steady to avoid any possible errors. The chillers are up and runnning. So, if it's a hot day, we should be able to keep the temperatures in the car well down into the safety zone.

Dave Burditt, from Datron Technology, is comiing up middle of next week, to install GPS and other data recording equipment. For the first time we'll be able to collect hard data on exactly what the car and rockets are doing in the real world.

Nerves are already starting to jangle... so slow and steady is the order of the day.

David has been a very busy boy. We've wanted to put brakes on the front for a while, and now it's been done. Putting brakes on wheels that were not designed for brakes is no easy matter. We've put on quite small twin, opposed-pot motorcycle brakes. The front tyres are too skinny for anything more. They will help with stopping and especially holding the car still when the rockets first fire up.

We've added a big chiller unit to the transporter, so now we can chill both the gases and the car itself. The colder it all is the better it runs and the safer the operation is.

We've also added more ballistic protection around the throttle area and under my backside.

That's just about all the jobs we wanted to get done before we take her out for a full four-rocket burn. As this is being written, we're trying to organise a date and location to be able to have the best crew available for a a private test,

We haven't yet decided which fuel combination we are going to run. The kerosene rocket has so much power, we may have to work up to running four of those.

So, sometime, in the next month, we'll see what she can really do!!

Fingers and toes crossed!

Rocket Analysis

David and I dismantled the Kerosene rocket yesterday, and (as usual) cut the fuel grain along its length.

Having given every component a thorough check we can report:

No damage, blistering or heat effect was observed on any of the metal parts of the rocket.

The fuel grain had burned evenly along almost its entire length. The inner liner was only half consumed (which is less burning of cardboard than any previous rocket).

If you watch the video carefully you'll see the rocket shut down very quickly - this is because the movement caused the safety solenoid to shut down. I must emphasise that nobody was put in harm's way.

Someone, posing as Tog from Eurodragster, made some very disparaging comments on Youtube. We know who this person is, he has been conducting a very sad hate campaign for several years. The comments have been removed.

Conclusion: A very repeatable rocket!!

Rocket Test Friday 19th March 2010

Angus, the farmer, has got out of the pig-breeding business. He’s been very busy tidying up the old place and our old testing site has been ploughed up, in readiness for a new Skeet shooting facility.

Angus very kindly moved us to the old muck-heap. That’s not so bad as it sounds as it’s a huge concreted area well away from anything we could damage, and almost all the muck is long gone. The only problem was – how to secure the testing-rig on a concrete surface? Angus came to the rescue by placing a huge old piece of agricultural equipment on the concrete using his great big front-loader. Basically a very heavy steel construction, with a fairly open framework, it was ideal for strapping our rig to it. We tried shifting the thing – it was going nowhere. ‘There’s no way one rocket is going to move this’, we observed.

In the morning we got everything ready, rocket, piston accumulator, and all the bits needed to fuel-up and fire the beast. Dave Rose, long-time team member and next-door neighbour turned up straight after work, about 3:00pm, along with Fred Thomas and Dave Burditt. Dave works at Datron Technology and he wanted to observe a test so that he can provide instrumentation for accurate data recording at future tests.

Dave Rose and I fuelled the rig up using our (now standard) protocol and got the bus and all the cars well out of harm’s way. Cameras were set rolling and I made the circuit to set off the pyro. I retreated to what I believed to be a safe distance and position, and pulled on the lanyard to open the pilot N2O valve. The rocket sprang into life. I couldn’t see the flame from my position, so I judged, by the sound, that she was well lit. One firm tug and the main throttle was opened. Loud, it was –very loud. ‘That’s going well’ I thought then I saw the whole steel framework starting to spin clockwise towards me. Carolynne exited stage-left at speed.

The rocket burned for just over five seconds with a very strong and steady exhaust jet. It was far and away the most powerful rocket we’ve yet fired. On reviewing the video, we could clearly see the shockwaves projecting from the bright heart of the flame. Four big and heavy men then tried to apply the same force the rocket had applied to the steel structure –they couldn’t budge it! We estimate that it took in excess of 1300lbs of thrust to do what that rocket did. I think we’re going to need more brakes in the car!!

The rocket itself was perfectly intact at the end of the burn, a little warm to the touch – but not hot.

For those of you who like the hard facts:

We wanted to run fully converted Bio-Diesel, but had an availability problem, so we soaked the cardboard with Kerosene, which is almost identical in terms of calorific value and volatility. The rocket had absorbed a little over 2 litres of kerosene.
We did not use an external Nitrogen tank. For the purpose of this firing, the Nitrogen capacity of the accumulator alone was more than adequate. We pumped in 8lbs of Nitrous oxide and pressurized to 1,000psi with Nitrogen.

Ambient temperature was 11.5 deg C, which gave us excellent fluid density in the N2O.

The burn duration was five seconds. All N2O was exhausted, the final pressure in the accumulator was 700psi (50 lbs over the vapour pressure of the N2O).

Now it’s getting very close to the time for getting out on the track and letting her rip!! We have the reliability and we have more power than we can safely use – so it’s time to learn how to get her stopped!!

VIDEOS NOW POSTED!