Rick Newlands 2013 - 2017

The booster rocket:

When it comes to rocket safety, stick to what you know. I know nitrous hybrid rocket engines, I’ve built and flown many.

A hybrid rocket engine works like a log fire: you blow a draft of air over the logs and this burns away the logs. Here, I blast a fierce draft of nitrous oxide (which contains oxygen) down the inside of a tube of plastic fuel (plastic is made from oil, it burns really well).

For more information on hybrids, see the Aspirespace website.

Nitrous self-pressurises like camping gas: no pumps are needed. And it isn’t toxic, and neither is the rocket exhaust provided I use pure polythene plastic: I don’t intend to clog up the stratosphere with tons of soot.

So the booster is simply a big tank of nitrous (around 460 litres) which feeds two tubes of plastic, that burn and their exhaust is carried into one common nozzle. Just one nozzle is better than several incase one of the several breaks and stops thrusting. Then you get a catherine-wheel!

The picture on the far right shows a rocket-powered test model I launched to check the stability of the system.

You’d be forgiven for thinking that this rocket system is small, and you’d be right, it’s basically all nozzle. Starting at 35 kilometres up from a big balloon gives a huge reduction in the propellant required, the nitrous tank is only about a metre in diameter.

Just after ignition, the rocket engine delivers a peak thrust of 22 kiloNewtons (22,000 N). What’s a Newton? It is the correct metric unit of force but you may well not have heard of it because the Eurocrats hate it cos Isaac Newton hated foreigners! The weight of an apple has a force of about 1 Newton.

1 Newton = 0.2248 pounds-force (old money).

Then the thrust slowly decreases throughout the burn as the tank empties. This decreasing thrust is another handy feature of nitrous hybrids as it keeps the gees from getting too high during ascent. All the while during the burn the booster gets progressively lighter as propellant is used up: The gees would increase dangerously if the thrust didn’t decrease to match the decreasing booster mass. Peak gees are four gees just before engine burnout.

I part company with the booster at high altitude as shown in the photo on the right which is taken from a rocket test flight. The booster simply parachutes back down.

Thrust vectoring

At the high altitudes the rocket will be firing at, simple fins don’t work; we’re above the atmosphere.

So like you trying to balance a pencil upright on your fingertip, something is required to give continuous little sideways nudges to the booster to keep it upright as it ascends. This is called thrust-vectoring, and is the standard steering method for rocket vehicles.

However, the cost and complexity of thrust-vectoring systems varies enormously. A cheap and simple method of vectoring is called a jetavator. This is essentially a napkin-ring shaped metal hoop that encircles the end of the nozzle. Tilting the hoop angles the exhaust flow.

The jetavator will be tilted using ultra-reliable electric motors. They won’t get that hot; nitrous hybrids burn warm rather than hot, the gasses leaving the end of the nozzle will only be at about 500 C temperature.

Vital statistics

For those of a technical bent, here are the engine parameters:

Two tubes of fuel result in a much shorter booster than one very long tube of fuel, which cuts down the weight, and I can test one of the tubes just on its own for reduced cost to tune its length correctly to get the best burn.