Part 1, 7 April, 2007
By Mark Uitendaal and Leon Krancher

Related articles:

Disclaimer: all liability waved! The contents of this page is presented for informational purposes only. Do not try to recreate any experiments presented in this page. The NAVRO and the author of this article cannot assume responsibility for any use readers make of this information. In The Netherlands it is forbidden by law to own this type of propellant if you do not have an exemption of the "Wet Explosieven Civiel Gebruik" (WECG).

The start of a new project

After the flight of Spectre IIb a couple of problems arose. It was clear some investigation was needed to understand why this bird didn't fly straight. The Spectre sure had a somewhat lower tower exiting velocity, so some initial "kick angle" can be given. But it can be seen clearly from the footage that the damping of the flight is slightly negative! This cannot happen with a statically stable rocket, so how is this possible? Did we screw-up our static margin, or where other things to blame?

The static margin of the Spectre IIb was surely positive. Initially the static margin was 2.8 calibres. Of course, with so much propellant weight in the tail (almost 2.1 kg!) the static margin should run up to maximum 4.3 at motor burnout. This is even a higher number, with a higher associated damping-ratio. Another look at the rocket took us in a new direction.

The Spectre IIb is very long in respect to its diameter. The fuselage wasn't stiff enough so the large angles of attack created a thrust misalignment! Dawm, a coupled situation!

Also, this situation would not have occurred when the angle of attack was lower. This can be done by increasing the tower exit velocity. This will also lead to a decreased rotation due to gravity turn, which is beneficial to the staging manoeuvre and safety.

Also, the manufacturer of PVC tubing decided to alter his product, thereby decreasing its allowable stress, so our PVC-motor designs are worthless.

So the Spectre II airframe and motors weren't usable anymore...
hmmm, what should we do next?

Let's be constructive by stating what we learned from the Spectre II project:

  • A long and thin rocket can cause problems.
  • The project consumed a huge amount of time, time which we didn't had.
  • Fancy things only work at home.
  • Motor performance is not always consistent.
  • Suppliers change their material properties in such way that some old designs fail.

With these lessons in mind it was time to start on a new project: Quick and Dirty.

The rocket should be:

  • A two stage rocket.
  • Less prone to unstable situations.
  • Easy to construct.
  • Low cost.
  • Low time-consuming.
  • The pilot for a whole new generation of rocket projects, so growth potential and optimization possibility for different techniques will be the key issues.

Also a new motor design has to be made, which isn't highly dependant on a supplier. To save time, the whole project will be designed in one single 3D CAD-program: CATIA.

The rocket design

The whole rocket will be in the order of 2.3 metres long and will have a diameter of about 101.6 mm.

The nose cone will house several systems:

  • Primary flight computer: PaDS or R-DAS
  • Backup timer: TRAX-ART
  • Recovery transmitter: standard NAVRO 433 MHz. transmitter
  • Sound beacon: 129 dB piezo-electric buzzer

All systems will have a dedicated power supply, so four 9V batteries will be part of it. The whole electronics canister will be housed in the nose cone, including the pyro-canisters, which will deploy the parachute.

The parachute system

To lower the cost and to lower the time consumption and ease of construction the project should be kept at its most basic form. No fancy onboard camera systems will fly in this rocket, just a rocket in its purest form. The rocket will not be equipped with a two-stage parachute system. This will save length (thereby increasing the rockets specific stiffness) and weight. Both factors will decrease the risk of instability. Also, the complexity will be lower, thus saving time.

The downside of this is that the rocket will have to fly a ballistic trajectory and deploy at very high speeds (80 m/s). We are planning to deploy at 700 metres with a very high velocity drogue parachute. In order to reduce stress on the recovery system the descent velocity will be high, in order of 15 to 17 m/s. This means the impact stress on the rocket will be very high.

The booster of this rocket won't have a parachute system onboard. This will also save weight and length.

The fuselage

The fuselage will be made of PVC reinforced with glass fibre. This is of course a very dirty solution, because it's heavy (1.25 kg/m). But since we lack the experience and moulds for making a pure glass fibre fuselage (~ 0.5 kg/m). This is the quickest and cheapest solution. Also, in this way, the fuselage will require the same construction technique as the motor casings.

The coupler will also be reinforced with glass fibre. The fins of the second stage will not interfere with the coupler, because they are positioned slightly higher up the second-stage fuselage. This will ease construction and save time and weight. The downside is that the fins are less efficient and therefore will have more surface area and thus more friction drag.

The motors

In order to speed up the design process the motor casings will use the same construction techniques as the fuselage. Since the casings are made out of one piece, the nozzles couldn't be cast in the casing, as with the concrete nozzles of the PVC motors. Therefore, the nozzles were made out of steel.

The motor designs will practically be the same as the PVC motor and therefore lowering the risk of this part of the project dramatically. Since all parts are in the same design program, the motors can be fitted in the virtual rocket long before it is actually built.

The motors will be fitted with a retention bolt at the top end. This will eliminate the need for a motor retainer, thereby decreasing the complexity and weight of the second stage.

The motors built and tested!

The motors have been built and tested for the first time! This was done in complete darkness, because of time constraints. Too bad, because now only low quality video imagery exists. On the bright side now we know the composite casing concept works! First some pictures of the two types of motors we built and tested:

This is the booster motor, which will push the rocket from terra firma into the sky. It contains 2.4 kg of sorbitol propellant, which is configured into a six grains BATES configuration. The empty weight of this motor is approximately 1450 grams. The motor will produce approximately a total impulse of 2300 Ns because it's low operating pressure. This motor was tested in exactly the same form as we are tending to use in the Quick and Dirty.

The other motor is the sustainer motor of the upper stage. This motor also has a glass fibre reinforced casing. The motor we built contains 700 grams of propellant, configured in a four grain BATES configuration. The empty weight of this motor is 600 grams. This motor will be altered with a smoke generator of 15 seconds added. Therefore the motor will have a fifth grain without a core. This extra grain will act as a cigarette burner and produce the smoke which will enhance the visibility and reduce the air resistance.

The tests proved the motors work. The thrust-up of the 70 mm motor (first in the video) is very rapid, insuring a high tower exit velocity. The second motor also proved a good performing motor, although the motor spits out a part of a liner in the final phases of the burn.

The video images itself are very blurry, because the camera had nothing to focus on in the dark.

Test aftermath

Back home again the motor and the nozzles were examined. The 70 mm composite casing was only scorched at the attachment point of the nozzle. The thermal loading at this point is huge, because the steel nozzle warms up very rapidly due to the heat flowing through it.

The nozzles itself are fully reusable and suffered no damage, only a good thorough cleaning was necessary to prevent corrosion.

So now the motors are validated, it is time to put them on the static test bench and look at their performance. This will be done when the opportunity occurs.

The rocket

The upper stage, including nose cone, is completed in record breaking time. After only five building evenings the whole nose cone and fuselage, including fins, is ready for painting.

The booster itself is very little work in comparison with the rocket. This will be just a tube and a coupler with a bulkhead in it. The fins will be attached on the fuselage with a very strong bonding epoxy in the same style as the booster fins of the Spectre IIb.

Next stop: The instrumented test.

Next part: Quick and Dirty, part 2, 16 April, 2007

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