Sounding rockets and satellites have almost constantly been launched with launchers driven by solid or liquid motors, or in combination. A third alternative has always existed, but has not been used to any appreciable extent – the hybrid engine. As the name indicates, it is a combination of the two first types.

Now, the hybrid engine seems to be a more realistic alternative. For years, the Norwegian company, Nammo Raufoss AS, has been working on developing a hybrid engine based on well-known principles and currently it has reached its launching phase of a full-scaled test of different types of the engine, adjusted to the needs of potential customers.

In 2004, the company started running laboratory tests and developing different types of fuel and oxidisers in addition to developing the necessary injection and ignition systems. Much of the work has been carried out in cooperation with the Swedish company Saab Dynamics due to their experience with the liquid oxidizer Hydrogen Peroxide (H²O²).

The theoretical calculations and small-scale tests are as of now almost done and full scale tests can start. Building a dedicated test centre has shown us that the company has great expectations for the development and is willing to spend substantial amounts of resources to complete the mission. In this test centre all sizes of planned engines can be burned, from small thrusters to large engines for satellite launchers.

 Can be Used all Over

Although the principle of the hybrid has been known for several decades, it has not yet proved successful, however, the advantages in relation to the traditional rocket engines are clearly formidable, and thus, working on the principle was intensified and non-stop. Virgin Galactic is so far the best-known user, though not the only one, and along with Spaceship II, they will bring tourists close to space. The European Space Agency has also found the engine principle interesting, both for sounding rockets, large launchers and for small thrusters. Hybrid rockets have also proved interesting as engines for ESA’s Mars Sample Return Mission.

One of the great advantages is the non-explosive and non-toxic fuel and oxidisers. A rocked can be fully manufactured, assembled and stored for a short or long time without special efforts. Either the oxidisers or the fuel is toxic and, if the flow control is functional, the fuel cannot burn.

Hybrid Rocket

The hybrid engine principle

A hybrid rocket has an engine which uses propellants in two different states of matter – one solid and the other either gas or liquid, almost similar to the traditional liquid rockets, but with less complicated construction.

Hybrid rockets exhibit advantages over both liquid rockets and solid rockets:

Non-explosive. Unmixed oxidiser and fuel.

• Non-toxic propulsion package.
• Solid fuel is cheap, the oxidiser is easy to handle
• Can be throttled, shut down and restarted during flights
• Fast turnarounds. Boosters are rechargeable and components are simple
• High-performance, several hybrid rocket propellants produce specific impulse levels superior to that of solid propellants

Because it is nearly impossible for the fuel and oxidizer to be mixed intimately (being different states of matter), hybrid rockets tend to fail more infrequently than liquids or solids. Like liquid rockets and unlike solid rockets they can be shut down easily and are simply throttle-able. The theoretical specific impulse performance of hybrids is generally higher than solids and roughly equivalent to hydrocarbon-based liquids. Hybrid systems are slightly more complex than solids, but the significant hazards of manufacturing, shipping and handling solids offset the system’s simplicity advantages.

In its simplest form a hybrid rocket consists of a pressure vessel (tank) containing the liquid propellant. The combustion chamber contains the solid propellant and a valve isolating the two. When thrust is desired, a suitable ignition source is introduced in the combustion chamber and the valve is opened. The liquid propellant (or gas) flows into the combustion chamber where it is vaporized and then reacted with the solid propellant.

The Norwegian version of the hybrid engine is a combination of their monopropellant hot gas rocket engine and solid fuel package with nozzle. What is clearly unique when talking about the advatanges of the engine is the lack of ignition system. A catalyst unit converts liquid Hydrogen Peroxide into hot gas through catalytic decomposition. The hot gas itself ignites the solid fuel and only to open the valve for the oxidizers the engine starts.

The Fuel

Much of the work has been concentrated on different types of fuel and oxidizers and the best combination of this. As fuels, several types and variants of HTPB have been tested as well as oxidizers, such as gaseous oxygen, liquid oxygen, hydrogen peroxide, nitrous oxide and nitrogen tetroxide. At last, the company has chosen to further develop the 87.5% Hydrogen Peroxide as oxidiser and Hydroxyl Terminated Poly Butadiene as fuel.

Hydrogen Peroxide (H²O²) is a common ordinary existing liquid most known as bleaching agent. The liquid is room storable with a low freezing point. Europe has a high-level of knowledge and qualified expertise regarding the liquid and large-scale industrial production and availability are found within Europe. As oxidiser in a hybrid rocket H²O² has an attractive system-specific impulse, as monopropellant a specific impulse about 1600 m/s.
HTPB – Hydroxyl-Terminated Poly Butadiene is a polymer of butadiene terminated at each end with a hydroxyl functional group. It reacts with diisocyanate to form polyurethane, a stable and easily stored synthetic material. The properties cannot be precisely stated, because HTPB is manufactured in various grades to meet specific requirements. HTPB is thus a generic name for a class of compounds. It consists of many butadiene molecules linked together into a polymer forming polybutadiene. Both ends of the chain are terminated with a hydroxide ion [OH-], thus giving it the name Hydroxyl-Terminated Polybutadiene. Although it is used in rocket fuel it does not ignite easily – it requires temperatures in excess of 773K  (400⁰ C) to combust and it is very safe to handle.

The combination is regarded as a Green Propellant and will therefore contribute to the vision about “Clean Space”.

The Next Steps.

The first step is to develop different types of sounding rockets. So far, the rocket engine is planned in two versions, the UM1 with a thrust at 25 kN and UM2 with a thrust of 100 kN. Through variation of number of this in a rocket, the company can offer thrust from 25 kN to a thrust at 400 kN. All these combinations are well suitable for sounding rockets with different needs for payload capacity. However, the ambitions go beyond this, one of which is to combine different types of hybrid rockets in a first and second and a Hot Gas Thruster as the third stage, it will be possible to launch small satellites to polar orbits – one has developed the Norwegian “North Star Launch Vehicle”.

Though a lot of work remains, launching satellites from Norway may still become a reality.

Featured photo: The Northern Light from ISS. Credit:  ESA/NASA