The new European launcher Ariane 6 has started to become reality as the launch complex ELA4 is taking shape at the spaceport Kourou. And first hardware for the launch vehicle is existing already. And some of this hardware is already at the European spaceport Kourou. There are already two test units of the solid rocket motors P120C at the site. This kind of motors will be used as boosters for Ariane 6.
Ariane 6 will use 2 or 4 P120C motors for the first 2 minutes of the flight. The small ESA launcher Vega-C will use one P120C motor for the first stage. Using the same P120C motors for Vega-C and Ariane 6 is one of the synergies in the new launcher strategy of ESA with the main goal to reduce costs for the independent access to space significantly. Manufacturing larger quantities of motors (30-40 units per year are planned) will allow a series production minimizing the costs per booster unit. Obviously, this also means a reduction of the costs per launch.
One of the questions always ask concerning solid rocket motors is why are we still using this old technology with origin in ancient China nowadays? Well, solid rocket motors have a few advantages if they are used for some special applications. The working principle is very simple. You have a big combustion chamber (also called motor case) filled with solid propellant. You are burning a powdered metal. Everybody knows “black powder”. Iron powder is used in that case. It is more efficient to burn Aluminium powder. Therefore Aluminium powder is used in all modern solid rocket motors. Additionally, you need an oxydizer for the burning process: ammonium perchlorate in powdered state. Therefore you need a binder to bring the powders into a solid state allowing a controlled burning process. In the case of the P120C motor the binder HTPB (Hydroxyl-terminated polybutadiene) is applied. It has the advantage of not being only a binder, it also serves as a fuel at the same time. Mixing these three main components yields a solid state with some minor rubber-like touch. This mixture can be stored for several years until usage.
The solid propellant is filling up the entire motor case. There is only one gap in the central part over the entire length of the motor case. That gap can have different shapes. Often it is circularly shaped or has a star-like profile. As only the free surface can burn it is actually this profile that defines the thrust of the motor.
The ignition of the propellant is caused by an igniter sitting in the upper part of the gap. When commanded the igniter sends a jet of flames down the entire gap igniting all the free surface of the propellant at once. In this way you guarantee to have full thrust for lifting off. In the following the propellant is burning down from the inside to the outside. Problems can only arrive in the case that gaps or holes appear in the propellant. In that case more surface of the propellant is available to burn creating a spike in the thrust. In the worst case this can lead to rupture of the booster casing and uncontrolled explosions. With modern X-ray technology it is nowadays possible to detect these gaps or holes inside the propellant to avoid catastrophic failures with solid rocket motors. Therefore no launch failure of the last 20 years was caused by a solid rocket motor. Thus, it is not wrong to state that solid rocket motors are simple, robust and safe systems nowadays. The remaining critical issue is to avoid unwanted activations of the igniter.
There are two main purposes to use solid rocket motors for launchers. Due to their simplicity they are often used for small satellite launchers like the Vega launcher. Or they are used as boosters for larger launchers to move these fast out of the thicker layers of the atmosphere to avoid frictional losses due to interactions of the launchers with the atmosphere. For this purpose you need a booster with high thrust and relatively short burn time. As mentioned earlier you can design a booster for the thrust curve you need by shaping the propellant grain and adopting the mixture ratio for the components of the propellant. Thus, for Ariane 5 the two solid rocket boosters are designed to generate 90 percent of the initial thrust to lift the launcher fast to thinner layers of the atmosphere.
The same methology will be used for Ariane 6: solid rocket motors will generate the majority of thrust during the initial lift phase. The two P120C motors of Ariane 62 will generate more than 80 percent of the thrust during liftoff. The four motors of Ariane 64 will deliver more than 90 percent of the initial thust.
Solid rocket motor technology has been advanced in recent years due to introduction of carbon fiber composites as material for the booster cases. Steel was used as classical material to form the cases of solid rocket motors. The most promiment cases are the segmented steel cases of the Space Shuttle or Ariane 5. Steel is a heavy material and therefore the booster cases have a relatively high mass. The segmented steel cases for Ariane 5 with a minimal wall thickness of 8 mm have a dry mass of about 37 tonnes while holding 239 tonnes of propellant. Therefore about 86 percent of the mass of a loaded Ariane 5 booster is propellant. For Ariane 6 P120C boosters carbon fiber composites are used for the cases. This improves the mass percentage for propellant to almost 93 percent. The dry mass will be about 11.3 tonnes and 141.6 tonnes propellant will be loaded. This improvement in the ratio between propellant and total mass of the booster will yield a positive effect on the mass of the payload that can be launched.
Surprisingly the cases for Ariane 6 are made in one piece of carbon fiber composite (CFC) material. With the a length of 11.5 m and a diameter of 3.4 m they are the largest monolithic booster cases in the world.
The application of CFC for launcher booster cases was introduced in Europe by the Italian company Avio. They developed three different solid stages for the Vega launcher. The P120C motor is more or less an advanced and upgraded version of the P80 motor, the first stage of the ESA launcher Vega. The cases are produced by winding carbon fibers and applying binding epoxy at the same time in the desired shape. That is quite a complicated process. The German company MT Aerospace is developing a simplified production process for booster cases jointly with the German Aerospace Center DLR. Here winding of the carbon fiber is performed as a first step and application of the binder is following as a separate step. MT Aerospace hopes to be able to deliver half of the booster cases for Ariane 6 starting in 2023. Additional information about that topic is available here.
Visiting the BIP building with the first test unit of the P120C motor inside was very interesting and impressive. This test unit is used in Kourou to practice prelaunch processing. The unit has a real CFK made case and was filled with inert non-ignitable propellant (sugar based!) in September 2017. The unit with the propellant has an impressive mass of about 150 tonnes. The inert propellant is shaped like the real one. Therefore it was possible to practice the installation of the igniter. The installation of a mock-up nozzle is next. During our visit we were able to see some preparations for this procedure. The installation is carried out vertically with help of a robotic device. Laser measurement systems monitor any movement of the robotic system to avoid any deviation from the planned position. Safety first! Any unwanted contact with certain areas of a booster with real propellant could cause unpleasant results.
By the way, for our tour of the BIP we had a very special guide. It was Francesco Capri of the company Europropulsion. I had the pleasure to know him already in the virtual world of Twitter as a supporter of my Vega and Ariane 6 model launches. It looks like he volunteered to be our guide as I had announced our visit in Kourou in advance. It was a great pleasure to meet Francesco for the first time in real life. Many thanks for the great tour! We have learned a lot during your tour. Mille grazie, Francesco!
What are the next steps for developing the P120C motor? The first test unit we were able to see will soon go to a test stand in Kourou after the installation of the mock-up nozzle was finished. At the test stand called ARTA preparing a static test firing of a P120C motor will be practised.
The second P120C test unit in Kourou was filled with propellant already. This time it is the real active propellant. At the moment the propellant is curing. Next will be to install additional compoments to the booster case like igniter and nozzle. The first static firing of this P120C motor is currently planned for May 2018 at the ARTA test stand. As we know now all processing for this event was and is still being tested with the first test unit we saw. We are looking forward to this test firing! Fingers crossed. We hope there will be interesting reports about this important milestone. Maybe the hashtag #Ariane6FiresUp could be used for this event – in good tradition! Obviously I would like to volunteer to report about this test firing! 😉
The first flight of the P120C motor is planned for mid 2019. Obviously it will not be an Ariane 6 launch. The P120C will serve as the motor of the first stage of the new Vega C launcher. All the best for that flight, too, and also for the additional static firings planned for P120C motors. All the best!
Dr. Rocket (a.k.a. SpaceHolgar) 😉
P.S.: Back to the hashtag #Ariane6FiresUp: The test firing of the P120C motor planned for May will actually not be the first test firing of an Ariane 6 component. In January 2018 a first test of the Vulcain 2.1 engine for the Ariane 6 core stage was successfully conducted at the DLR test stand P5 in Lampoldshausen, Germany. Many more tests like this one are planned. Again, I would be very happy to report for you about this test firing! 😉