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Quality assurance in satellite engineering at 10-5 millibars

HOW OHB TESTS SATELLITES FOR THEIR SUITABILITY FOR SPACE

Before being sent into orbit on board a launcher, satellites undergo extensive testing. To this end, they are placed in a vacuum, heated and cooled. Dr. Dieter Birreck, director of the Heinrich Hertz satellite mission, explains how OHB tests its satellites.

 

Dr. Birreck, why does OHB test satellites in a thermal vacuum chamber?

We need to expose satellites to the type of conditions in which they will operate later on. As there is no vacuum on earth in contrast to space, we create it artificially in a vacuum chamber for testing. We want to prove that the satellite will not experience any problems when it is in orbit later on.

Where are these chambers located?

Individual components are first tested separately in the thermal vacuum chamber. These chambers can be found in many different places. We also have a small TVAC chamber at OHB in Bremen. But the truly exciting moment is when the entire satellite has been assembled and is then tested. In Europe there are a number of very large thermal vacuum chambers which can also accommodate complete satellites, such as the one at IABG in Ottobrunn near Munich. We also test smaller satellites here at OHB.

How is the test set up?

The thermal vacuum chamber comprises a gigantic chamber, i.e. a tube in which the satellite is inserted. First of all, we pump the air out of the chamber until we obtain a vacuum of around 10-5 millibars.

Then we can cool or heat the chamber. This is necessary because the side of the satellite facing the sun can become very hot, while the side facing away from it is correspondingly cold. The temperature extremes range from around –150 °C to +100 °C.

We monitor the test environment as well as the satellite, which is now activated, and perform a thermal balance test. The satellite is brought to a constant temperature and we test whether it reaches all temperatures predicted in the models during operations. The models demonstrate theoretically that the satellite is never too hot or too cold in any given place. The thermal balance test verifies the model.

What happens next?

Then the satellite is exposed to extreme, very hot or cold temperatures. We also test the transitions from hot to cold because if it is always too hot at a certain time of year, this gives rise to problems in that the satellite will always experience this each year.

If the satellite survives the first year, it won’t see any other temperatures in the second year. That’s why we do this test.

How exactly do you measure the temperature?

A satellite has hundreds of sensors which are also used in space to determine whether it is working properly or not. Further sensors are specifically installed for testing purposes. In individual cases, there may be up to three or four hundred additional sensors depending on what the thermal specialists want to observe. The tests allow us to intervene when a temperature becomes critical and ensure full operations.

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What can you do if temperature problems arise?

In that case, we adjust the satellite’s thermal system to achieve optimum temperatures, perhaps by installing additional heaters for example. Frequently, it is enough to insulate the satellites more effectively or to open the insulation. We use multilayer metallized plastic films as insulation. In some cases, we may also cover parts of the radiator surfaces reflecting the sun’s rays.

Temperatures are the main problem in orbit. In what way does the vacuum adversely affect the components?

Some waveguides may experience problems when moving from air to vacuum. Many satellites are activated before they are launched into space to solve problems if they cannot be switched on later on. In this way we can check whether the satellite is fully functional a few minutes before the launch.

However, the transition to vacuum conditions may cause problems in the electric fields as well as voltage discharges. That’s why we test this first. Although we can upload new software later on, we no longer have access to anything that is not software once the satellite has left the earth.

How long does testing take?

We spent three months in the thermal vacuum chamber for our new telecommunications satellites range. In the case of normal telecommunications satellites, we usually require around 30 days, while testing smaller satellites generally takes two weeks. Although testing is complex, telecommunications satellites are increasingly growing in size, making it more difficult to launch them. That makes a large and powerful satellite an attractive proposition. The tests ensure that everything is working perfectly.

How large is the team?

The thermal vacuum chamber is the most complicated test that we use. The team works in three shifts around the clock. Each team has 10 to 15 members, making a total of up to 45 people. These include OHB technicians who are responsible for the thermal design and for checking the temperatures. Then we have people who operate the satellite. They activate boxes, send signals to the satellite and verify all the functions. In addition, we have a team on site to run the thermal vacuum chamber itself and, for example, to adjust the temperature and monitor the vacuum.

What difficulties may arise?

Difficulties occur if the modeling is not good enough. This may concern the satellite’s response or the special equipment fitted to the chamber to heat up the satellite. If, for example, the modelling of the external irradiance is inadequate, we cannot heat the satellite properly.

Obviously, the components may also fail during testing, in which case we may have to open the chamber again and repair the faulty part.

Could satellites also be repaired in orbit?

No, that’s not possible. For this reason, every satellite has redundant systems. Multiple components of the same type are installed so that if one component fails another fully functional one can step in as a replacement.

At the same time, research is being conducted into ways of refueling satellites to extend their service lives. A satellite carries with it a certain quantity of propellant which it requires to reposition itself in orbit. Once the propellant has been used up, it can no longer hold its position. Consequently, the quantity of propellant plays a key role in determining the satellite’s life expectancy.

Do you have to document the tests?

Yes, we must submit documentary evidence of the test results to the buyer of the satellite. And the operator of the launch vehicle also needs to study some of the test results as the two systems have to harmonize with each other. We frequently use the European space center in Kourou. So the local operators have to be sure that the satellite does not pose any risk to the launcher during lift-off.

Are there certain testing standards?

In Europe we have the ESA’s satellite engineering standards, the ECSS standards. We build satellites in Europe in accordance with these standards. Other countries have different standards but they are all mutually compatible. The standards define the materials that must be used to ensure the defined service life and quality. In addition, they set out the testing procedure. Thermal vacuum testing is only one of the prescribed procedures. In addition we perform vibration and acoustic tests. This is because the launch produces a great deal of noise and vibration which may possibly impact the satellite.

Our head system engineer also always insists that we suspend the satellites and move all the wheels to see whether they all turn properly.

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