Hardened Hydrazine the source of Galileo satnav FAIL

Russian aerospace firm's kit fails on 46th mission

A “shortcoming in the system thermal analysis performed during stage design” (for the Fregat launch vehicle’s fourth stage) was the reason two of Europe's Galileo satnav craft ended up in the wrong orbit following a launch earlier this year.

As we reported back in August, two failure meant two Gallileo sats landed in the wrong orbit, causing much hand-wringing at the European Space Agency. The mess was later blamed on a software bug.

But Arianspace, the commercial launch operator that sent the birds aloft, now says that wasn't the case and that the mission's fourth stage was built to fail.

An internal investigation found that the three stages of the Soyuz launcher all performed as expected. But Fregat (built by Russian aerospace outfit NPO Lavochkin) struck problems “at the beginning of the ballistic phase preceding the second ignition of this stage”.

Here's what Arianspace says went down up there:

  • The orbital error resulted from an error in the thrust orientation of the main engine on the Fregat stage during its second powered phase.
  • This orientation error was the result of the loss of inertial reference for the stage.
  • This loss occurred when the stage's inertial system operated outside its authorized operating envelope, an excursion that was caused by the failure of two of Fregat's attitude control thrusters during the preceeding ballistic phase.
  • This failure was due to a temporary interruption of the joint hydrazine propellant supply to these thrusters.
  • The interruption in the flow was caused by freezing of the hydrazine.
  • The freezing resulted from the proximity of hydrazine and cold helium feed lines, these lines being connected by the same support structure, which acted as a thermal bridge.
  • Ambiguities in the design documents allowed the installation of this type of thermal "bridge" between the two lines. In fact, such bridges have also been seen on other Fregat stages now under production at NPO Lavochkin.
  • The design ambiguity is the result of not taking into account the relevant thermal transfers during the thermal analyses of the stage system design.

All of which sounds a bit like someone didn't properly account for how cold the launch vehicle would get, which froze its fuel, which in turn meant the rockets didn't fire enough or soon enough to get the satellites into the desired orbit.

Fregat systems have been used in 45 previous missions without this problem arising, so while Arianspace is clearly unhappy about this mess, its statement on the matter contains a fair bit of bonhomie. Indeed, it is expected that Fregat can be fixed in time for a planned launch in December.

The ESA, meanwhile, has had nothing to say about the prospects of the two stricken satellites since September, when it said “The potential of exploiting the satellites to maximum advantage, despite their unplanned injection orbits and within the limited propulsion capabilities, is being investigated.” ®

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