ESA’s Future Launchers Preparatory Program and France’s CNES space agency have test fired the Pressure Oscillation Demonstrator eXperimental (POD-X) motor at the French military procurement agency test site. The test relates to providing information on pressure oscillations in solid motors, an issue that was highlighted during the development of the since-defunt Ares launch vehicle.
Solid Motors and Oscillations:
Thrust Oscillation (TO) – also known as ‘Resonant Burning‘ – is a recognized characteristic of solid rocket motors, technically described as vortices that shed within the solid rocket motors during combustion due to the shearing of internal flow at propellant discontinuities.
The issue relates to when the frequency of thrust oscillations is coincident with the acoustic modes of the motor cavity.
It became the focus of the Constellation Program (CxP), when resonant modes in the Ares I structure were thought to be capable of harming the astronauts riding in the Orion spacecraft via vibrations in the latter part of first stage ascent.
The Ares I rocket involved a five segment solid motor as the first stage, with a J-2X driven liquid upper stage, and Orion riding atop of the stack.
While it was later revealed the TO concerns were somewhat overblown via highly conservative safety parameters, CxP engineers were tasked with designing an array of mitigation hardware, from dampers to isolation mounts, reducing the amount of vibrations riding up into the Orion spacecraft.
Ares I – along with the Constellation Program – was cancelled before the mitigation techniques could be tested during flight, as NASA was refocused on to a human-rated Heavy Lift Launch Vehicle (HLV).
SLS – with its two solid motors on either side of the stack – naturally disconnects the concerns relating to oscillations suffered by the Orion spacecraft.
European Testing:
For the purpose of gaining further understanding into thrust oscillation in solid motors used by Europe’s rockets, an experimental simulator was test fired at the French military procurement agency test site in Saint Jean d’Illiac, France.
Known as the Pressure Oscillation Demonstrator eXperimental (POD-X), the test – which was conducted on Tuesday – allows for the studying of controlled oscillations that will aid engineering understanding of the underlying physics, with a goal to devise countermeasures that will drastically reduce the effect in future motors.
The POD-X simulator – under the cooperation of ESA’s Future Launchers Preparatory Program and the CNES space agency – was designed and built by prime contractor Europropulsion with partners Herakles and Avio SpA, with France’s DGA military procurement agency responsible for operations and testing.
According to ESA, the test mimicked the firing of Ariane 5’s solid boosters, at a reduced scale of 2:9. The firing lasted 28 seconds at a pressure of 65 bars.
The six meter long, 70 cm diameter motor was loaded with 2700 kg of solid propellant inside its case components made of high-strength steel. The motor is designed to be reused 30 times.
Kitted out with 263 sensors, the resulting data will help to validate the numerical modelling tools for mastering the phenomenon, that – per ESA’s overview – will allow for multiple configurations and innovative technologies to be tested in a representative way.
Some of the concepts envisaged for Europe’s next generation launcher, the Ariane 6, will be tested in this simulator over its forward test path.
The Future Launchers Preparatory Programme (FLPP) was set up by ESA in 2003 to anticipate the needs of developing launcher technology, be it for Ariane, Vega – which also uses solid motors – and future launch vehicles.
“Launcher technology matures by combining different technologies and ultimately testing them using large integrated demonstrators,” according to ESA. “FLPP is the link between concept and demonstration to accelerate the move to maturity and to mitigate the risks.
“Improving propulsion techniques is important for maintaining and widening our gateway to space. It takes extreme speed to reach orbit and precisely controlled thrust to get a mission where it needs to be. Without propulsion technology, nothing goes anywhere.”
The advancement of solid booster technology is continuing in the United States, with ATK pressing forward for the next test firing of the five segment motor, two of which will aid SLS’ debut ride uphill in 2017.
However, per extensive notes in L2, full scale test articles – utilizing mitigation technique – are being built, with initial results proving to be positive.
An article will follow on this progress later this month.
(Images: Via ESA, NASA, ATK and L2 content – To join L2, click here: http://www.nasaspaceflight.com/l2/)