Shape Memory Alloy valves can be used in space for reducing the risks of spacecraft break-ups. ESA is investigating in SMA valves that could be activated at the end of the missions to vent all the remaining propellant.
The number of debris in orbit requires an urgent reaction if we want to keep benefiting from it. New national and international legislations and guidelines are imposing the rules to take that path.
SMA actuated valves are devices considered to mitigate the risks of in-orbit break-ups, therefore supporting the compliance with these new sets of requirements.
Except for a few collisions (less than 10 accidental and intentional events), the majority of the 200 break-ups observed in-orbit were explosions of spacecraft and upper stages – typically due to leftover fuel, material fatigue or pressure increase in batteries.
Break-ups account for circa 35% of debris with a further 30% of debris from unidentified sources.
In the propulsion systems, break-up occurs due to the internal pressure resulting from the stored energy in propellant and pressurant tanks, in the form of pressurant gas or residual propellants.
The ESA’s Annual Space Environment Report – released first semester 2017 – lists a few cases of sub-classes for rocket stages that showed repeated breakup events:
- Delta upper stage There were several events for Delta second stages due to residual propellants until depletion burns were introduced in 1981.
- SL-12 ullage motor The Blok D/DM upper stages of the Proton rocket used two ullage motors to support the main engine. They were released as the main engine performed its final burn.
- Titan Transtage The upper stage of the Titan 3A rocket used a hypergolic fuel oxidizer combination.
- Briz-M The fourth stage of the Proton rocket which is used to insert satellites into higher orbits.
- Ariane upper stage Breakups for the H8 and H10 cryogenic stages were observed, most likely due to overpressure and subsequent bulkhead rupture. Passivation was introduced in 1990.
- Tsyklon upper stage The third stage of the Tsyklon-3 launcher used a hypergolic fuel oxidizer combination.
- Zenit-2 upper stage The second stage of the Zenit 2 launcher used an RP-1/Liquid oxygen propellant.
These examples show how crucial it is that future spacecraft fly using passivation systems that will allow to perform a depletion of the stored energy. To reduce the risk of such breakups, it is necessary to create venting paths so that pressurant gas and propellant leftovers can be safely removed throughout the disposal phase.
Several methods exist to passivate:
- at spacecraft level: depletion burns can be performed in order to consume as far as possible propellants left on board. However, for hydrazine systems for example, thrusters are not qualified to operate at low enough pressures to fully vent the remaining propellant and pressurant gas at end of life. Moreover it requires long operations and the presence of gas bubbles in the pipes may cause sudden thrust variations hard to predict.
- at subsystem level: venting valves or pipe perforators can be used to open a path to outer space permanently in order to vent the pressure from the propellant and pressurant tanks.
According to studies done with the European satellite integrators, the most efficient way to properly vent a satellite is through the use of a nominally closed and leak tight passivation valve such as the ones using pyrotechnic actuators or Shape Memory Alloy. Presently pyro-valve actuators are qualified for a 8 years lifetime which is too short for the application one is looking for. Indeed, most spacecraft have a lifetime of 10 to 15 years on top of the ground storage time.
SMA is an alloy that remembers its original shape. It is deformed for some applications and when needed it will returns to its pre-deformed shape when heated. Even though there is less heritage in Europe, the Shape Memory Alloy (SMA) valve provides many potential advantages:
- unlimited lifetime,
- lower shock,
- low cost,
- no pyro handling constraints and
- longer time of activation allowing the operator to intervene.
ESA is therefore promoting the development of an SMA valve with ASL that would do the following:
- when the Phase Transformation Temperature (PTT) is reached then the actuator shall elongates to its initial state. The PTT can vary from -40°C to 450°C even if the baseline is around 100°C.
- the force will then act on a notch until it breaks up, leading to the opening of the valve.
However, in order to confirm the benefits of the SMA valves for this application and prove it’s compatibility with propellants and pressurant gases, extensive testing need to be performed to fully qualify this valve for the use of passivation of satellites, that is required in every orbit. This activity should be completed already beginning 2019.