All space-crafts and satellites are equipped with propulsion systems for changing route, orbit or attitude. Propulsion in space is obtainable only as a reaction to an expulsion of substance: the thrust T acting on the space-craft is equal to the product of the mass flow rate m ejected times the emission speed v :
T=m·v
In other words, the same amount of mass ejected generates a thrust proportional to the speed of the jet: the highest speed is obtained by the use of "ion motors", where the substance to be ejected is first ionized, then fast accelerated through an intense electric field.
However, the high value of the thrust is neither the only nor the most important quality of a thruster: especially in the case of new ESA scientific missions, as GAIA and LISA Pathfinder, the most stringent requirements are related to the ability to produce extremely precise "amounts of thrust" in the range (1-500) µN. In comparison with ion motors, the so-called "cold gas" technique seemed to ensure these requirements with higher reliability level, so that it was selected by ESA for these missions.
The "cold gas" technique is based on the emission of nitrogen through a regulating valve followed by a nozzle where the flowing gas expands and accelerates. The effectiveness of this solution depends on the ability of the valve mechanism to accurately generate the required flows, and on the design of the nozzle to expand the gas with a minimum loss of energy.
Fig.1: scheme of INRIM thruster.
In cooperation with Thales Alenia Space, INRIM developed a new design for these valves, based on the use of piezo technology instead of the classic electro-magnetic technique, thus overcoming its limits essentially as regards energy consumption and inability to operate with the required accuracy. In fig. 1 the drawing of the INRIM thruster is given, while the picture of fig. 2 shows the thruster subjected to the vibration-test at TÜV laboratories.
Fig. 2: INRIM thruster under vibration test.
The thruster-prototype fulfills the following ESA requirements:
- burst-test pressure: 1 MPa;
- resistance to sine & random vibration up to 32 g ;
- internal leakage in closed-valve condition: < 10 -7 scc/s with He @ 1 MPa upstream;
- external leakage: < 10 -8 scc/s with He @ 1 MPa upstream;
- flow rate regulated between 0.1 and 50 scc/min;
- mass: < 100 g.
The conical plunger is operated by the actuator made of 5 piezo-benders supplied by a (0-200) V voltage range, acting in series and capable of a whole stroke of 0.1 mm. The actuator is divided into two sections, operated independently: the first 4 piezo benders, so called base-section , are devoted to compensate the wear and the thermal plays that can occur during the different phases of the mission; the last piezo bender is the only one that actually regulates the flow, being a part of the closed loop controlled by a flow rate sensor. The stroke of this last piezo (20 µm) can, alone, cover the whole flow range, from zero to 50 scc/min.
Detailed analysis of the flow field within the nozzle is required in order to be able to compute the performance of the system and therefore to design the nozzle profile. Because of the extremely reduced size (throat opening as small as 0.4 mm), only integral measurements are possible; therefore the analysis was executed via numerical simulation. This approach, though, could not be performed using standard methodologies. Indeed, while on the one hand the flow within the nozzle is extremely rarefied, on the other hand the propeller inlet features a high-density continuum flow.
These two different conditions require the development of a coupled Euler/DSMC method for the flow analysis. In the present phase, the coupling is performed interactively by using two separate codes for the computation of the two regions, and using the results of the continuum region as the boundary condition for the rarefied computation.
Fig. 3: velocity field within the nozzle.
The computations enabled us to obtain complete maps of all the flow field quantities in a number of test cases (in fig. 3 a velocity field within the nozzle is shown). These results can afterwards be post-processed in order to obtain the computed value of any quantity of interest; for instance, delivered thrust and specific impulse can be computed by integrating the momentum variation through the nozzle. The cited quantities, in particular, allowed validation of the numerical method, which was performed as follows. A nozzle prototype (fig. 4) was manufactured and tested on a microbalance in ESA-ESTEC laboratories, Noordwijk (Netherlands). Results with the quantities accessible to measurement (thrust and specific impulse), once compared to the numerical simulations, provided satisfactory agreement.
Fig. 4: the thruster nozzle (section).
Once the method was validated, it was used to improve the nozzle design; actually it is much easier and cheaper to obtain numerical results especially in the case of varying nozzle shape, as the experimental verification would require us to build and test several prototypes. Various nozzle profiles were therefore computed and allowed the selection of an optimum shape. The latter is currently being tested experimentally on the nano-balance of Thales Alenia Space in Torino: the first results are consistent with the required value of 60 s for the specific impulse in the (5-50) scc/min flow rate.
In the middle of 2007, on the basis of the above theoretical and experimental results, ESA chose the INRIM-Thales thruster for the propulsion of GAIA mission: qualification models are at present in phase of construction, while flying models are expected by the end of 2008.
G. Matticari, G. Noci, P. Siciliano, G. Martini, A. Rivetti, N. Kutufa: "New generation propellant flow control ...", 29 th IEPC-23, Princeton University, November 2005.
P.G. Spazzini, R. Arina, A. Rivetti, P. Siciliano: "Numerical prediction...", 17 th IMACS World Congress, T1-I57, Paris (France), July 2005.
A. Rivetti, G. Martini, G. La Piana, F. Alasia: "Thruster PVN02 ..." INRIM Technical Report no. 10, April 2006.