GALILEO GALILEI"  (GG)
A Small Satellite to Test the Equivalence Principle of Galileo, Newton and Einstein
Proposal to ESA, F2&F3 Competition, January 31 2000

3. The Payload

The GG payload is constituted by: the PGB laboratory (Pico Gravity Box) enclosing, in a nested configuration, two cylindrical test bodies with the read-out capacitance plates for very accurate sensing of their relative displacements, the small capacitance sensors/actuators for sensing relative displacements and damping the whirl motions, the suspension springs and the coupling flat gimbals, the FEEP thrusters for drag compensation (physically located on the spacecraft outer surface), the inchworms and piezo-ceramics for fine mechanical balancing and calibration. All suspended bodies are provided with a locking mechanism to withstand launch accelerations and to be unlocked once the nominal attitude (perpendicular to the orbit plane) and spin rate (2Hz) have been achieved at the nominal orbit (circular, equatorial, 520 km altitude). In addition, all bodies have a locking mechanism made of inch-worms for finer control of their unlocking at the beginning of the mission26, Ch. 2.1.6. The payload apparatus includes the electronics (for calibration, signal measurements, FEEP control and whirl damping), although a large part of it is located at the s/c level outside the PGB. The PGB also carries a small mirror, in correspondence of a photo-detector mounted on the inner surface of the spacecraft, for the measurement of small residual phase lags between the spacecraft and the PGB which might remain despite the passive mass compensation mechanism26, Ch. 2.1.2 and Fig. 5.7; residual phase lags will be reduced to acceptable levels using the FEEP. No such phase lags will arise between the test bodies due to the thermal stability achieved inside the PGB26, Ch. 4.4.

The total mass of the payload is 79.6 kg (with system margins), including electronics, capacitors, inch worms, rods etc… The breakdown is given in26, Ch. 4.1, with mention to the materials used. See also Table 4.1. A 3D view of the payload internal to the PGB is shown in Figure 3.1 where the locking/unlocking mechanisms (LUM) of the test bodies are well visible.

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Figure 3.1 Overall, 3D view of the GG experimental apparatus internal to the PGB laboratory (of 55 cm height; only the central tube of the PGB is shown, in dark pink). Of the actual apparatus to be located inside the PGB everything is shown here except the outer test cylinder, which would hide all the components inside it. The locking/unlocking mechanisms are shown in gray; the cylindrical bullets and their actuators, also in gray, are well visible only for the outer test cylinder; the inner test cylinder is locked. It is shown in blue, partially covered by the capacitance plates of the read-out (in light pink). The inch-worms for the mechanical balancing of the read out capacitance plates are schematized as small cylinders (in light blue). Note that these plates are rigidly connected (through the inch worms) to the PGB tube. There is one inch-worm per radial shaft, each plate has 2 shafts, 1 at the top and 1 at the bottom, amounting to 8 shafts and 8 inch-worms for the total read-out system. The small plates of the active dampers are shown in light blue. Each damper consists of 2 halves: 1 is connected to the inch-worm case, hence to the PGB, and the other to the test body. In this way it is possible to sense the relative position of each test body with respect to the PGB, and to actuate in order to damp their whirl motions (details in26, Ch. 6).

 

Each LUM (there is one at the top and one at the bottom) has: 4 mechanical arms connecting it rigidly to the PGB laboratory, 8 fixing cylindrical bullets, 4 for holding the inner test cylinder and 4 for holding the outer test cylinder, which are inserted into the test bodies during the GG assembling (note that the inner test cylinder has smaller height than the outer one and therefore the bullets for holding it need to be mounted on higher supports); 8 actuators, 4 per test body, to pull out the cylindrical bullets whereby unlocking the test bodies; a transmission mechanism. At the time of unlocking the actuators, by means of the transmission mechanism, pull out the cylindrical bullets from the test bodies whereby freeing them. The actuators taken into consideration for this purpose are either electric motors or paraffin actuators (as used already in BeppoSAX). The electric motors would be located on the mechanical arms far away from the test bodies; in any case, they need to be used only once, at the beginning of the operation phase. After unlocking, the motors would be switched off and, in case of paraffin actuators, the required heat would be dissipated and the heat source turned off. Note that, once freed, the test bodies are constrained by mechanical stops which allow them to perform only little movements (about 0.5 cm room). This is well visible also in Figure 3.1, showing the shafts sticking out of the PGB tube (8 per test body, 4 at the top and 4 at the bottom, each arm at 90º from the next). Inside the PGB tube they are connected together and suspended by a spring to the arms that couple the test bodies via flat gimbals. In addition, a finer locking is available26, Ch. 2.1.16, Fig. 2.20 by means of inch-worms equipped with pressure sensors (8 per test body, 4 at the top and 4 at the bottom) which allows fine release and ensures a safe transient from the initial unlocking to the dynamical state at regime simulated numerically in26, Ch. 6. See26, Ch. 2.1.7 for the calibration procedure.

A 2D view of the coupled system of the test cylinders is shown in Figure 3.2 and discussed in the caption. There are active dampers also between the PGB tube and the spacecraft (shown in Figure 2.4). Each damper is made of two little plates, just like those described here for the test bodies; the inner plate is rigidly connected to the PGB tube while the outer one is rigidly connected to a short spacecraft tube; in addition, axial movements between the PGB and the spacecraft can be sensed and adjusted, which requires 8 additional small dampers between the PGB and the spacecraft, 4 at the top and 4 at the bottom. In total, the PGB has 16 small dampers. It is worth adding that power and signals are brought inside the PGB via its helical suspension springs (3 wires each spring), as described in26, Ch. 2.1.1, Fig. 2.5. No electric signal goes through the helical springs which suspend the test bodies (2 for each body, see Figure 3.2). The piezo used for calibration and for adjusting the arms which couple the test cylinders (also located inside the PGB tube) need to be commanded: this requires 3 wires, which are selected to be 3 of the 6 wire sectors of each of the two flat gimbals through which the coupling arms are connected to the PGB tube, so that they can be insulated at their clamping rings (Figure 2.6).

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Figure 3.2 (to scale). Section through the spin axis of the GG test cylinders (10 kg each; Be and Pt/Ir in this drawing; the height of the outer Be cylinder is 21 cm) and the capacitance plates of the read-out in between. The lower density cylinder (21 cm in height) encloses the higher density one. Inside the inner cylinder is a narrow tube rigidly connected to a laboratory (also of cylindrical shape) called PGB (Pico Gravity Box) enclosing the test bodies and the read-out capacitance plates for sensing the relative position of the test cylinders. The PGB in its turn is mechanically suspended inside the spacecraft (not shown; see Figure 2.4 for an overall view). For coupling the test bodies there are two "coupling" arms (shown in light blue) located inside the PGB tube but not in contact with it; the inner test cylinder is suspended from the coupling arms at its center by means of two helical springs; the outer one is also suspended from the arms with helical springs, one at the top and one at the bottom of its symmetry axis. The only connection between the coupling arms and the PGB laboratory is via two flat gimbals (Figure 2.6) at the midpoints of each arm. Being pivoted on torsion wires the gimbals allow conical movements of the coupling arms around their midpoints, e.g. in response to a differential force between the test bodies. The piezoelectric actuators shown next to the gimbals are for adjusting the length of the two halves of each coupling arm. The capacitance plates of the read- out are shown in between the test cylinders; they are connected to the PGB tube and have inch- worms for adjusting their distance halfway between the surfaces of the test cylinders (mechanical balancing of the capacitance bridge) so as to improve sensitivity to differential displacements. On the PGB tube are shown the mechanical stops which constrain the test bodies to only slight movements. The small capacitance sensors/actuators (with plates of about 2 cm2) are for sensing and damping the slow whirl motions of the test bodies with respect to the PGB26, Ch. 6.

The orbit of the GG satellite is such that it spends almost half of its time in eclipse (shadow of the Earth) and half in sunlight: equilibrium temperatures in the two different conditions may be very different. Due to the fast spin of the satellite, the temperature has azimuthal symmetry, but temperature gradients between the dark side and the lightened side of the spacecraft (radial gradients) may still be significant. However, temperature gradients can be made negligible if the satellite is properly insulated: in fact, the temperature of the payload becomes stable if we are able to reduce the heat flowing from the inner surface of the satellite towards PGB and Test Masses. The thermal control of the GG P/L is passive. It is based on thermal insulation as good as possible among test masses, PGB and S/C, and on vacuum inside the satellite, so that energy is transferred only by radiation among the large surfaces inside the satellite, and not by thermal conduction. Thermal conduction is limited to the small connecting elements between the spacecraft and PGB, and between the PGB and test masses: the smallness of the connecting elements makes it feasible to reduce the heat flux inside them. Moreover, only the preamplifiers are located at the PGB level: the rest of the electronics is located at the s/c level. The analysis reported in26, Ch. 4.4 shows that passive thermal control allows us to fulfill the requirements posed by an EP test to 10-17, namely a temperature drift smaller than 0.2 K/day (see Sec. 2.5).

The GG payload electronics is composed by two major sub systems: the Payload Control Electronics and the Payload Data Processor. The former is in charge of performing active damping to the PGB and the test masses, as well as of performing data acquisition from the sensors; the latter is in charge of processing the data received from the Payload Control Electronics, TLC/TLM management, S/C interface and FEEP thrusters actuation for drag free control. The payload electronics and the FEEP control electronics have been designed for GG by Laben during the ASI Study of the mission to Phase A Level26, Ch.4.2.2, Ch. 4.3 based on the control laws for whirl damping and drag compensation developed by Alenia Spazio during the same Study26, Ch.6. 6 FEEP thrusters are needed26. Fig. 5.14; their firing noise is at the spin frequency and is attenuated by the PGB suspensions (see transfer function26, Fig. 2.6).

In the GGG prototype the electronics for the read-out and the active damping of whirls has already been manufactured and testedSec. 5.

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