Payload System Engineering

At present the baseline for Planck is that it should be launched in the first quarter of 2007 together with the FIRST satellite, in a configuration often referred to as the "Carrier" (because FIRST is "carried" by Planck in the launcher - see picture at upper left). After launch, Planck and FIRST will separate and will be placed in different orbits around the second Lagrangian point of the Earth-Sun System. The launch configuration of Planck and FIRST (Figure 1) constrains the payload design considerably.

**Figure 1:** A sketch of Planck and FIRST in the Ariane fairing and after separation. The main elements of each spacecraft are outlined. The adapter which mates FIRST to Planck remains attached to FIRST after separation. The size of the cylindrical adapter is fixed by that of the standard Ariane clampband separation device. The telescope and shield must fit within the adaptor.

Figure 2 shows in more detail the the main elements and general configuration of the payload. Thermal aspects are among the main drivers of the design of the Payload Module (PLM), most significantly the requirement to radiatively cool the focal plane environment to a temperature near 60 K. This requirement leads to the existence of two thermal environments on the Planck spacecraft, one at 60 K (the telescope enclosure), and one at 300 K (the Service Module or SVM, which holds the cooling systems and the payload electronics). The two environments are decoupled by means of a series of three so-called V-grooves, specular conical structures which evacuate very efficiently to space the heat into the PLM originating in the SVM.

**Figure 2:** A crosssectional cut of the Planck payload showing its main elements. The extraction from the FIRST mating adaptor requires that the telescope and radiation shield fit into a conical volume defined by the clampband diameter (2.624 m), and a 9^o clearance angle. The size of the solar arrays (located at the bottom of the Service Module, and which also shield the payload from solar illumination) is constrained between the diameter of the Ariane fairing, and a minimum required to allow a 10^o degree inclination of the spacecraft with respect to the Sun direction (with no direct solar illumination of any part of the payload). Also shown are the three nested conical radiators which thermally decouple the payload from the Service Module.

The telescope line-of-sight is oriented at an angle of 80° with respect to the axis of symmetry of the spacecraft. Figure 3 shows how the elements of the payload (telescope, shield, V-groove radiators) are brought together into this configuration. Not shown are the instrument units located in the SVM (e.g. LFI Back End Unit, sorption cooler compressors, electronics, etc), and the linkages between the two (waveguides, cryogen piping, harness, etc). These linkages also constrain (mechanically, thermally) the payload design quite significantly.

The mass and power budgets of the Planck payload are summarized in Table 1. In the following sections, we discuss system engineering issues which are of importance in determining the final performance of the payload.

**Table 1:** Payload Mass & Power Breakdown
Item		Mass (kg)	Power (W)
Telescope (Prim. + Sec.)		17.5+4.8
Tel. Structure		40
Shield & Baffle		34 + 1.2
LFI	FEU (inc. SCCE and struts)	25.6	0.55
	Waveguides, coax, and pipes	6
	BEU, RAS, SCP, SCC, REBA, Harness	85.4	609.5
	Total	117	610
HFI	FPU (excl. LFI FEU)	15	0.03
	J-FET box	2	0.1
	All other units	200	325
	Total	217	325.1
V-grooves		51
SVM shield		27
GFRP struts		18.2
I/F ring		34
Equipment platform		17
Thermal Control		2
Miscellaneous		1
Total		582 kg	935 W
Total (w/ 20% margin)		698 kg	1122 W