The payload (PL) subsystem includes the essential hardware and instruments needed for the tether experiment, the most essential one being the tether itself. The space for the payload is in the centre of the upper panel of the cube. The spin axis of the satellite should be perpendicular to the tether deployment plane. The payload compartment is 15 mm high and (excluding the satellite walls) and 95 mm  95 mm wide. It has three openings for the tether, the camera and the electron gun. It is sensible to put the reel motor next to the capstan where its torque is needed, and if the reel drum is also motorised then it must be close to the motor as well. The camera and the electron gun must be mounted at the surface. The electron gun should not be very close to the tether.

Tether reel
The proposed reel is a 6-cm diameter, 1-cm wide drum, which is capable of holding a 9 mm wide Hoytether. The mass of the reel is 11,3 g, estimated by assuming that the drum is made of 1 mm thick plastic (density 1,5).

Tether reel bearings and xing are estimated 2 cm³ of Al equivalent needed, mass 5,4 g. Estimated mass is 10 g. The baseline approach is that the capstan is motorised but the reel drum is only braked with passive friction. This design is not capable of retracting the tether. Tether reel launch lock will prohibit the reel from rotating during launch. Estimated mass is 5,65 g (50% of reel mass). Tether capstan mechanism is a rotating motorised reel made of elastic rubber-type material, which presses towards a passively rotating metal axis, a design mimicked from cassette tape decks. If the capstan reel is 1 cm in diameter and 1,5 cm in length, its mass is 1,8 g if made of plastic (density 1,5). The metal axis mass is 1 g if made of steel, assuming a 0,3-cm diameter. If one reserves 1 g each for reel and axis bearings, the total mass is 5 g after rounding upward.

Tether
The proposed tether is a 9 mm wide 4-fold Hoytether with a 30^o angle between the parallel and diagonal wires, diagonal wires not bonded together where they cross. The wire material is Al99Si1. The parallel wire thickness (diameter) is 50 m and the diagonal wire thickness is 25 m. The length of the tether is 10 m and mass 0,137 g (the wire material density 2,7 g/cm3). Tether end mass 0,25 g is close to optimal. At this end mass, if the initial spin is 1 revolution per second and the tether length is 10 m, at the end of deployment the spin period is 18 s and the tether tension at the tip is 30 mg (equivalent to 2,2 m of tether hanging at earth). If the end mass is heavier, the original spin period will be longer and the tension lower, which may not be enough to straighten the tether. On the other hand if the end mass is lighter, the tension at the start of deployment may be dangerously low. The end mass must be fixed during the launch at the tether opening and released at the start of the deployment. Since we do not know yet how the fixation will be implemented, let us reserve 2 g for it (four times larger than the end mass itself), plus 1 g for cabling and connector since the release signal must be transmitted from the controller.

High voltage (HV) source
A 1 W, 200 V voltage source is needed in the tether experiment. Estimated total mass is 25 g. For example, EMCO produces a device capable of outputting 0,5 W and weighing only few grams and available also as lowoutgassing- epoxy version. This HV source is also responsible for producing the cathode heater voltage (1,5 V) for the electron gun, if any is required. The tether is connected to the electron gun anode plate through a conducting path. Otherwise the tether is insulated from the satellite body, e.g. the reel drum is made of insulating material. The electron gun cathode is grounded to the satellite body. The HV source is used to force a potential dierence of selectable polarity (plus or minus) between the tether and the satellite body. Insulated single wire, 2,75 g/m. Total of 3 g reserved.

Electron gun
The goal is to have up to 200 V and up to 5 mA (1 W acceleration power) electron gun capability. At least two guns will be included for redundancy. Currently, three gun options are under consideration: (1) a gun based on
a commercial space-qualied indirectly heated barium impregnated tungsten cathode, (2) a gun based on a self-made directly heated tungsten lament cathode, (3) a gun based on a self-made carbon nanotube or a carbon nanotube based cold cathode. Option (1) has relatively low cathode heating power requirement, but is somewhat expensive. Option (2) currently has a larger heating power requirement and requires more environmental testing. It also has some cathode operational lifetime issues. Option (3) is the most advanced. It requires no heating power at all and the cathode could be longlived. Each gun should ideally fit in about 5 mm by 5 mm by 15 mm box. The electrons exit from a 0,6 mm 13 mm slit. The beams are inclined to shoot away from the satellite and the tether spin plane. We reserve 3 g for each gun. In Figure 26, the internals of the gun have been magnied and drawn in vertical instead of the actual horizontal placement.

Tether current measurement could be almost free depending on how the HV source is constructed, but 2 g is reserved for it. Camera with electronic adapter estimated mass is 15 g. Most of the mass is for the electronics.