Thermal

External loop A

Loop A flow rate S1000001 · in kg/h

Ammonia circulates between the heat exchangers and the radiators — nominally 3,700 kg per hour on this loop and 4,000 on loop B, the difference being the hydraulic resistance of two differently shaped plumbing runs rather than a fault. Between them the two loops reject up to 70 kW.

Loop A pump outlet pressure S1000002 · in kPa

The pump module keeps the ammonia pressurised well above its vapour pressure so it stays liquid all the way round the loop; a bellows accumulator backed by nitrogen gas takes up its expansion and contraction as the heat load changes. Software stops the pump if this outlet pressure climbs too high.

Loop A pump outlet temp S1000003 · in °C

The pump’s control valve blends cold ammonia returning from the radiators with warm ammonia that bypassed them, holding the supply at a set point of 2.8 °C — just above the freezing point of the water it meets in the heat exchangers. An over-temperature at this sensor, about 18 °C, stops the pump.

External loop B

Loop B flow rate P1000001 · in kg/h

The port loop. Its pump turns a little faster than loop A’s — 14,700 rpm against 14,000 — and moves about 4,000 kg of ammonia an hour. Each loop is rated to reject 35 kW, so if one fails the station carries on cooling at reduced capacity rather than none.

Loop B pump outlet pressure P1000002 · in kPa

Same arrangement as loop A, on the port side: the pump module holds the ammonia under enough pressure to keep it liquid, and its accumulator works with the ammonia tank on the P1 truss to absorb volume changes and make up small losses. The two loops are routed apart so that one piece of debris cannot cut both.

Loop B pump outlet temp P1000003 · in °C

Held to the same 2.8 °C set point as loop A by mixing radiator return with bypass flow; when the loop is carrying too little heat to reach it, heaters on the bypass line make up the difference. Each loop serves up to five heat exchangers, spread across Destiny, Harmony and Tranquility.

Radiators

Starboard TRRJ position S0000001 · in °

The radiator beam is turned twice an orbit, edge-on to the Sun while the station is lit and face to the Earth during eclipse. It is not simply chasing the coldest sky: the goal is −40 °C at the radiator outlet, cold enough to reject the heat and warm enough that the ammonia does not freeze in the manifolds.

Port TRRJ position S0000002 · in °

The port radiator beam, on loop B. Unlike the solar alpha joints this one never goes round: ammonia crosses it through flexible hoses, so it swings within about ±105° of neutral at up to 45° a minute — edge-on to the Sun in daylight, face to the Earth in eclipse.

Loop A TRRJ mode S0000007 · a state

Directed position means the joint is holding still; autotrack means it is chasing the best angle for rejecting heat; shutdown means the motor is disabled.

Loop B TRRJ mode S0000006 · a state

The same vocabulary as loop A — directed position is holding still, autotrack is following the computed goal angle, shutdown is the motor off; standby, checkout, restart, blind and switchover are the remaining states the software can report. The two loops are independent by design, so this mode need not match the starboard one.

Internal water loops

Destiny low-temp coolant USLAB000056 · in %

The low-temperature loop cools the delicate hardware — science racks and payloads. Water is used inside the modules rather than ammonia: it will not poison the cabin if a line leaks.

Destiny moderate-temp coolant USLAB000057 · in %

The moderate-temperature loop takes the hotter, hardier equipment: avionics and power electronics. Both loops hand their heat to the external ammonia loops through an interface heat exchanger.

Harmony low-temp coolant NODE2000002 · in %

Harmony’s heat exchangers cool more than Harmony: the node was launched with six of them so that Columbus and Kibo, berthed on its sides, could reject their heat through it. The figure is how full the loop’s accumulator is, as a percentage of its capacity.

Harmony moderate-temp coolant NODE2000001 · in %

The moderate loop takes the avionics. In Destiny the low and moderate loops can be valved together and run as one — to spare a pump, to save its power, or to cover for one that has failed. The figure is the accumulator fill, as a percentage of its capacity.

Tranquility coolant 1 NODE3000017 · in %

Coolant 1 is Tranquility’s moderate-temperature loop. Node 3 houses the life-support racks — water recovery, oxygen generation, the toilet, the exercise machines — and its heat exchangers were connected to the external ammonia loops when the node arrived. The figure is the accumulator fill, in percent of capacity.

Tranquility coolant 2 NODE3000019 · in %

Coolant 2 is Tranquility’s low-temperature loop, the colder of the two: in Destiny the equivalent loop is designed to run at 4 °C and cools the cabin air conditioner among other things. The figure is the accumulator fill, as a percentage of its capacity.

Where it lives on the station

• Port TRRJ Thermal Radiator Rotating Joint

  Rotates the port radiator to keep it edge-on to the Sun.

• Starboard TRRJ Thermal Radiator Rotating Joint

  Rotates the starboard radiator for the same reason.

• Port Radiator Port Heat Rejection Subsystem Radiator

  Three white panels that dump waste heat into space — around 70 kW for the station as a whole.

• Starboard Radiator Starboard Heat Rejection Subsystem Radiator

  The mirror-image radiator, fed by the second ammonia loop.

• Destiny U.S. Laboratory

  The American laboratory and nerve centre of the US segment: the station is commanded from here, and most experiments run here.

• Harmony Node 2

  The forward node: it connects the European and Japanese laboratories and receives vehicles docking at the front.

• Tranquility Node 3

  The node housing the life-support systems: water recycling, oxygen generation, toilet and exercise equipment.

Each link opens the 3D twin on that part, with its live readings beside it.