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Oxygen Candles

Required hygiene water mass

6 Life Support Systems

6.1 Atmosphere Control and Supply

6.1.3 Oxygen Candles

Figure 6-6: Oxygen candle system schematic [50, p. 79]

6.1.3.1 Description

Oxygen candles (or chlorate candles) yield O2 through an exothermic reaction. Most used in space applications are lithium perchlorate LiClO4 candles on MIR and Potassium perchlorate KClO4 on ISS. The chemicals are enclosed in a long cylindrical canister and are ignited electrical. As can be seen in Figure 6-6, the candles for repressurisation are enclosed in a storage tank outside the pressurized section to enhance safety and reduce volume penalties. Candles for cabin leakage are stored, ignited, and replaced inside the habitat of a spacecraft, like the ones on the ISS. This is necessary because only small amounts of oxygen for leakage are needed and replacing the candle safes mass and volume, because then only one ignition control unit and filter are needed. [2, 50, p. 76]

6.1.3.2 Data and Sizing

A typical oxygen candles produces 0.79 kg O2 from 2.2 kg LiClO4 (see also chemical equation in Eq. ( 6-11 )) and burns between 5 and 10 minutes. KClO4 would be a little bit more effective with 0.4 kg O2 per kg KClO4 compared to 0.36 kg O2 per kg LiClO4. But not enough data could be found about that system and therefore a LiClO4 system is analyzed. [66, 67, 68, p. 119]

𝐿𝑖𝐢𝑙𝑂4 𝑦𝑖𝑒𝑙𝑑𝑠→ 𝐢𝑙𝐿𝑖 + 2𝑂2 Eq. ( 6-11 )

Table 6-8: LiClO4 system properties

Parameter Value Unit Source

LiClO4 per cartridge 2.2 [kg] [67] calculated by applying the stowage factor for a cylinder (𝑓𝑆𝑃𝐹,π‘π‘¦π‘™π‘–π‘›π‘‘π‘’π‘Ÿ) to the cartridge volume (π‘‰π‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’), as can be seen in Eq. ( 6-12 ). As can be seen in Table 6-8, the produced O2 per cartridge is 0.79 kg. Because the assumed leakage of O2 is 0,042 kg per day, only every 19 days a cartridge would be needed. A potential rise of the oxygen partial pressure when the cartridge is ignited can be neglected due to the size of the SpaceHab. By dividing the mission time through the calculated 19 days, between 5 and 11 cartridges are required (𝑛LiClO4,π‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’π‘ ) for mission length of 88 and 211 days respectively. With this value, the total cartridges volume (π‘‰π‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’π‘  π‘“π‘œπ‘Ÿ π‘™π‘’π‘Žπ‘˜π‘Žπ‘”π‘’) needed for the leackage subsystem can be calculated with Eq. ( 6-13 ), which is between 1.8*10-2 mΒ³ and 3.96*10-2 mΒ³ for the SpaceHab and Evolved-SpaceHab respectively.

π‘‰π‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’,𝑀,𝑆𝑃𝐹= π‘‰π‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’ 𝑓𝑆𝑃𝐹,π‘π‘¦π‘™π‘–π‘›π‘‘π‘’π‘Ÿ Eq. ( 6-12 ) π‘‰π‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’π‘  π‘“π‘œπ‘Ÿ π‘™π‘’π‘Žπ‘˜π‘Žπ‘”π‘’= π‘‰π‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’,𝑀,𝑆𝑃𝐹 𝑛𝑛𝑒𝑒𝑑𝑒𝑑 π‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’π‘  Eq. ( 6-13 ) The total mass of one cartridge (π‘šπ‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’) stated in Table 6-8 is calculated by Eq. ( 6-14 ). Similar to the volume calculation, the total cartridge mass (π‘šπ‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’π‘  π‘“π‘œπ‘Ÿ π‘™π‘’π‘Žπ‘˜π‘Žπ‘”π‘’) needed for the leackage subsystem can be calculated with Eq. ( 6-15 ), to get 13.5 kg or 29.7 kg for the SpaceHab or Evolved-SpaceHab respectively.

π‘šπ‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’= π‘šπ‘ π‘‘π‘œπ‘€π‘Žπ‘”π‘’ 𝑒𝑛𝑖𝑑

π‘›π‘π‘Žπ‘Ÿπ‘‘π‘–π‘‘π‘”π‘’π‘  π‘π‘’π‘Ÿ π‘ π‘‘π‘œπ‘€π‘Žπ‘”π‘’ 𝑒𝑛𝑖𝑑 Eq. ( 6-14 ) π‘šπ‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’π‘  π‘“π‘œπ‘Ÿ π‘™π‘’π‘Žπ‘˜π‘Žπ‘”π‘’= π‘šπ‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’ 𝑛LiClO4,π‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’π‘  Eq. ( 6-15 ) For the repressurization system, it is highly unlikely that single cartridges are used and therefore it is not valid to calculate with single cartridges because this would lead to false conclusions. Instead the needed volume and mass of LiClO4 is calculated for one repressurisation first. Because it is known that one cartridge produces 0.79 kg O2 and the LiClO4 mass per cartridge is 2.2 kg, the required mass of LiClO4 per kg O2

(π‘šLiClO4 π‘π‘’π‘Ÿ π‘˜π‘” 𝑂2) can simply be calculated with Eq. ( 6-17 ) to 2.79 kg LiClO4 per kg O2. Eq. ( 6-17 ) then yields the total mass of required LiClO4 (π‘šLiClO4 π‘Ÿπ‘’π‘π‘Ÿπ‘’π‘ π‘ π‘’π‘Ÿπ‘–π‘ π‘Žπ‘‘π‘–π‘œπ‘›) for a repressurisation event. For the SpaceHab design, this would be 844.87 kg and for the Evolved-SpaceHab design it would be 1325.57 kg of LiClO4.

π‘šLiClO4 π‘π‘’π‘Ÿ π‘˜π‘” 𝑂2 =π‘šLiClO4 π‘π‘’π‘Ÿ π‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’

π‘šπ‘‚2 π‘π‘Ÿπ‘œπ‘‘π‘’π‘π‘’π‘‘ Eq. ( 6-16 )

π‘šLiClO4 π‘Ÿπ‘’π‘π‘Ÿπ‘’π‘ π‘ π‘’π‘Ÿπ‘–π‘ π‘Žπ‘‘π‘–π‘œπ‘›= π‘šLiClO4 π‘π‘’π‘Ÿ π‘˜π‘” 𝑂2 π‘šπ‘‚2 π‘Ÿπ‘’π‘žπ‘’π‘–π‘Ÿπ‘’π‘‘ Eq. ( 6-17 ) With the mass of LiClO4 per cartridge (π‘šLiClO4 π‘π‘’π‘Ÿ π‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’) and the respective density (𝜌LiClO4) of 2.42 g cm-3, the volume of LiClO4 in one cartridge (𝑉LiClO4 π‘π‘’π‘Ÿ π‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’) can be calculated by Eq. ( 6-18 ), to 9.09*10-4 mΒ³. Multiplying this value with the number of theoretical cartridges (see Eq. ( 6-19 )), the total volume of LiClO4 for repressurisation (𝑉LiClO4 π‘Ÿπ‘’π‘π‘Ÿπ‘’π‘ π‘ π‘’π‘Ÿπ‘–π‘ π‘Žπ‘‘π‘–π‘œπ‘›) is 0.35 mΒ³ or 0.55 mΒ³ for the SpaceHab or Evolved-SpaceHab So far, only the consumables were considered, but not the necessary ignition system, filter etc. The Backup Oxygen Candle System (BOCS) developed by NASA for the ISS is a passive system that utilizes KClO4 for oxygen generation. One BOCS O2 candle can produce 3.4 kg of O2 and therefore data from the BOCS in Table 6-9 must be scaled by dividing the cartridge mass of the leakage system (π‘šcartridge) through the BOCS cartridge mass (π‘šBOCS π‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’). The same approach can be done for the volume (Eq. ( 6-21 )). Adding these values to the mass and volume of the cartridges leads to the total mass and volume of the leakage system, which is given in Table 6-10.

π‘šπ‘™π‘’π‘Žπ‘˜π‘Žπ‘”π‘’ π‘ π‘¦π‘ π‘‘π‘’π‘š=π‘šπ΅π‘‚πΆπ‘† 𝑀/π‘œ π‘π‘Žπ‘Ÿπ‘‘π‘Ÿπ‘–π‘‘π‘”π‘’ LiClO4 systems have already been used in flight, which means the system has a TRL of 9.

The MTBF is high and therefore 100,000 hours are assumed. [67]

Like for the high-pressure system it is assumed that the system is managed automatic and consequently no crew time is needed for operation or maintenance.

Table 6-10: Properties of oxygen candle system for leakage

Parameter Value Unit

Number of required candles (SpaceHab) 5 [-]

Number of required candles (Evolved-SpaceHab) 11 [-]

Total expendables mass (SpaceHab) 13.50 [kg]

Total expendables mass (Evolved-SpaceHab) 29.70 [kg]

Total system mass (SpaceHab) 20.26 [kg]

Total system mass (Evolved-SpaceHab) 36.46 [kg]

Total expendables volume (SpaceHab) 1.8*10-2 [mΒ³]

Total expendables volume (Evolved-SpaceHab) 3.96*10-2 [mΒ³]

Total system volume (SpaceHab) 6.99*10-2 [mΒ³]

Total system volume (Evolved-SpaceHab) 9.15*10-2 [mΒ³]

Required power 0 [W]

TRL 9 [-]

Reliability (best-case) 0.9791 [-]

Reliability (worst-case) 0.9506 [-]

For the repressurization system, an equipment mass of 177.81 kg is assumed. [50, p.

78]

The volume is calculated by assuming a cylindrical shape for the LiClO4 storage. It is unlikely that all mass is ignited at the same time, which could produce a hazard. Instead it is assumed that the LiClO4 is stored in 8 separated sections each holding an equal mass and separated by a gap which is also used for passive cooling similar to BOCS.

The gap is assumed to be 1 % of the total volume for simplification. Therefore, the total repressurization system volume (π‘‰π‘Ÿπ‘’π‘π‘Ÿπ‘’π‘ π‘ π‘’π‘Ÿπ‘–π‘ π‘Žπ‘‘π‘–π‘œπ‘› π‘ π‘¦π‘ π‘‘π‘’π‘š) can be calculated by applying the cylinder factor (𝑓𝑆𝑃𝐹,π‘π‘¦π‘™π‘–π‘›π‘‘π‘’π‘Ÿ), as stated in Eq. ( 6-22 ). It is further assumed that the volume of valves, filters etc. are much smaller than the storage volume and therefore are included in the stowage volume.

π‘‰π‘Ÿπ‘’π‘π‘Ÿπ‘’π‘ π‘ π‘’π‘Ÿπ‘–π‘ π‘Žπ‘‘π‘–π‘œπ‘› π‘ π‘¦π‘ π‘‘π‘’π‘š=𝑉LiClO4 π‘Ÿπ‘’π‘π‘Ÿπ‘’π‘ π‘ π‘’π‘Ÿπ‘–π‘ π‘Žπ‘‘π‘–π‘œπ‘›1.1 𝑓𝑆𝑃𝐹,π‘π‘¦π‘™π‘–π‘›π‘‘π‘’π‘Ÿ Eq. ( 6-22 ) As stated above, the system is assumed to be passive. Power is only necessary during ignition, which is negligible.

Even when no identical unit like the presented one has ever been developed it is assumed that that the technology is comparable with the ones used on MIR and ISS.

To stay in compliance of the NASAΒ΄s Technology Maturity Assessment (TMA), the TRL is reduced to 5.

It is assumed that the system is managed automatic except for ignition during an emergency and consequently no crew time is assumed for operation or maintenance.

The breakdown of the calculations above for the repressurisation system can be seen in Table 6-11.

Table 6-11: Properties of oxygen candle for repressurisation

Parameter Value Unit

Total expendables mass (SpaceHab) 844.87 [kg]

Total expendables mass (Evolved-SpaceHab) 1,325.57 [kg]

Total system mass (SpaceHab) 1,022.68 [kg]

Total system mass (Evolved-SpaceHab) 1,503.38 [kg]

Total expendables volume (SpaceHab) 0.35 [mΒ³]

Total expendables volume (Evolved-SpaceHab) 0.55 [mΒ³]

Total system volume (SpaceHab) 0.49 [mΒ³]

Total system volume (Evolved-SpaceHab) 0.77 [mΒ³]

Required power 0 [W]

TRL 5 [-]

The oxygen candle system is not considered for a storage system, since it has the worst mass performance.

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