Formula: **{Volume_Tank_xi = 50 liters = 50 000 cm**^{3} = ca. 12 U.S. gallon}

**One internal (25 l/6 gal) and one external (25 l/6 gal) liquid hydrogen (LH2) tank provide the total LH2 fuel for Xi.**

**Volume LH2 tanks** = **V**_{LH2MainTank} + **V**_{LH2ExtraTank} - Xi is aerodynamic even with two LH2 tanks

~ **V**_{LH2MainTank} = p×13.3^{2}×46.3 = 25 717 cm^{3} = ca. 25 liters = ca. 6.6 US gal - main tank (internal)

~ **V**_{LH2ExtraTank} = p×12.1^{2}×63.7 = 29.285 cm^{3} = ca. 29 liters = ca 7.6 US gal - external tank (replaces one sensor rack)

**Weigh of LH fuel itself:** - LH2 as a fuel is not too heavy for Xi even with two LH2 tanks

~ **d**_{LH2} = 0.07 kg/l - the density of liquid hydrogen

~ **w**_{LH2} = **d**_{LH2} × **V**_{LH2Tank} = 3.5 kg = 7.7 lbs

**Weigh of LH2 tanks:** - LH2 tanks are not too heavy for Xi thanks to new composite materials

~ **c**_{LH2} = 0.11 - LH2 mass presents typically 11% of the total mass of the LH2 tank

~ **w**_{LH2} = 3.5 kg = **c**_{LH2} × **w**_{LH2Tank} <=> **w**_{LH2Tank} = 31.82 kg = 70 lbs

**Total Weigh penalty LH2:** - the weight of LH2 liquid plus tanks required is not too heavy and these figures are pessimistic, see XCopter

~ **w**_{LH2Total} = **w**_{LH2} + **w**_{LH2Tank} = 35.32 kg = 77.7 lbs

**Efficiency of LH2 as a fuel:** - liquid hydrogen offers performance for aggressive and agile manoeuvres

~ **b**_{LH2} = 4 - constant depicting the fact that four times more LH2 liters are needed compared to benzin

~ **e**_{LH2} = 1.33 - constant depicting the fact that LH2 fuel is 1.33 times more efficient than benzin

~ **E**_{LH2Tank} = **V**_{LH2Tank} / **b**_{LH2} × **e**_{LH2} = 50.0 / 4 × 1.33 = 16.625 liter = 4.39 gal - benzin energy offered by LH2

**Requirements for Xi:** - are being met

~ Maximum_Flight_Time_xi

~ Maximum_Speed_xi

~ if Xi consumes LH2 50 l/h = 13.2 gallon/h, that equals to two tanks of LH2

~ above means that Xi can fly 100 kilometers in one hour (ca 60 miles/hour) with two LH2 tanks

catch ~Tank_xi
~Fuel_xi
~Volume_Tank_XCopter
~Xi!