Formula: {Volume_Tank_xi = 50 liters = 50 000 cm³ = 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×13.3²×46.3 = 25 717 cm³ = ca. 25 liters = ca. 6.6 US gal - main tank (internal)
~ V_LH2ExtraTank = p×12.1²×63.7 = 29.285 cm³ = 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!