January 8, 2021, ©. Leeham News: In our Corner before Christmas we discussed the hydrogen tank placement at the rear of the aircraft for Airbus’ ZEROe concept turbofan aircraft.
We now calculate how the weight transfer when emptying the tanks in the rear affects the ZEROe’s efficiency.
We discussed the hydrogen tanks’ placement for the ZEROe turbofan aircraft (Figure 1). It has two LH2 tanks (liquid hydrogen tanks, see previous Corners why it uses cryogenically cooled liquid hydrogen, LH2) placed in the rear, Figure 2.
It’s two tanks for safety reasons. A tank can start leaking or lose its insulating vacuum, which means losing hydrogen as it boils off faster than expected. We also assumed the aircraft has two separate fuel systems that feed both engines, once again for safety reasons.
The drawback of the design is a varying Center of Gravity (CG) during flight, as the fuel consumed in the rear placed tanks gradually makes the aircraft more nose-heavy.
Airbus has given data for the aircraft in Figure 1. It’s a six abreast single-aisle aircraft with around 160 seats in a single class high-density layout, just below the passenger capacity of the A320, and a maximum range of 2,000nm.
In Figure 3, I have sketched the aircraft to find the distance between the Center of Gravity (CG) and the consumed fuel. The placement of the tanks in the rear of the fuselage instead of in the wingboxes, as for today’s airliners, makes the design more rear heavy. On a statical level, this is fixed by placing the wing further back, so the distance between the center of gravity and the aircraft’s aerodynamic center is kept, thus keeping positive pitch stability.
The problem is the movement of the CG during flight. As the LH2 is consumed in the two tanks, the rear gets lighter and the aircraft gradually gets more nose-heavy.
We need to check to what extent such a design creates problems. We start with the influence of the CG shift on efficiency. To this, we need to calculate the increase in nose-down movement around the CG during flight.
A moment is force times distance. Our performance model gives us the weight change of LH2 in the tanks. Now we need the distances. The front tank is 8m from the CG, and the rear tank 13m. We then run the model to get the hydrogen consumption at max range (2,000nm) and the typical flight of 800nm.
The LH2 consumption at the max range is 3.3t with 0.8t in reserves. The change from 4.1t of hydrogen divided between the tanks down to 0.8t means the aircraft’s nose down moment increases by 340,000Nm. Airbus might schedule the forward to tank to take the main consumption so CG change is minimized, but there must always remain sufficient LH2 in the tank so the aircraft can land safely should the rear tank run into problems. We, therefore, assume an equal consumption in the two tanks to ease the calculation.
If we divide the increased nose-down moment with the distance between the CG and the horizontal tailplane’s aerodynamic center of 19m, we get an additional tail downforce at the end of the mission of 17,900N/4,000lbf (Figure 4 shows the main forces involved in an airliner).
This increases the induced drag for the horizontal tailplane, and it also increases the induced drag of the wing as its lift force must increase with the same amount. Our performance model gives the efficiency loss from this change; it’s 1.4% for the aircraft’s max range flight.
For the typical 800nm flight, the hydrogen consumption is 1,500kg. This gives a nose-down moment increase of 154,500Nm, which increases the tail downforce by 8,100N/1,800lbf. Our performance model now shows an efficiency loss of 0.5%.
We can see that the effects on the efficiency of having a rear-mounted LH2 tank system in a domestic airliner with a maximum range of 2,000nm are marginal. The rear-mounted tank system works from an efficiency point of view for this type of aircraft. What other considerations must be made? This we discuss in next week’s Corner.