December 18, 2020, ©. Leeham News: After discussing the risk-reducing research programs we need to do before a program launch in 2027, we focus the next Corners on the hydrogen airliner’s biggest problem, the liquid hydrogen tank.
In this Corner, we start with the placement and discuss how it affects aircraft performance.
We have discussed the hydrogen tanks’ placement for an airliner in previous parts of the Hydrogen Corner series. Here we will focus on the placement proposed in the Airbus ZEROe concepts, presented on the 21st of September.
Figure 1 shows the ZEROe turbofan from the presentation. It’s 33 window positions suggest a capacity of ~200 seats. Note, the rear door is far from the tail. It’s because the tailcone houses two cylindrical LH2 tanks, Figure 2.
Figure 2 shows the tanks are not part of the fuselage structure. They are designed as two separate isolated tanks, filling the aircraft’s non-pressurized tailcone aft of the cabin pressure bulkhead to the trim jack for the horizontal tailplane.
Why two tanks? Probably for safety reasons, should there be a leak in one tank or its fuel system should develop a problem. We can assume the tanks have separate fuel systems, feeding both engines.
There are several advantages to this placement. The cabin is ahead of any leaking tank from a hard landing or accident. As hydrogen leaks and burns straight up, evacuating passengers are separated from the danger area.
Structurally the fuselage is kept close to round, which is good for fatigue reasons. The major fatigue problem of an airliner is the cabin pressure stretching the fuselage skins every flight. Then a round shape is ideal.
The same is valid for the hydrogen tanks. They have a slight overpressure to ambient air, and the thinner air at cruise flight levels cycles the skin stress level of the hydrogen tanks.
The drawback of the design is a varying center of gravity (CG) during flight. A typical narrowbody flight is around 800nm or two hours. It consumes about 4.2t of Jet fuel during such a flight. If we assume our hydrogen airliner has the same efficiency, it consumes 1.4t of LH2 on the trip. With reserves, we have about 2t LH2 in the tanks (today’s jet 6t Jet-A1).
When the ZEROe turbofan lines up for takeoff, it has a weight somewhere around 65t. During the flight, 1.4t is consumed. We can assume Airbus designs the system so this consumption is focused on the forward tank, with the rear acting as a safety tank and holding the reserves.
The consumption of fuel closer to the center of gravity limits the effect of center-of-gravity travel. The loss of 1.4t of LH2 at the rear of the aircraft makes the ZEROe more nose-heavy than at the start of the flight.
An airliner uses the horizontal tailplane to balance the aircraft in pitch, Figure 3.
At the beginning of the mission, we can assume the horizontal tailplane downforce to be on the aft side of the middle position of Mean Aerodynamic Chord (MAC). For the Airbus A320, the mid position is 28% MAC, Figure 4. So say around 33% if the ZEROe used a CG range diagram similar to the A320.
As we approach landing, the CG would have moved forward. How much we discuss next Friday. The forward movement of the CG must be within the acceptable range for the aircraft. It’s about the horizontal tailplane’s authority in controlling the pitch during the landing and a possible go-around.
But the forward travel of the CG will also decrease the efficiency of the aircraft. A nose-heavy jet consumes more fuel as the trim drag of the horizontal tailplane increases, and the wing must carry a higher load.
How detrimental these effects will be for hydrogen airliners with the tanks in the rear we calculate in the next Corner.