Technical Field The invention relates to a tank suitable for containing liquid hydrogen and suitable for use on a high altitude long endurance aircraft.

Background to the invention High altitude platforms (aircraft and lighter than air structures situated from 10 to 35 km altitude) - HAPS, have been proposed to support a wide variety of applications. Areas of growing interest are for telecommunications, positioning, observation and other information services, and specifically the provision of high speed Internet, e- mail, telephony, televisual services, games, video on demand, mapping services and global positioning.

High altitude platforms possess several advantages over satellites as a result of operating much closer to the earth’s surface, at typically around 20 km altitude.

Geostationary satellites are situated at around 40,000 km altitude, and low earth orbit satellites are usually at around 600 km to 3000 km altitude. Satellites exist at lower altitudes but their lifetime is very limited with consequent economic impact.

The relative nearness of high altitude platforms compared to satellites results in a much shorter time for signals to be transmitted from a source and for a reply to be received (the“latency” of the system). Moreover, HAPS are within the transmission range for standard mobile phones for signal power and signal latency. Any satellite is out of range for a normal terrestrial mobile phone network, operating without especially large antennas. HAPS also avoid the issues of rocket launches for satellites: high acceleration and vibration, as well as the high failure rates with their attendant impact on satellite costs. Payloads on HAPS can be recovered easily and at modest cost compared to satellite payloads. Shorter development times and lower costs result from less demanding testing requirements. US patent 7,046,934 discloses a high-altitude balloon for delivering information services in conjunction with a satellite.

The patents, US 20040118969 Al, WO 2005084156 A2, US 5518205 A, US 2014/0252156 Al, disclose particular designs of high altitude aircraft.

To avoid the costs and lack of availability that would be engendered by short flight endurance for HAPS aircraft, endurance of many weeks or months, rather than hours, is necessary. In such aircraft, energy is supplied by solar cells with a battery storage system to provide power overnight, or by hydrogen fuel. This energy is used for the propulsion system and payload power. Such aircraft are referred to as‘High Altitude Long Endurance’ (HALE) aircraft in the following text.

Aerodynamic drag consumes energy and reduces the available payload energy, and can curtail the aircraft operating speed: altitude and latitude. It is therefore highly desirable to minimize the HALE aircraft aerodynamic drag.

Much of the world’s population resides away from the tropics at high latitudes where sufficient solar power is not available during winter months to allow practical HALE aircraft to operate overnight. For this reason, hydrogen powered HALE aircraft, utilising internal combustion engines or fuel cells, have been trialled. The

AeroVironment‘Helios’, the Boeing‘Phantom Eye’, and the Lockheed‘Global Observer’ are all examples of prototype HALE hydrogen powered aircraft. Hydrogen for such aircraft is stored either as a pressurised gas, or as a liquid at low temperature - typically at between 20 to 30 Kelvin.

Hydrogen can be contained in vessels within the fuselage or wing of the aircraft, or in pods mounted beneath the aircraft wing. The latter arrangements provide a more uniform weight distribution and therefore smaller bending moments in the main wing spar. A lighter aircraft structure and greater endurance or increased payload capacity results, both of which are highly desirable for HALE aircraft. The‘Helios’ aircraft was an example of such an approach. The low altitude‘Antares’ aircraft developed by DLR had pressurised hydrogen contained in pods mounted beneath the wing.

Liquid hydrogen is much more dense than gaseous hydrogen at practical storage pressures. This higher density allows for a lower tank volume than systems utilising gaseous hydrogen, and hence a potentially greater endurance. For HALE aircraft, the fraction of the fuel weight as a proportion of the tank system weight, including the tank structure and associated operating and control systems (such as, but not limited to, valves and pipework), is particularly important.

Tanks containing liquid hydrogen in aircraft have, in the past, been mainly spherical and constructed out of aluminium. This technology is well established. Typically, they have a relatively modest weight of hydrogen as a fraction of the total weight of the tank and baffles inside to reduce the impact of sloshing of fuel on aircraft flight stability.

Aircraft fuel tanks that are intended to contain liquid fuel, such as liquid hydrogen, contain baffles to reduce the wave energy that results from aircraft acceleration or rotation. Examples are quoted in US1310517A and US20080230654A1 and

US3009671A. The transit and reflection of these waves at the vessel ends can generate impulses of a frequency that can interfere with the flight control dynamics, stability, and aeroelasticity of the aircraft.

Conventional baffles, mitigating the effect of acceleration or rotation of the aircraft on the tanks, span the internal diameter of aircraft tanks, and dissipate energy from wave movement by creating turbulence. Axial accelerations in the direction of flight and variation of pitch create waves that travel parallel to the direction of flight along the tank and create the greatest impulses affecting flight stability. Sideways accelerations can also cause stability issues. If there are tanks placed on both wings, the left and right tanks can oscillate symmetrically and antisymmetrically, both modes need to be considered in the analysis of the stability of the aircraft. However, for all their benefits, baffles also provide stresses between the baffle and the inner wall. Firstly the temperature of the baffle will be different from that of the inner wall during tank heating and cooling, leading to differential expansion. Secondly the inner wall will expand with internal pressure, and thirdly the baffles will be subject to sideways loads with wave motion.

Such forces can be reduced by dishing the baffle but the problems are still present. These can be more pronounced when the tank is made of a lightweight material, which may be particularly desirable when storing liquid hydrogen, particularly for HALE aircraft.

Therefore, there is a need for an improved design of liquid hydrogen tank suitable for a high altitude long endurance aircraft.

Summary of the invention

In a first aspect, the invention provides a tank suitable for containing liquid hydrogen and suitable for use on a high altitude long endurance aircraft, wherein the tank has a non-spherical external surface and at least one internal liquid hydrogen storage chamber having at least one opening, characterised in that internal hydrogen storage facing surface is elongate such that the internal surface comprises an elongate body terminated by two facing ends, thereby defining a main axis passing through points on the two respective facing ends that are furthest apart from each other, and for at least one end of the tank, the ratio of the diameter of the internal surface perpendicular to the axis, at the position where the angle of the tank wall is 45° to the axis, to the maximum diameter of the internal surface is less than 2/3.

Thus, the invention provides a liquid hydrogen tank wherein the shape of the internal surface prevents wave reflections and creates wave-breaking effects that dramatically reduce the interference of the aircraft motion by wave action and completely, or partially, eliminate the need for baffles connected to the tank wall. As a result, either no baffles, or substantially smaller baffles, are required.

Preferably for both ends the ratio is less than 2/3. Preferably the internal surface of the storage chamber is axisymmetric about the axis. For the purposes of this invention, an‘axisymmetric surface’ is defined as a surface whose deviation is less than 10% of a diameter from an axisymmetric surface over 20% of the area of a surface.

An important function of the tank wall, of a liquid hydrogen fuel tank, is to prevent permeation of hydrogen. This can be improved by the provision of a liner on the inside surface of the internal liquid hydrogen storage chamber.

The provision of a suitable impermeable barrier, as part of the inner vessel wall, to store liquid hydrogen has been discussed. For example, in“Permeation Barrier for Lightweight Liquid Flydrogen Tanks” by Daniel SchultheiB 16. April 2007 OPUS Augsburg Dissertation zur Erlangung des Doktorgrades der Mathematisch- Naturwissenschaftlichen Fakult at der Universit at Augsburg der Online- Publikationsserver der Universit at Augsburg.

Generally, thin continuous films of metals or plastics provide a very low permeation rate at cryogenic temperatures below about 80 K. Continuous (i.e. free from microcracks) films of less than 0.2 mm are known to provide an adequate barrier against permeation. These barriers are also well known for the operation of high pressure tanks for hydrogen. However, a liner presents issues if internal structures are bonded to the internal wall, such as baffles. In particular such baffles will create difficulty in having a perfect seal of the liner as the baffles are mechanically attached to the inner storage chamber wall. This can present the opportunity for hydrogen leaks, which is undesirable. The present invention is therefore ideal for use with a lined fuel tank. Therefore the storage chamber preferably comprises a hydrogen-impermeable liner on its internal hydrogen storage facing surface. Preferably the ratio is less than ½. This has been found to provide a greater degree of wave mitigation.

Although baffles are not preferred, the tank can contain baffles, but preferably where the area of each baffle, projected onto the plane normal to the axis of the tank, occupies less than 20%, more preferably less than 1%, and most preferably less than 0.1% of the cross-sectional area of the tank. This reduces the stresses placed on the storage chamber.

If baffles are present they may be conveniently supported at the opening to the storage chamber.

However it is preferred that the tank contains no baffles.

If present, it is preferred that the liner is composed of a metal such as, but not limited to, aluminium, stainless steel, or Invar. The liner can also be made of an unfilled plastic.

In a second aspect, the invention relates to a high altitude long endurance aircraft comprising a hydrogen fuel cell and at least one tank according to any of the preceding claims.

HALE aircraft may have wingspans exceeding 30 m, but generally they are of low weight. Their weight is typically less than 10,000 kg, more typically less than 2,000 kg and sometimes even lower than this.

The invention will now be illustrated by way of example, and with reference to the following figures, in which:

Figure l is a cross sectional view of a hydrogen fuel tank comprising an internal liquid hydrogen storage chamber according to the present invention.

Figure 2 is a cross-sectional view through a hydrogen fuel tank comprising an internal liquid hydrogen storage chamber and falling outside the present invention. Figure 3 is a cross-sectional view through a hydrogen fuel tank comprising an internal liquid hydrogen storage chamber and falling outside the present invention. Figure 4 is a cross-sectional view through a hydrogen fuel tank comprising three internal liquid hydrogen storage chambers and falling outside the present invention.

Figure 5 is a perspective view of the internal structure of an internal liquid hydrogen storage chamber comprising baffles. The nature of the ends of the storage chamber are not shown.

Figure 6 is an illustration of the inside of an internal liquid hydrogen storage chamber comprising a liner and also comprising baffles. Figure 7 is a chart showing the force experienced (arbitrary scale) versus time under test conditions of a half filled internal hydrogen storage chamber.

Figure 8 is a cross-sectional view through a hydrogen fuel tank comprising an internal liquid hydrogen storage chamber according to the present invention.

Turning to the figures, figure 1 shows a liquid hydrogen fuel tank according to the present invention. The liquid hydrogen tank has an external surface 1 and an internal liquid hydrogen storage chamber 2. The external surface 1 serves two functions, firstly to have a suitable shape to minimise drag for a given tank volume, and secondly to sustain a vacuum adjacent to the hydrogen storage chamber 2 to provide for a suitable thermal insulation.

An additional shroud 4 may be provided to avoid a separation between the tank internal shape and the external aerodynamic shape at the rear of the vessel. Flowever, in the interests of weight saving it is often preferable to minimise the area of the tank that has a separate additional shroud. Separate additional shrouds can also be used at the front of the vessel to reduce drag. A bellows 3 connects the inner chamber and outer surface, providing a closed vacuum space between the inner and outlet shells 1 and 2 to allow the conveying of fluids into and out of the tank and access for instrument and heating connections. The vessel attachments spacing the inner and outer vessels are not shown but have to allow for high g loadings as part of an aircraft system and ensure that suitable spacing of the multilayer insulation is provided so it does not get crushed with high g loadings on the system.

Figure 1 also shows the detail of the internal liquid hydrogen storage chamber 2 The inner chamber 2 A metal or polymer liner 5 prevents permeation of hydrogen through the inner wall 6 A vacuum space 12 is left to accommodate deflections between the inner and outer vessels and provide a space for vacuum pumping. The outer vessel wall is made up of a suitable foam such as Rohacell 10 Also shown in figure 1 is a line of axial symmetry 7 and a maximum diameter 8 of the storage chamber dmax. Also shown is a construction line 9 which is at 45° to the line of axial symmetry 7. The construction line 9 is positioned so it is tangential to the inner surface of the storage chamber 2 Also shown is the diameter of the storage chamber 11 at the point of contact between the construction line 9 and the inner surface of the storage chamber, referred to as d45.

In this case the value of d45/dmax (i.e. the ratio of the diameter of the internal surface perpendicular to the axis, at the position where the angle of the tank wall is 45° to the axis, to the maximum diameter of the internal surface) is approximately 0.6 and so this storage chamber falls within the scope of the present invention.

Figure 2 shows a liquid hydrogen fuel tank falling outside the present invention. The liquid hydrogen tank has an external surface 21 and an internal liquid hydrogen storage chamber 23 As for figure 1 there is a bellows 24 and an internal surface 22 of the storage chamber 23

In this case the value of d45/dmax is essentially 1.0 and so falls outside the scope of the present invention. It is to be noted that the flat ends to the tank provide good reflecting surfaces leading to low energy loss for wave reflections and an unsuitable design.

Figure 3 also shows a liquid hydrogen fuel tank falling outside the present invention. The liquid hydrogen tank has an external surface 31 and an internal liquid hydrogen storage chamber 32 As for figure 1 there is a bellows 34 and an internal surface 33 of the storage chamber 32

In this case the value of d45/dmax is approximately 0.71 and so this storage chamber falls outside the scope of the present invention. The hemispherical ends to the tank provide good reflecting surfaces leading to low energy loss for wave reflections and an unsuitable design.

Figure 4 also shows a liquid hydrogen fuel tank falling outside the present invention. The liquid hydrogen tank has an external surface 41 and three internal liquid hydrogen storage chambers 42 As for figure 1 there is a bellows 44 and an internal surface 43 of the storage chamber 42

As for figure 3 the value of d45/dmax for each of the three storage chambers is approximately 0.71 and so this storage chamber falls outside the scope of the present invention. Additionally, as the storage chambers are not elongate such an arrangement makes far less efficient use of the internal space for fuel storage and so is

unacceptable for this reason also.

Figure 5 shows a perspective view of the internals of a liquid hydrogen storage chamber. A conventional baffle arrangement used for a variety of purposes from the transport of fluids in road tankers to wing tanks can be seen. The dished ends 51 to the tank, and the baffles 52 inside the tank produce turbulence and energy absorption when waves travel from the front to back of the tank. Small holes 53 in the base of the baffles allow slow movement of liquid between the baffled compartments.

Figure 6 shows some of the issues with inserting a baffle inside a lined tank. The inner wall 61 is protected by a liner 62 from liquid hydrogen permeation. A baffle 63 spanning the tank is also shown. This also has to be protected by a liner 62 as well or delamination and failure will result from liquid hydrogen permeation.

Figure 8 shows a liquid hydrogen fuel tank according to the present invention. The liquid hydrogen tank has an external surface 81 and an internal liquid hydrogen storage chamber 83 As for figure 1 there is a bellows 84 and an internal surface 82 of the storage chamber 83

In this case the value of d45/dmax is approximately 0.9 and so falls inside the scope of the present invention. The ellipsoidal shape provides a tapering effect at the ends which provides for the dampening effect of waves.

Examples A variety of models of internal liquid hydrogen storage chambers were made, in order to test their response to acceleration on the waves generated in the tanks when half- filled with water.

The force is measured when a tank is moving at a constant velocity half full of fluid and then subjected to a sudden stop. Each tank has a strain gauge mounted which measures the force along the axis of the tank over time. The experiment allows the magnitude of the force to be measured as well as the period of oscillation.

From the requirements of the value of d45/dmax, only the ellipsoidal vessel and the fuel tanks (as shown in figure 1) are expected to produce a favourable response in the measured force.

The results can be seen in Figure 7. The vertical axis is force in arbitrary units, and each line has been offset vertically for clarity. From top to bottom, the curves relate to an ellipsoid (figure 8), a cylinder with flat ends (figure 2), a cylinder with

hemispherical ends (figure 3), the fore end of a cigar shaped tank (figure 1) and the aft end of a cigar shaped tank (figure 1). As can be seen from the results, summarised in Table 1, only the shapes with flat ends and hemispherical ends produced a significant spike in the initial amplitude, because they do not conform to the geometric requirements of the invention. The other shapes do conform to the criteria and experienced much smaller initial forces as a result. For convenience, embodiments falling outside the invention are marked with a *.

Table 1. Initial amplitudes caused by an impulse measured for various shapes.