Rocket propellant tank

Abstract

A rocket propellant tank 8 comprises an inner tank 10 and an outer tank 12 concentrically arranged with the inner tank nested within the outer tank wherein the inner tank contains liquid oxygen, and the outer tank contains liquid hydrogen. The outer tank may be toroidal and is defined by an inner wall (18, figure 5) and outer wall (20). The inner tank is defined by an outer wall (14) which is spaced radially inward from the inner wall of the outer tank to form a gap (24) which is filled with insulation, for example aerogel material. An inner or outer wall of the tank may be formed from composite material, for example carbon fibre, and may have a thickness of 4mm or less. A pole 30 may extend through a longitudinal axis of the inner tank connecting end caps (16, figure 3) of the inner tank and forming a conduit for carrying cables. Upper and lower bulkheads (32, 34) may be provided, spaced from the end caps, and connected to the inner and outer tanks by circular rings (36, figure 6).

Description

The present application relates to a rocket propellant tank. BACKGROUND A major hurdle for increasing the frequency of space travel is the cost. The cost of space travel can be improved in two ways: increasing the payload carrying capability of a rocket of given size, or reducing the mass of a rocket with a given payload. Tsiolkovsky’s Rocket Equation is a simple method of determining the ability of a rocket to reach orbit with a given payload. AV - * In ( AV is the change in velocity across the rocket burn and is the fundamental measure in the performance of any rocket - the higher AV the better. Isp is the specific impulse of the rocket engine and propellant choice which defines the efficiency; again, a higher value is better. Minitiai is the initial mass of the fully fuelled rocket before take-off, whereas Mfinai is the final mass of the rocket when all propellant has been burnt and thus corresponds to the mass of the rocket structure plus the mass of the payload. Therefore, reducing Mfinai provides a greater AV, and thus a lighter structure for a given payload needs to burn less fuel to reach orbit. It is therefore desirable to reduce weight in order to reduce the fuel costs. SUMMARY In accordance with an aspect of the invention, there is provided a rocket propellant tank comprising: an inner tank and an outer tank concentrically arranged with the inner tank nested within the outer tank; wherein the inner tank contains liquid oxygen and the outer tank contains liquid hydrogen. The outer tank may be toroidal and may be defined by an inner wall and an outer wall. The inner tank may be defined by an outer wall which is spaced radially inward from the inner wall of the outer tank to form a gap therebetween. The gap may be filled with a layer of insulation. The layer of insulation may be an aerogel material. One or more of the inner wall of the outer tank, the outer wall of the outer tank and the outer wall of the inner tank may be formed from a composite material. One or more of the inner wall of the outer tank, the outer wall of the outer tank and the outer wall of the inner tank may be formed from carbon fibre. One or more of the inner wall of the outer tank, the outer wall of the outer tank and the outer wall of the inner tank may have a thickness of 4mm or less. One or more of the inner wall of the outer tank, the outer wall of the outer tank and the outer wall of the inner tank may have a thickness of 1 mm or less. The rocket propellant tank may further comprise a pole which extends through the inner tank. The pole may extend along a longitudinal axis of the inner tank. The pole may connect to end caps of the inner tank. The pole may be hollow and form a conduit for carrying cables through the inner tank. The pole may be formed from carbon fibre. The rocket propellant tank may further comprise upper and lower bulkheads; wherein the upper and lower bulkheads are spaced from end caps of the inner and outer tanks to form cavities therebetween. The upper and lower bulkheads may be connected to the inner and outer tanks by circular rings. In accordance with another aspect, there is provided a rocket comprising a rocket propellant tank as described above. BRIEF DESCRIPTION OF DRAWINGS For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a front view of a rocket according to an embodiment of the invention; Figure 2 is a cross-sectional view of the rocket showing a rocket propellant tank; Figure 3 is a front cross-sectional view of the rocket propellant tank; Figure 4 is a top view of the rocket propellant tank; Figure 5 is a perspective cross-sectional view of the rocket propellant tank; and Figures 6 and 7 are enlarged cross-sectional views of the rocket propellant tank showing a bulkhead structure. DETAILED DESCRIPTION Figure 1 shows a rocket according to an embodiment of the invention. The rocket 2 generally comprises a lower engine section 4, a central tank section 6 and an upper nose section 9. As shown in Figure 2, the central tank section 6 comprises a propellant tank 8 which is formed by an inner tank 10 and an outer tank 12. The inner tank 10 contains liquid oxygen, whereas the outer tank 12 contains liquid hydrogen. The outer tank 12 is toroidal and thus has an annular cross-section. The outer tank 12 receives the inner tank 10 such that the outer tank 12 surrounds the inner tank 10. The inner tank 10 and the outer tank 12 are arranged concentrically and have a common longitudinal axis and extent. As can best be seen in Figures 6 and 7, the inner tank 10 is defined by a cylindrical outer wall 14 which is closed at either end by an end cap 16. The end caps 16 are circular and are domed outwards. The outer tank 12 is defined by a cylindrical inner wall 18 and a cylindrical outer wall 20 which are closed by end caps 22 at either end. The end caps 22 are annular and are domed outwards. The outer wall 14 of the inner tank 10 and the inner wall 18 of the outer tank 12 are separated by a gap 24 which receives a layer of insulation. The layer of insulation may be formed from a high-performance insulator material, such as an aerogel material. The insulation in the gap 24 reduces heat transfer between the liquid oxygen in the inner tank 10 and the liquid hydrogen in the outer tank 12. In this example, the aerogel material may have a thickness of 1.5mm. A layer of insulation may also be provided around the outer tank 12. As can be seen in Figure 5, one or more (two are shown here) baffle plates 26 are provided within the inner tank 10 and the outer tank 12. The baffle plates 26 are provided with a plurality of openings 28. The baffles plates 26 divide the inner and outer tanks 10, 12 into a plurality of sections, with the openings 28 in the baffles plates 26 providing fluid communication between the sections to allow the contents to move through the sections. Each of the walls 14, 18, 20 of the inner and outer tanks 10, 12 may be formed as a number of shorter sections which are joined to achieve the desired length. The baffle plates 26 may be used to connect these sections together. The baffle plates 26 may also provide additional stiffness to the inner and outer tanks 10, 12. The walls 14, 18, 20 of the inner and outer tanks 10, 12 may be formed from a high-performance structural monolithic or composite material. In this example, the walls are formed from a composite material, such as carbon fibre. The walls are thin and have a thickness of 4mm or less. In this example, the walls have a thickness of only 1mm (or less). The pressure of the contents of the inner and outer tanks 10, 12 may provide rigidity to the tanks 10, 12. Further, the cryogenic temperatures of the liquid oxygen and liquid hydrogen may also increase the rigidity of the walls 14, 18, 20. A mechanism may be provided to maintain a substantially constant pressure in the inner and outer tanks 10, 12 as the liquid oxygen and liquid hydrogen is used to propel the rocket 2. For example, the liquid oxygen and liquid hydrogen may be replaced by a gas, such as helium, held in a separate tank. As shown in Figures 2 and 5, a pole 30 extends through the inner tank 10. The pole 30 extends along the longitudinal axis of the inner tank 10 and passes through the end caps 16 (and also the baffle plates 26) of the inner tank 10. The pole 30 is connected to the end caps 16 (and optionally the baffle plates 26) where it passes through and thus fixes the distance between the end caps 16. The pole 30 therefore maintains tension in the outer wall 14 of the inner tank 10 and thus also in the inner and outer walls 18, 20 of the outer tank 12. The pole 30 is hollow and therefore provides a conduit for carrying cables (e.g., electrical, optical, hydraulic, pneumatic, mechanical) through the inner tank 10. The pole 30 is formed from carbon fibre. In this example, the pole 30 has a diameter of 100mm and a wall thickness of 3mm. In other examples, the pole 30 may be offset from the longitudinal axis. Further, multiple poles may be used which may include one or more poles located in the outer tank 12. As can be seen in Figures 5 to 7, the propellant tank 8 is located between a lower (engine) bulkhead 32 and an upper (payload) bulkhead 34. The lower and upper bulkheads 32, 34 are connected to the propellant tank 8 by circular rings 36. The circular rings 36 are connected to and extend from a corresponding one of the outer wall 14 of the inner tank 10, the inner wall 18 of the outer tank 12 and the outer wall 20 of the outer tank 12. In other examples, the walls 14, 18, 20 may extend beyond the end caps 16, 22 to form the circular rings 36. Further, in other examples, a single ring may be provided for the outer wall 14 of the inner tank 10 and the inner wall 18 of the outer tank 12. The circular rings 36 transfer vertical loads from the inner and outer tanks 10, 12 to the lower and upper bulkheads 32, 34. The lower and upper bulkheads 32, 34 comprise a plurality of beams 38 which extend diametrically and intersect one another midway along their lengths at the centre of the bulkhead. In particular, in this example, the lower and upper bulkheads 32, 34 comprise four beams which are angularly offset at 45-degree intervals (or eight radial beams). The beams 38 support a platform wall 39. The platform wall 39 corresponds to the cross-section of the central tank section 6 and thus forms a dividing wall. The circular rings 36 are slotted into the lower and upper bulkheads 32, 34 (e.g. into slots formed in the beams 38) to lock the lower and upper bulkheads 32, 34 in place. Cavities 40 are formed between the end caps 16, 22 and the opposing lower and upper bulkheads 32, 34. These cavities 40 may be used to house ancillary components. For example, as shown in Figure 2, the cavity 40 formed between the end caps 16, 22 and the upper bulkhead 34 may be used to house hypergolic fuel spheres 44 which are connected to a Reaction Control System (RCS) to provide attitude control. The propellant tank 8 may be located within a protective outer skin, which may be formed from a material such as Kevlar. The upper nose section 9 is formed by a payload fairing 42 which forms a cavity which is closed by the upper bulkhead 34. The payload fairing 42 may house a payload which may be released into orbit by the rocket 2. The payload fairing 42 may be formed from a material such as Kevlar. The lower engine section 4 houses the engine of the rocket 2 which is supplied with propellant from the propellant tank 8. The engine may be of any suitable design which is able to run on hydrolox fuel. In particular, the engine may be as described in our copending application GB2401822.8. The volume requirement for the oxygen tank is lower than the volume requirement for the hydrogen tank. Therefore, arranging the oxygen tank within the hydrogen tank reduces the diameter / circumference of the outer wall 14 of the inner tank 10 and thus reduces the overall weight of the propellant tank 8. As described above, the rocket 2 uses hydrolox fuel. Hydrolox provides the highest specific impulse of conventional rocket propellants, and it has the significant added benefit in that no carbon dioxide is released when it is burnt. The power-to-weight ratio of the rocket 2 may be sufficient to allow it to be used solely during the take-off of a rocket without the need for supplemental power from solid rocket boosters and thus without generating any carbon. 5 It will be appreciated that positional references such as upper, lower, upwards, downwards, top, bottom, etc. are made with respect to the rocket being arranged vertically. The invention is not limited to the embodiments described herein, and may be modified 10 or adapted without departing from the scope of the present invention.

Claims

1. A rocket propellant tank comprising:an inner tank and an outer tank concentrically arranged with the inner tank nested within the outer tank;wherein the inner tank contains liquid oxygen and the outer tank contains liquid hydrogen.

2. A rocket propellant tank as claimed in claim 1, wherein the outer tank is toroidal and is defined by an inner wall and an outer wall.

3. A rocket propellant tank as claimed in claim 2, wherein the inner tank is defined by an outer wall which is spaced radially inward from the inner wall of the outer tank to form a gap therebetween; wherein the gap is filled with a layer of insulation.

4. A rocket propellant tank as claimed in claim 3, wherein the layer of insulation is an aerogel material.

5. A rocket propellant tank as claimed any of claims 2 to 4, wherein one or more of the inner wall of the outer tank, the outer wall of the outer tank and the outer wall of the inner tank is formed from a composite material.

6. A rocket propellant tank as claimed in claim 5, wherein one or more of the inner wall of the outer tank, the outer wall of the outer tank and the outer wall of the inner tank is formed from carbon fibre.

7. A rocket propellant tank as claimed in any one of claims 2 to 6, wherein one or more of the inner wall of the outer tank, the outer wall of the outer tank and the outer wall of the inner tank has a thickness of 4mm or less.

8. A rocket propellant tank as claimed in claim 7, wherein one or more of the inner wall of the outer tank, the outer wall of the outer tank and the outer wall of the inner tank has a thickness of 1mm or less.

9. A rocket propellant tank as claimed in any one of the preceding claims, further comprising a pole which extends through the inner tank.

10. A rocket propellant tank as claimed in claim 9, wherein the pole extends along a longitudinal axis of the inner tank.

11. A rocket propellant tank as claimed in claim 9 or 10, wherein the pole connects to end caps of the inner tank.

12. A rocket propellant tank as claimed in any one of claims 9 to 11, wherein the pole is hollow and forms a conduit for carrying cables through the inner tank.

13. A rocket propellant tank as claimed in any one of claims 9 to 12, wherein the pole is formed from carbon fibre.

14. A rocket propellant tank as claimed in any one of claims, further comprising upper and lower bulkheads; wherein the upper and lower bulkheads are spaced from end caps of the inner and outer tanks to form cavities therebetween.

15. A rocket propellant tank as claimed in claim 14, where the upper and lower bulkheads are connected to the inner and outer tanks by circular rings.

16. A rocket comprising a rocket propellant tank as claimed in any one of the preceding claims.

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