GB2153913A - Rocket motor - Google Patents

Abstract

The liquid propellants supplied from reservoirs 12, 13 to combustion chamber 10 are atomised by ultrasonic vibration means 20 which includes a transducer 21 and focus element 22. The motor may be used for satellite altitude control with thrusts below twenty neutrons. <IMAGE>

Description

SPECIFICATION Improvements in or relating to rocket motors The present invention relates to rocket motors, and in particular to liquid propellant supply apparatus in small liquid fuel rocket motors.

The realisation of liquid fuel rocket motors having a thrust below about twenty Newtons is encumbered by difficulties of supplying liquid fuels consistently at the required low rates, and more especially of obtaining sufficient atomisation and mixing of the fuels for consistent combustion.

The present invention provides a liquid fuel rocket motor having liquid propellant supply apparatus adapted for consistent delivery, atomisation and mixing of propellants at low thrusts.

According to the present invention in a liquid fuel rocket motor having combustion chamber, an exhaust nozzle and a liquid propellant supply apparatus, the said propellant supply apparatus comprises: a liquid propellant reservoir, a liquid propellant supply duct connecting the reservoir with a supply orifice in the combustion chamber, an ultrasonic source, and focussing means arranged to focus ultrasonic vibration on the liquid in the supply duct, whereby in operation of the motor the liquid is atomised by ultrasonic vibration.

In one embodiment of the invention the fuel duct includes a delivery tube attached to the focusing means in transverse array and the supply orifice is at the end of the delivery tube at a distance from the focussing means substantially equal to (2n- 1)N 4 where n is an integer and A is the wavelength of the natural vibration of that portion of the tube, where the natural vibration of the said portion of the tube is resonant with the ultrasonic source.

In a second embodiment of the invention the source is in the form of a circular shell of part spherical form, the focussing means includes a cone contiguous at the base with the source, bonded thereto, and having a part spherical cavity at its apex, the distance between the base and the cavity wall being an integer multiple of half wavelengths at the resonant frequency of the source, and the fluid duct includes a cavity part bounded by the part spherical cavity at the apex of the focussing cone and arranged so that the focal point of the focussing cone is at the supply orifice and at a distance from the part spherical cavity wall equal to an integer multiple of half wavelengths at the source resonant frequencies. This distance is preferably as small as possible. It will be appreciated that in this embodiment liquid in the cavity forms part of the focussing means.

In this second embodiment the duct may be arranged to pass through the source and the focussing cone, to afford some cooling of the ultrasonic source and cone.

According to a feature of the invention the ultrasonic source and the focussing means may be mounted in the rocket motor via a nodal zone in the focussing means, in order to minimise the transfer of ultrasonic vibration to the rest of the rocket motor.

A particularly suitable material from which the focussing means may be made, especially the focussing cone of the second embodiment, is titanium alloy, which combines excellent low loss accoustic transmitting properties with physical strength and chemical compatibility with most liquid propellants, and is advantageously light in weight.

The ultrasonic source may comprise a lead zirconate-titanate peizo-electric ceramic material and an electrical power supply arranged to excite the material at its resonant frequency to produce a mechanical vibration.

A bi-propellant rocket may have two sets of ultrasonic source and focussing means, one set for each propellant. Alternatively it may be possible for the two liquid supply ducts to be arranged for liquid atomisation by the one set.

As it is preferred that mixture of the liquids occurs after atomisation, it is the first embodiment which is especially amenable to a shared ultrasonic source and focussing means arrangement. In such an arrangement each fuel duct may have a delivery tube, both of which are attached to the same focussing means.

Moreover the delivery tubes may be arranged to converge one towards the other towards the supply orifice, to aid mixture of the liquids.

Rocket motors in accordance with the present invention will now be described by way of example with reference to the accompanying drawings, of which: Figure 1 is a schematic part-sectional view of a first type of rocket motor, Figure 2 is a view on ll-ll in Fig. 1, Figure 3 is a schematic part-sectional view of a second type of rocket motor, and Figure 4 is an exploded view of part of Fig.

3 but with an alternative fuel supply arrangement.

The bi-propellant liquid rocket motor illustrated in Figs. 1 and 2 comprises a combustion chamber 10 with an exhaust nozzle 11, two liquid propellant reservoirs 12, 1 3 and associated fuel flow controllers 14. 1 5 and ultrasonic propellant atomisation means 20.

The atomisation means 20 comprises an ultrasonic source 21, a focussing mandrel 22 attached to the source 21 at one end and carrying at the other end a pair of propellant supply tubes 23, 24. The propellant supply tubes project transversely through the focussing mandrel 22 and end in supply orifices in the combustion chamber 1 0. The length of each tube 23, 24 from the mandrel 22 to the orifices is equal to (2n - 1)A 4 where n is an integer and A is the vibration frequency of the ultrasonic source, so that upon vibration of the tubes an amplitude maximim occurs at these orifices. The tubes 23, 24, which are respectively connected to the flow controllers 14, 15, converge as they pass through the mandrel 22, as shown in Fig. 2, to assist admixture of the emerging propellant liquids.The ultrasonic source 21 is connected to an electrical supply 25. The length of the focussing mandrel 22 between the source 21 and the tubes 24, 25 is equal to an integer multiple of the source frequency half wavelenght. The mandrel 22 is attached to the rocket motor at a nodal point of the ultrasonic wave.

In operation of the rocket motor described with reference to Figs. 1 and 2, liquid is fed from the reservoirs 12, 1 3 to the combustion chamber 10 via the flow controllers 14, 1 5 and the tubes 23, 24. Ultrasonic vibration of the source 21 excited by the supply 25 is amplified by the mandrel 22 conveying it to the tubes 23, 24. The vibration is further amplified along the tubes to atomise the liquid as they leave the supply orifices.

The rocket motor illustrated in Fig. 3 has a combustion chamber 30, two liquid propellant reservoirs 31, and associated flow controllers 32, and two ultrasonic atomisers. The atomisers each comprise a part spherical ultrasonic bowl 33, a focussing cone 34, and a propellant cavity 35. Cavity inlet ducts 36 are connected via the appropriate flow controller 32 to a reservoir 31, while each cavity has an outlet orifice 37 leading to the combustion chamber 30. The sources 33 are connected to an electrical supply 38.

Each focussing cone 34 is formed of titanium alloy and its apex is part spherically recessed where it bounds the cavity 35. The length of the cone between its two part spherical surfaces is an integer multiple of half wavelength of the ultrasonic vibration.

The cavities 35 enable a continuation of ultrasonic vibration focussing through liquid therein to a point at the orifices 37, the available length for this being also arranged to be an integer multiple of half wavelength to the ultrasonic vibration.

The atomisers are supported in the rocket motor by diaphragm 39 attached to the cones 34 at a nodal point, so that the transmission of vibration out of the cones is minimised.

The supply tubes 36 pass through the sources 33 and the cones 34 for cooling purposes.

In operation of the rocket motor illustrated in Fig. 3 liquid propellants from the reservoirs 31 are fed via the controllers 32 and the tubes 36 to the cavities 35. Ultrasonic vibration of the source bowl 33 excited by the supply 38 is amplified in the cone 34 on route to the cavity 35, and therefore in the liquid in the cavity to the mouth of the orifice 37 where it is atomised on passing into the combustion chamber 30.

Fig. 4 illustrates a variation of the atomiser construction depicted in Fig. 3, where like elements have like reference numerals. The variation consists in ducting the propellant supply tube 36 through the motor structure into the cavity 35 instead of through the cone 34.

In particular examples of rocket motors as hereinbefore described, for use in satellite attitude control, the liquid propellants are mono-ethyl hydrazine and dinitrogen tetroxide.

The ultrasonic source is formed of a lead zirconate-titanate piezoelectric ceramic (PZT).

Such a motor may need less than twenty Newtons thrust and be quite small; for example the combustion chamber may be 2 cm long.

Rocket motors in accordance with the invention afford considerable advantage, especially in the small size and thrust range. For example the ultrasonic vibration also tends to minimise the risk of blockage of small injector passages, and propellant atomisation is independent of propellant flow velocity and can be made particularly efficient both in terms of droplet size and percentage quantity of liquid atomised. Moreover a variation of the electrical power can be arranged to afford a flow rate control via both cavitation and discharge coefficient variation.

Claims (9)

1. A liquid fuel rocket motor having combustion chamber, an exhaust nozzle and a liquid propellant supply apparatus, the said propellant supply apparatus comprising a liquid propellant reservoir, a liquid propellant supply duct connecting the reservoir with a supply orifice in the combustion chamber, an ultrasonic source, and focussing means arranged to focus ultrasonic vibration on the liquid in the supply duct, whereby in operation of the motor the liquid is atomised by ultrasonic vibration.

2. A rocket motor as claimed in claim 1 and wherein the fuel duct includes a delivery tube attached to the focussing means in transverse array and the supply orifice is at the end of the delivery tube at a distance from the focussing means substantially equal to (2n - 1)A 4 where n is an integer and -A is the wavelength of the natural vibration of that portion of the tube, where the natural vibration of the said portion of the tube is resonant with the ultrasonic source.

3. A rocket motor as claimed in claim 2 and which is a bi-propellant rocket motor, wherein two delivery tubes are so attached to the focussing means, one tube for each fluid.

4. A rocket motor as claimed in claim 3 and wherein the two tubes converge are toward the other.

5. A rocket motor as claimed in claim 1 and wherein the source is in the form of a circular shell of part spherical form, the focussing means includes a cone contiguous at the base with the source, bonded thereto, and having a part spherical cavity at its apex, the distance between the base and the cavity wall being an integer multiple of half wavelengths at the resonant frequency of the source, and the fluid duct includes a cavity part bonded by the part spherical cavity at the apex of the focussing cone and arranged so that the focal point of the focussing cone is at the supply orifice and at a distance from the part spherical cavity wall equal to an integer multiple of half wavelengths at the source resonant frequencies.

6. A rocket motor as claimed in claim 5 and wherein the duct is arranged to pass through the source and the focussing cone.

7. A rocket motor as claimed in any one of claims 1 to 6 and wherein the ultrasonic source and the focussing means are mounted in the rocket motor via a nodal zone in the focussing means.

8. A rocket motor as claimed in any one of the preceding claims and wherein the focussing means is made of a titanium alloy.

9. A rocket motor substantially as hereinbefore described with reference to the accompanying drawings.