Gas driven gyroscope with extended rundown time

A gas gyroscope 11 is provided with an aspirator pump 71 in order to evacuate gas from the main chamber 19 of the gyroscope 11. The evacuation of gas reduces windage and other air friction encountered by the gyroscope's rotor 21 and thereby extends the rundown time of the gyroscope. The aspirator 71 may be energized by a dedicated compressed gas source such as a compressed gas chamber 91. Alternatively, the aspirator 71 may be provided with ram air or bleed perpellant gases.

BACKGROUND OF THE INVENTION 
This invention relates to a gas driven gyroscope (gyro) with an integrally 
contained gas source. More particularly it relates to improvements in gas 
driven gyroscopes which reduce internal friction in order to extend the 
rundown time or maintain higher speeds of the rotor during a specified 
time. 
Gas driven gyroscopes with integrally contained gas sources are known which 
have a gas driven reaction rotor in one chamber and compressed inert gas 
in another chamber. Gas is released from one chamber by means such as 
puncturing a thin wall between the chambers. The gas, when released, flows 
through the hollow bore of the rotor shaft and outwardly through reaction 
passages in the rotor to cause it to spin. Changes in the gas pressure may 
also be used to uncage the gyroscope gimbals after the rotor is spinning. 
Such gyroscopes are sometimes called pyrotechnic gyros because pyrotechnic 
techniques are frequently used to puncture the wall. 
An example of the foregoing type of gas driven gyroscope is detailed in 
U.S. Pat. No. 3,393,569, issued July 23, 1968, now assigned to the present 
assignee. The U.S. Pat. No. 3,393,569 is incorporated herein by reference 
and any of the various species of gas driven gyros shown in that patent 
might employ the improvement of the present invention. 
One advantage of a gas driven gyro, as compared to a spring driven gyro, is 
that the gas driven gyro has a high ratio of usable energy in relation to 
its volume and weight. The potential energy in the compressed gas is 
quickly translated to rotational kinetic energy of the spinning rotor. The 
"rundown" of the rotor refers to the gradual decrease in rotor speed over 
time after the gas is expelled through the rotor passages. Various loss 
factors affect the rundown of such gyros. The most significant of these 
factors relates to energy absorbed by the gyro's interaction with that gas 
which remains in the gyroscope after gas release is completed. This 
invention is directed to the reduction of such air friction losses. 
One such air friction loss is caused by the reaction rotor acting as a pump 
rather than as a reaction turbine and acting to suck gas through the end 
of the shaft to pump it out the rotor exhaust ports. This reduces kinetic 
energy and causes more rapid slowing of the rotor. This problem is 
specifically addressed by U.S. Pat. No. 4,271,709, issued Jan. 9, 1981, 
now assigned to the present assignee and incorporated herein by reference. 
That patent discloses a poppet valve which prevents such pump action. The 
prevention of such pumping, of course, does not eliminate other air 
friction losses from the gyroscope which is spinning at high speeds, 
preferably in excess of 30,000 RPM. 
Accordingly, one object of the present invention is to improve gyroscope 
performance. It is therefore an object to provide an improved gas driven 
gyroscope with an extended rundown time, or having a higher average speed 
during a specified rundown time. Another object of the invention is to 
provide an improved gas driven gyroscope with reduced losses during 
rundown. Another object of the invention is to provide a gas driven 
gyroscope in which running friction is reduced during rundown. It is a 
further object to provide the gyroscope in which air is provided to the 
gyroscope in order to provide kinetic energy for the gyroscope rotor, and 
the air is evacuated from the gyroscope after initial run-up in order to 
extend rundown time of the gyroscope particularly at high rotor speeds. It 
is therefore an object to reduce air friction in a gas driven gyro. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, air friction acting on a gas 
driven gyroscope rotor is reduced in order to extend the rundown time of 
the gyroscope. The gas is preferably vented during a time in which working 
gas is admitted to the chamber. After the venting, the chamber is 
essentially sealed and an aspirator evacuates remaining gas from the 
chamber. This evacuation of the chamber reduces air friction during 
rundown. The aspirator may be powered by gas from propulsion of the 
vehicle carrying the gyroscope, by ram air from the forward motion of the 
vehicle or from a compressed gas chamber. Alternatively, the aspirator may 
be powered by electrical or mechanical means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1 of drawings, a gas gyroscope 11 includes a main housing 
13 and a compressed gas housing section 15 attached thereto by bolts 17. 
The main housing 13 defines a chamber 19 enclosing a gas reaction rotor 21 
mounted on a hollow rotor shaft 23. The shaft 23 is mounted in precision 
high speed bearings 25 supported by an inner gimbal 27. The inner gimbal 
27 is rotatably supported in an outer gimbal 28 which, in turn, is 
supported in bearings 31 in housing 13. 
The rotor 21 and gimbals 27, 28 are "caged" prior to release of a gas by 
means of a caging mechanism shown generally at 33. Details of the 
operation of the caging mechanism 33 may be had by reference the aforesaid 
U.S. Pat. No. 3,393,569. It should suffice to note that the caging 
mechanism 33 includes a piston 35 which is initially released and 
positioned by gas pressure and then caused to hold a caging plug 37 upward 
against a beveled portion 39 of the rotor shaft 23. When a gas pressure 
differential across the piston falls to a predetermined value, the caging 
plug 37 is withdrawn and the rotor 21 and gimbals 27, 28 are uncaged. The 
caging mechanism 33 also serves as a conduit to conduct compressed gas 
from an intermediate passage 41 to the hollow rotor shaft 23. However when 
the caging plug 33 is withdrawn, the conduit is disconnected from the 
rotor shaft 23, which now has free access to the interior of housing 13. 
The compressed gas housing 15 defines a compressed gas chamber 43 which 
communicates with the hollow rotor shaft 23 via the caging mechanism and 
the intermediate connecting passage 41 when a thin disc 45 is punctured in 
a known manner by an electrically operated pyrotechnic propelled piston 
47. The operation of the puncturing mechanism which operates to release 
compressed inert gas from the compressed gas housing 15 into passage 41 is 
immaterial to the present invention, but details of several suitable types 
of mechanisms may be found by reference to the aforesaid U.S. Pat. No. 
3,393,569. 
The rotor 21 includes a plurality of circumferentially spaced nozzles 51 
and the shaft 23 also includes a plurality of circumferentially spaced gas 
admission ports 53 freely communicating with nozzles 51. 
In operation, an inert gas, preferably nitrogen, at 3000 pounds per square 
inch is contained in the compressed gas housing 15 and is communicated via 
passage 41 and caging mechanism 33 to the interior of shaft 23 and thence 
to rotor nozzles 51 via ports 53. This causes the rotor 21 to spin at a 
high speed, preferably 36,000-50,000 R.P.M. depending upon the size of the 
gas chamber and the particular use to which the gyroscope 11 is put. When 
the gas pressure within the main housing chamber 19 falls to a 
predetermined value, the caging mechanism 33 releases the rotor and 
gimbals, also disconnecting the conduit leading to the intermediate 
connecting passage 41. 
Gas escaping from the rotor nozzles 51 is received within the main housing 
chamber 19. In order to avoid a significant build-up of air pressure 
within the main chamber housing 19, this air is permitted to escape 
through an exhaust port 61 and a flapper valve 63. Where the rotor 21 is 
uncaged by the caging mechanism 33, gas in the compressed chamber 43 is 
depleted. 
In accordance with the present invention, gas from the main housing chamber 
19 is then further evacuated by an evacuation pumping means such as an 
aspirator 71. The purpose of further evacuating gas from the main housing 
chamber 19 is to reduce windage gas friction which would result from the 
rapid rotational speed of the rotor 21. 
The aspirator 71 is selected to provide a vacuum sufficient to reduce air 
friction for the duration of time that extended run down is called for. A 
significant criteria is the amount of compressed gas which must be 
consumed to operate the aspirator. A preferred vacuum aspirator 71 for use 
in the embodiment of FIG. 1 is model AVR-093H Vacuum Transducer Pump, 
manufactured by Air-Vac Engineering Co. of 100 Gulf St., Milford, Conn., 
U.S.A. 
Flapper valve 63 communicates with an exhaust conduit 73. Flapper valve 63 
is fixed to the exhaust conduit 73 and functions as a check valve in order 
to permit gas to escape from the exhaust conduit 73 while preventing gas 
from reentering the exhaust conduit 73. Thus, when the aspirator 71 is 
able to draw a vacuum from the main housing chamber 19 through the exhaust 
conduit 73, flapper valve 63 is closed. When gas from the compressed gas 
housing 15 is provided at a volume which exceeds the capability of the 
aspirator 71 to extract the gas from the main housing chamber 19, flapper 
valve 63 opens. Likewise, a second flapper valve 75, associated with the 
caging mechanism 33, opens to enable the piston 35 to operate the caging 
mechanism 33, and closes when pressure within chamber 19 drops. 
The aspirator 71 is of conventional design and converts compressed air to 
vacuum by using venturi (Bernoulli's Law) principles. Compressed gas 
enters the aspirator 71 through a compressed gas inlet port 81 and is 
exhausted through a gas outlet port 83. This flow of gas causes a vacuum 
to be created at a vacuum inlet port 85, and gas to flow through the 
aspirator 71 from the vacuum inlet port 85 to the outlet port 83. 
As shown in FIG. 1, the compressed gas source for the compressed gas inlet 
port 81 may be an additional compressed gas housing 91. The additional 
compressed gas housing 91 is caused to release a compressed gas charge a 
short time after gas is initially released from compressed gas housing 15. 
This may be accomplished by a simple delay circuit 93. A restrictor valve 
95 may be used to control the flow of air from the additional compressed 
gas housing 91 to the compressed gas inlet port 81. Alternatively, the 
compressed gas source may be a pyrotechnic device (not shown). 
Referring to FIGS. 2 and 3, the compressed gas inlet port 81 may be 
connected to an external compressed air source 95. The external compressed 
air source 95 may be a ram air source which provides compressed air to 
operate the aspirator 71 as a result of motion of a vehicle such as a 
missle (not shown) into which the gyroscope 11 is mounted. Alternatively, 
an external compressed air source 96 may be pressurized propellant gas 
from the vehicle's propulsion source. 
As shown in FIG. 4, it is further possible to use some of the compressed 
gas from a compressed gas housing 15' in order to operate the aspirator 
71. This would be accomplished by use of an appropriate diverter circuit 
97 which would divert gas from the piston 35 to the compressed gas inlet 
port 81. 
In order to extend operating time, it is possible to use a combination of a 
compressed gas housing and an external compressed gas source to operate 
the aspirator pump 71. This can be accomplished by valving the aspirator 
71 at the compressed gas inlet port 81 so that when compressed gas to 
operate the aspirator 71 is depleted, the external compressed gas source 
can provide gas to operate the aspirator 71. This is particularly useful 
where ram air may not be available at initial launch of a missile. 
In order to determine the feasibility of using compressed gas to operate 
the aspirator 71, an aspirator was used to maintain a vacuum from a 52 
cm.sup.2 container for 2 seconds. The aspirator drew a vacuum of 0.7 atm 
(28 in. Hg). The exhaust gas from the aspirator was captured in a rubber 
balloon and was found to be approximately half the volume of the 
container, or approximately 25 cm.sup.2. From this it was determined that 
the use of compressed gas to maintain a gyroscope of like volume at a 
vacuum in a working environment is practical. 
As shown in FIG. 5, it is possible to substitute other vacuum sources 99 
for aspirator 71. For example, bleed vacuum from a vehicle propulsion 
system may be used. Mechanically or electrically driven suction pumps may 
also be used. 
The above description of the preferred embodiments is given by way of 
example only, as it is anticipated that modifications of the preferred 
embodiment may prove to be economically feasible. Accordingly, the 
invention should be read as limited only by the appending claims.