Development of apparatus for simulating zero gravity condition

Lackner, H. G.

English

NASA Technical Reports Server

METHOD AND APPARATUS OF SIMULATING ZERO GRAVITY CONDITIONS 
.- .-. ..... ~ ....._....... 
IG. i 
IQVEN TOR 
HELMUT G. IACKNER 
ATTORN E YS 
https://ntrs.nasa.gov/search.jsp?R=19710006748 2020-03-17T02:37:22+00:00Z
3.526.140 
United States of America as -represented-by the Ad- 
ministrator of the National Aeronautics and space 
r. 11, l969, Ser. No. 806,149 
bt. el. GO11 1 /oo 
U.S. (61. 73-432 6 Claims 
A method and apparatus for determining the effects of 
acoustic radiation pressure on a liquid suspended in a 
substantially frictionless environment. The apparatus in- 
cludes an ultrasonic transducer excited by the output of 
a power amplifier. The power amplifier amplifies the out- 
put of an oscillator which has been adjusted to deliver an 
output at a frequency of approximately 100 kHz. The 
transducer is oriented so that the acoustic energy or pres- 
sure generated thereby is directed in a beam toward drop- 
lets of a cryogenic liquid floating on a precisely leveled 
plate. The droplet floats on a vapor envelope that sur- 
rounds and thermally insulates the droplet. Such a droplet 
can sustain for some time up to a minute. This apparatus 
and method makes it possible to investigate the transfer be- 
havior of cryogenic liquids under zero gravity. 
ORIGIN OF THE INVENTION 
The invention described herein was made by an em- 
ployee of the United States Government and may be man- 
ufactured and used by or for the Government of the 
United States of America for governmental purposes 
without the payment of any royalties thereon or therefor. 
BACKGROUND OF THE INVENTION 
The development of the capability to orbit various type? 
of space vehicles has been accompanied by a need to in- 
vestigate the transfer behavior of cryogenic liquids such 
as rocket propellants within a fuel tank. These propellants 
under zero-gravity can move freely about within the tank 
and may become dispersed at random in irregular glob- 
ules throughout the tank. It is necessary in space vehicles 
that the liquid propellants in the tanks cover the inlets 
to propellant pumps and suction lines so that during re- 
starting at least the inlets will always be in communication 
with a supply of propellant. Otherwise the flow of pro- 
pellant to the rocket engine could be erratic and result m 
faulty engine operation. Thus, it is desirable to provide 
some means for assuring proper positioning of the pro- 
pellant within a propellant tank. 
A source of ultrasonic force or pressure has been 
suggested as a means of positioning the propellant within 
a tank. Ultrasonic forces have disruptive effects on ma- 
chinery and structures and therefore it is desirable to use 
only the force intensity that is required. Also, a device 
capable of delivering great force has increased heat diu. 
persion, and increased electrical power requirement which 
means larger batteries, fuel cells or other power sourcs; 
all of which increase the $weight of an orbiting vehicle. 
Thus, it is desirable to limit the size and complexity of 
an ultrasonic source of energy to that level sufficient to 
accomplish a desired purpose, i.e., position propellants 
within a tank. The determination of what output is re- 
quired is complicated however by the fact that the only 
way to subject a tank of cryogenic propellants to zero- 
gravity is to place such tank in orbit. This is very expen- 
sive and it is much more desirable to develop a method 
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and technique for obtaining such information in a labora- 
tory on earth. 
SUMMARY OF THE INVENTION 
The present invention provides a method and apparatus 
for determining in 8 laboratory the ultrasonic forces neces- 
sary to move masses of cryogeaic liquids in the fuel tanks 
of orbiting space vehicles. This is accomplished by direct- 
ing acoustic energy or radiation pressure of a known ultra- 
sonic frequency toward a droplet of liquid nitrogen or 
other cryogenic liquid floating free of Lriction on a leveled 
plate. This frictionleqs support of the droplet, which 
makes it possible to simulate movement of a mass in zero- 
gravity, is explained by Leidenfro$’s phenomenon. Leiden- 
frost’s phenomenon is the fact that a liquid droplet placed 
on a surface with p temperature higher than the boiling 
temperature of the liquid, builds up an envelop of vapor 
around the droplet which is a bad heat conductor and 
thus thermally insulates the droplet. Such a droplet can 
sustain for some time up to a minute. The vapor en- 
velop in addition to thermally insulatieg the droplet also 
provides an ideal gas bearbg between the droplet and the 
surface of the level plate, thus providing a friction free 
support for the droplet. The source of acollstic energy or 
radiation pressure is a transducer excited by the ampli- 
fied output of an oscillator operating at about 100 kHz. 
By appropriate calculations it is possible to extrapolate 
from experimental data obtained just what radiation pres- 
sure and power is necessary to move a particular pro- 
pellant mass in zero-gravity. 
BRIEF DESCRIPTION OF DRAWING 
FIG. 1 is a schematic illustration of an apparatus for 
FIG. 2 is an enlarged view of a cryogenic liquid drop- 
simulating zero-gravity conditions. 
let surrounded by a vapor envelop. 
DETAILED DESCRIPTION OF THE INVENTION 
Referring to the drawing wherein an apparatus for 
simulating zero-gravity in a horizontal plane is shown. 
The apparatus includes a plate 10 having a polished upper 
surface supported over a base 12 by means of suitable 
level adjustment mechanisms 14. Mechanisms 14 permit 
leveling of plate 10 to provide a precisely level horizontal 
plane. The plate and base, as well as other components 
of the invention, are positioned on a table or platform 16. 
An oscfllator 18 is connected to a power amplifier 20 
and the amplified oscillator output is connected to an 
ultrasonic quartz transducer 22. Transducer 22 is sup- 
ported on a mast and oriented so that acoustic energj 
radiated therefrom when the transducer is excited by 
the amplifier output is directed toward a droplet 26 of B 
cryogenic liquid that has been placed on the plate by 
any suitable dispensing device (not shown). 
Zero-gravity in three dimensional space means that 
there is no gravitational acceleration in any direction and 
therefore there ace no gravitational forces. To simulate 
this condition in the laboratory a two dimensional hori- 
zontal cross section or plane is needed where under pre- 
vailing gravity the horizontal forces are zero. This can 
be accomplished by carefully leveling plate 10 so that there 
is no tendency for a droplet of liquid placed on the plate 
to move about on the plate. In other words there are no 
gravitational forces acting in the harizontal plane of pos- 
sible motion. Gravitational forces would be present if the 
plate were not level. 
It is also necessary to consider the fact that an object 
under zero-gravity can move freely through a space with- 
out any friction forces between it and a supporting sur- 
face. A frictionless engagement between droplet 26 and 
plate 10 can be obtained by making use of Leidenfrost’s 
3,526,140 
3 
phenomenon. Leidenfrost’s phenomenon is that a liquid 
droplet placed on a surface with a temperature higher than 
the boiling temperature of the liquid, builds up an envelop 
of vapor which is a bad heat conductor and thus thermally 
insulates the liquid droplet. Such a droplet can sustain for 
some time up to a minute supported on the plate. FIG. 2 
illustrates the vapor envelop 28 that forms when a droplet 
of liquid nitrogen or other cryogenic liquid is placed on 
a plate that is at room temperature. The layer of vapor 
between the liquid and the plate forms an ideal gas bear- 
ing that enables the droplet to be moved about free of 
friction. 
Experiments utilizing the apparatus have proven that 
droplets of liquid nitrogen resting stationary on a level 
plate can be accelerated by the external forces produced 
by acoustical radiation pressure generated by an ultra- 
sonic transducer. It was found that the wavelength of the 
acoustical energy produced by the transducer had to be on 
the order of or smaller than the geometrical dimensions 
of the droplet being moved over the plate. Movement of 
the droplet was obtained by acoustic energy in the fre- 
quency range between 60 kHz. and 10.0 kHz. However, 
best results were obtained with ultrasdnic frequencies of 
about 100 kHz. At lower frequencies, those below 60 
kHz., the radiation pressure acts on all sides of the drop- 
let and there is no movement of the droplet. At these lower 
frequencies the wavelength of the acoustic radiation is 
larger than the diameter of the droplet and therefore 
propagates to all surfaces of the droplet. This can be 
further explained by making an analogy to the reception 
of electromagnetic medium frequency radiation and 
microwave radiation. Medium frequency radiation can be 
received behind an obstacle because it bends around all 
objects due to the fact that most objects are smaller than 
these long RF wavelengths, while microwave radiation 
cannot be received behind an obstacle which is large 
compared to the wavelength. 
At frequencies above 100 kHz. the directional diagram 
of the transducer; i.e. the beam of acoustic energy gener- 
ated by the transducer, becomes so narrow that it is not 
efficient. Such a beam has a needle-like shape and it is 
difficult to orient the transducer so that the beam hits the 
droplet to be moved. In contrast to this the beam has a 
wider lobe or span at 100 kHz. and the beam can readily 
be directed so that it impinges on a droplet. The exact 
operating frequency in this range is determined by the 
resonance frequency of the transducer which delivers 
maximum power. 
How the present invention is practiced is readily ap- 
parent. A droplet of a cryogenic liquid is placed upon the 
plate and the oscillator and amplifier activated so that the 
transducer is excited and acoustic energy is directed to 
the droplet on which acoustic radiation pressure builds up. 
Since the droplet is suspended on a level plate in a fric- 
tionless manner the only parameters which determine how 
much acceleration is imparted on the droplet are: the 
intensity of the acoustic radiation pressure, the effective 
cross-sectional area of the droplet, and the mass of the 
droplet. By making observations and measurements using 
the present invention it is possible to accumulate data 
from which calculations can be made to determine the size 
and type of acoustic energy source required for positioning 
propellants in a tank in zero-gravity. It is contemplated 
that transfer or movement of a propellant within a tank 
would require more than a single transducer since a single 
transducer has a rather narrow lobe which would not be 
very efficient in moving liquid within the tank. It is be- 
lieved that high speed scanning of the liquid to be posi- 
tioned, with ultrasonic energy, would create a local pres- 
sure that changes its position so rapidly that a quasi-con- 
tinuous unidirectional pressure would be applied onto the 
floating liquid within the tank. Rather than moving the 
transducer mechanically this beam scanning could be ac- 
complished with an array of ultrasonic transducers ar- 
ranged in the form of a matrix where each transducer is 
fed with power that is phase shifted with respect to the 
power fed to an adjacent transducer. This technique would 
provide scanning in X and Y directions similar to the 
scanning pattern used in a television screen. This tech- 
nique is well known and used in electronically-scanned 
antennas known as phased arrays. Such antennas consist 
of mechanically fixed radiation apertures of which the 
phase of excitation is varied so as to change the direction 
of radiation. 
What is claimed is: 
1. A method for determining the effect of acoustic 
15 radiation pressure on a mass of cryogenic liquid pro- 
placing a droplet of a cryogenic liquid on a precisely 
leveled plate at room temperature so as to form a 
vapor envelop around said droplet that floats said 
droplet in a substantially friction free manner over 
the plate; and 
directing acoustic energy from an ultrasonic transducer 
at said droplet so as to exert acoustic radiation pres- 
sure thereon that moves said droplet in a horizontal 
25 direction and provides data for determining the 
amount of acoustic energy needed to position cryo- 
genic fuels within the fuel tank of a space vehicle. 
2. The method recited in claim 1 wherein the acoustic 
energy directed at the droplet has a frequency in the range 
30 of 60. kHz. to 100 kHz. 
3. The method recited in claim 1 wherein the acoustic 
energy directed at the droplet has a frequency of about 
100 kHz. 
4. An apparatus for determining the effect of acoustic 
35 radiation pressure on a mass of cryogenic liquid in a zero- 
a precisely leveled plate means adapted to have a drop- 
let of cryogenic liquid placed thereon whereby said 
droplet will float friction freely on said plate on a 
a source of ultrasonic energy positioned adjacent said 
leveled plate means and oriented so that acoustic 
radiation from said source can be directed at said 
droplet of cryogenic liquid to accelerate and move 
said droplet across said plate means. 
5. The apparatus recited in claim 4 wherein said source 
an ultrasonic quartz transducer; 
an oscillator adapted to operate in the frequency range 
of 60 to 100 kHz.; and 
a power amplifier connected to said oscillator and said 
transducer for amplifying the output of said oscillator 
and feeding it to said transducer, whereby said trans- 
ducer is excited and an ultrasonic radiation pressure 
is applied to said droplet. 
6. The apparatus recited in claim 5 wherein said plate 
eferenees Cited 
UNITED STATES PATENTS 
3,264,467 8/1966 Mann et al. _ _ _ _ _ _ _  7 3 4 3 2  XR 
3,273,388 9/1966 Webb _ _ _ _ _ _ _ _ _ _ _ _  7 3 4 3 2  XR 
3,330,159 7/1967 Ongaro _ _ _ _ _ _ _ _ _ _ _ _ _ _  7 3 4 3 2  
65 3,339,418 9/1967 Paynter et al. _ _ _ _ _ _ _ _ _  73-432 
pellants in zero-gravity comprising the steps of: 
20 
gravity environment comprising: 
40 layer of vapor; and 
45 
of ultrasonic energy includes: 
50 
55 
means has a polished upper surface. 
60 
S. CLEMENT SWISHER, Primary Examiner 
H. C.  POST III, Assistant Examiner 
70 U.S. C1. X.R. 
181-.5 


Method and apparatus of simulating zero gravity conditions Patent

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