Some of the ram accelerator research topics being investigated at the University of Washington include:
Single stage ram accelerator operation with low projectile entrance
velocity
(700 to 1000 m/s) is under investigation. The primary objectives are
development of propellant mixtures and projectile geometries, which
allow ram
projectile starting at low entrance velocities (and correspondingly low
Mach
numbers) in conjunction with maximum acceleration in a single stage. A
successful
ram projectile start is defined as obtaining supersonic flow past the
throat
while initiating and stabilizing combustion behind the throat.
Theoretical and
computational studies will outline the basis for propellant mixture and
projectile geometry selection of the experimental ram accelerator
shots.
Propellant mixtures with low sound speed values and projectile
geometries with
flow throat area variations will be utilized. The initial effort of
this
research will be conducted from October 1995 through March 1996.
- Last Updated 30 June 1996
Optimization of velocity in the thermally choked mode of the ram
accelerator
will be investigated. Thermally choked operation, also referred to as
subdetonative operation, is defined as the velocity regime where the
ram
projectile is moving faster than the sound speed of the local gas but
slower
than its Chapman-Jouget detonation speed. Earlier research towards the
goal of
velocity optimization in this regime was performed in 1994 and 1995 by
Imrich,
wherein he systematically varied the projectile geometry to determine
an
optimum shape that would allow the projectile to be accelerated to near
the detonation
speed while minimizing the projectile's mass. Assuming the thrust is
constant,
this maximizes the projectile acceleration and, hence, the velocity. In
the new
research, the optimized projectile will be accelerated through a
variety of
propellant mixtures to determine what combination of fuel, oxidizer and
diluent
will produce the greatest thrust.
- Last Updated 30 June 1996
Preliminary experiments with projectiles fabricated from aluminum and titanium alloys have demonstrated that acceleration is possible at velocities greater than the Chapman-Jouguet (CJ) detonation speed of a gaseous propellant mixture. Projectile materials were found to play a significant role in these experiments. Theoretical modeling was successful in predicting projectile drag in nonreactive gas mixtures at hypersonic velocities. When this drag was subtracted from the ideal thrust of a supersonic combustion ram accelerator, the net thrust closely matched that measured in the experiments. The dependence of the maximum operating Mach number on both the projectile diameter and propellant heat release was examined. The peak velocity capability of the experimental projectile geometry is predicted to be about 1.5 times the CJ speed of the propellant mixture. It was found that the drag resulting from an increase in projectile diameter was more than offset by the corresponding enhancement in thrust, and that velocities of nearly twice the CJ speed are possible.
The conditions under which a supersonic blunt projectile will
initiate a
detonation wave are investigated experimentally. A blunt body of
sufficient
size and velocity injected into a combustible mixture can initiate a
detonation
wave which, if the projectile velocity is less than the mixture
detonation
velocity, will propagate ahead of the projectile. If the projectile
does not
initiate an unsupported detonation, the mixture may still react in a
combustion
wave which is not fully coupled to the projectile bow shock. The
boundary
between these two phenomena is the subject of this investigation. Spheres were
launched
into a stoichiometric mixture of hydrogen and oxygen with 70% argon
dilution.
The sphere's diameter varied from 9.5 mm to 16 mm and the sphere's
velocity
ranged from Mach 2 to Mach 6. The mixture fill pressure varied from 0.4
bar to
7.5 bar. The results of the experiments were monitored via pressure
transducers
mounted on the chamber wall. It was necessary to isolate the experiment
from
the effects of the launcher, the diaphragm thickness, and the
interaction of
the projectile bow shock with the test chamber wall. The results
indicate that
a very distinct boundary exists between immediate detonation initiation
by the
sphere and no detonation. A simple theory due to Lee and Vasiljev which
equates
the energy required to initiate a cylindrical detonation with the work
done by
the drag force of the projectile predicts the fill pressure at which a
projectile traveling at the CJ velocity will initiate a detonation. The
theory
fails at projectile velocities below the CJ velocity, however, as it
predicts
detonations to occur at fill pressures where none are experimentally
observed.
--Last updated 03 December 1995
A control volume analysis code was developed which incorporates a
generalized equation of state to model the flow conditions inside the
ram
accelerator. Most prior performance analyses were conducted with 1-D
codes
using an ideal equation of state. However, the ram accelerator operates
at
higher pressures where the ideal gas equations are no longer valid. Not
surprisingly, these codes typically underpredict the experimental
results for
the projectile's thrust and velocity. It has been recently demonstrated
by
other researchers that using a high pressure equation of state will
provide
excellent performance characteristics when compared with experiments.
The code
currently includes the following equations of state: ideal gas,
Boltzmann,
Percus-Yevick, and a Virial Expansion in which the virial coefficients
are
obtained by the Lennard-Jones and Stockmayer potentials.
--Last updated 03 July 1997
Normal operation of the ram accelerator at the University of
Washington
involves the use of a perforated tube to vent the gun gases into an
evacuated
dump tank before the projectile enters the ram accelerator tubes. The
need for
a large dump tank and the equipment to evacuate it makes this venting
process
impractical for some potential applications of the ram accelerator.
Experiments
were conducted with the perforations in the vent tube plugged to
evaluate the
effect of launch tube gases, both in front of and behind the obturator,
on the
starting process. Fill pressures in the accelerator section were varied
from 25
atm to 50 atm. The standard mixture developed at the University of
Washington,
2.8CH4 + 2O2 + 5.7N2, was used in the ram accelerator section. The
unvented ram
accelerator operated nominally for pressures of 30 atm and above.
However, at
25 atm, the projectile repeatedly failed to start as a result of a
significant
amount of gun gases that were not relieved in the unvented system. A
large
shock was formed in front of the obturator and caused the projectile to
immediately unstart. This shock was present in the higher pressure
shots as
well, but because its magnitude was significantly less than the fill
pressure,
an unstart did not result. It was also observed that the ventless
configuration
had an effect on the pressure in the launch tube after the projectile
had
passed. After the projectile entered, gas from the ram accelerator
section
expanded into the launch tube more quickly than in the vented case.
Another
effect of ventless operation was decreased obturator separation
relative to
vented operation. It was concluded that the ram accelerator at the
University
of Washington can operate nominally without venting of the gun gases
for fill
pressures of 30 atm and above.
--Last updated 20 July 1996
Experimental and computational efforts are underway at many research facilities around the world to map out detonation limits in propellant mixtures of interest to the ram accelerator community. Knowledge of the ignition characteristics of high pressure gaseous fuel and oxidizer mixtures is useful for optimum propellant selection. Subdetonative operation of the ram accelerator requires that a detonation not be initiated by the projectile, obturator, or combination thereof. On the other hand, detonable propellants may be highly desirable for superdetonative ram accelerator operation. Piston initiated detonation studies seeking to define the conditions necessary to create detonations in the ram accelerator have been conducted in nitrogen and carbon dioxide diluted mixtures of methane and oxygen at pressures up to 50 atm in a 12m long, 38.1mm bore tube. Detonation limits were mapped out for various piston velocities, piston masses, and mixture compositions. The effects of finite rate chemistry on the shock heated flow are numerically simulated to identify key parameters influencing the detonation initiation process.
Detonation limit envelopes can be applied to the unsteady ignition
process
or to the quasi-steady normal operation of the ram accelerator. In
order to
apply a detonation limit envelope to the ram accelerator, it was
necessary to
create a model characterizing the effects of the projectile and
obturator on
the propellant. Propellant envelopes for the ram accelerator created
with
detonation limit information were compared to the phenomena observed in
ram
accelerator projectile shots in these mixture classes. This paper
summarizes
the detonation limit envelopes generated to date, presents a model for
characterizing the effect of the projectile and obturator on the
propellant,
and discusses how both might be brought together and applied to the
operation
of the ram accelerator.
--Last updated 20 July 1996
Ram accelerator operation with low projectile entrance velocity
(<1000 m/s) is investigated. The ram accelerator is a hypervelocity
launcher in which a projectile, similar in shape to the centerbody of a
ramjet, travels supersonically through a tube filled with premixed
gaseous fuel and oxidizer. A conventional gun initially boosts the
projectile to supersonic ram accelerator entrance velocity. A
successful ram projectile start is defined as obtaining supersonic flow
past the throat, while initiating and stabilizing combustion behind the
throat. The primary objective of this study is to start the ram
accelerator projectile at low entrance velocities. Low velocity
starting is important for facilities with launch tube length
constraints or low pressure pre-launchers. Knowledge of the low
velocity start process also furthers the understanding of ram
accelerator starting at all entrance velocities. A study of detonation
initiation by piston was conducted to determine ignition
characteristics of methane/oxygen/carbon dioxide and
methane/oxygen/nitrogen propellants. Mapping of the detonation limits
for ram accelerator propellants provided an approximate propellant
envelope from which to attempt low velocity starting of ram accelerator
projectiles. Experiments were performed using carbon dioxide diluted
mixtures of methane and oxygen at 25atm fill pressure to examine low
velocity starting of five-fin projectiles having a flow throat to tube
area ratio of 0.42. Starting at low velocity in low sound speed
propellants was found to be extremely sensitive to perturbations in the
amount of diluent present, the entrance velocity, and the obturator
mass. Low velocity starting of three-fin projectiles having flow throat
to tube area ratios of 0.42, 0.504, and 0.588 was investigated in
nitrogen diluted mixtures of methane and oxygen at 50atm. The flow
throat area was increased to start at lower entrance velocities in
higher sound speed propellants. This paper provides the current
understanding of the ram accelerator starting process, summarizes the
results of the piston detonation initiation effort, and presents
experimental data for low entrance velocity ignition of the ram
accelerator. >
--Last updated 20 July 1996
