Successful Flight Demonstration Conducted by the Air Force
and United Launch Alliance Will Enhance Space Transportation
The Air Force and United Launch Alliance (ULA)
successfully completed numerous on-orbit
cryogenic-fluid-management demonstrations on
the Atlas V AV-017 mission following successful
insertion of the DMSP-18 spacecraft. Six
distinct demonstrations were performed
specifically designed to improve our
understanding of propellant settling and slosh,
pressure control, RL10 chilldown and RL10 two-
phase shutdown operations. Lessons learned
will guide further upgrades to ULAâs current
Atlas and Delta cryogenic upper stages
improving performance and allowing longer,
more demanding missions. These results also
directly benefit current operation of the Air Force
Evolved Expendable Launch Vehicle fleet,
development of ULAâs planned Advanced
Common Evolved Stage and other cryogenic
systems.
Special research and development
instrumentation was added to Centaur, the Atlas
cryogenic upper stage, to support the
demonstration including temperature
measurements on the LH2 sidewall, forward
bulkhead, LH2 feedline, RL10 pump housing
and aft bulkhead components. The
demonstrations commenced once Centaur had
maneuvered a safe distance away from DMSP-
18 to ensure no risk to the spacecraft. The
Centaur disposal burn was delayed by 2.4 hours
to allow for the low acceleration demonstrations.
The disposal burn itself provided a unique
opportunity to perform demonstrations without
an attached payload. The light weight of DMSP-
18 allowed 12,000 lbs of remaining LO2 and
LH2 propellant, 28% of Centaurâs capacity, for
the demonstrations.
A preliminary data review of the demonstrations
showed very favorable results. The majority of
instrumentation worked properly, providing a
wealth of data.
1)
Low-G Settling Demonstration:
Centaur and other cryogenic space
propulsion systems, such as the Delta
IV second stage and Saturn Vâs S-4B
stage, use on-orbit settling to separate
liquid from gas. This separation is
Lift-off of AV-017 carrying the DMSP satellite
for the Air Force.
Centaurâs sidewall was painted white and
numerous dedicated instruments were added
to support the on-orbit CFM demonstrations.
critical, enabling the stages to vent pure gas to control tank pressure. The required
magnitude of settling acceleration directly affects the stage performance and maximum
coast duration. For this demonstration, Centaur was settled by pulsing its four hydrazine
thrusters at a reduced duty cycle to provide half of the acceleration that is typically used
to support Centaurâs longer coast missions. Lower duty cycle settling reduces the
hydrazine consumption rate, allowing longer mission durations.
Results from the sidewall and bulkhead temperature measurements show that LH2
remained settled for the majority of the phase. A slosh wave did briefly cool the forward
bulkhead after initially achieving the lower settling level, but the LH2 remained adequately
settled for the remainder of the demonstration. The short sloshing period can be
accommodated on future operational missions by inhibiting venting during the start of a
coast. Ultimately, this lower acceleration level is shown to adequately support future long
coast missions.
2)
Solid-Body-Rotation Settling Demonstration:
For coasts longer than about 15 minutes, Centaur is rolled around its longitudinal axis to
ensure uniform heating. Typically, the roll direction is regularly reversed to prevent solid
body rotation of the propellant. For this demonstration, Centaur maintained a single roll
direction to ensure solid body rotation. Following the low axial settling period, settling
thruster firing was terminated with the objective to demonstrate that low level centrifugal
acceleration could adequately retain liquid slosh. The liquid slosh must be kept
sufficiently damped such that the hydrogen vent port, located on the forward door near
the tank centerline, remains clear of liquid.
Initial post flight review of the attitude control thruster firings indicates that the solid body
rotation did not induce nutation affects that would adversely affect future missions.
Likewise, Centaur hydrogen tank venting and bulkhead temperature measurements
Liftoff
Maximum Dynamic Pressure
Alt = 37,800 ft
Max Q = 482 psf
Atlas/Centaur Separation
Alt = 594,524 ft
= 97.8 nm
Down Range = 145.9 nm
SC Separation
Alt = 462.7 nm
Centaur MECO1
Alt = 462.6 nm
Down Range = 1,837 nm
T+ 917.1 sec
Centaur CCAM
MET = 1,086 to 2,086 sec
Demonstration Coast
MET = 2,086 to 10,686 sec
Centaur MES1
Alt = 666,437 ft
= 109.7 nm
Down Range = 166.1 nm
PFJ
Alt = 118.9 nm
Down Range = 182.2 nm
Centaur Disposal Burn
ALT = 454.2 nm@ MES2
T+ 10,685.7 sec
ALT = 495.0 nm @ MECO2
Liftoff
Maximum Dynamic Pressure
Alt = 37,800 ft
Max Q = 482 psf
Atlas/Centaur Separation
Alt = 594,524 ft
= 97.8 nm
Down Range = 145.9 nm
SC Separation
Alt = 462.7 nm
Centaur MECO1
Alt = 462.6 nm
Down Range = 1,837 nm
T+ 917.1 sec
Centaur CCAM
MET = 1,086 to 2,086 sec
Demonstration Coast
MET = 2,086 to 10,686 sec
Centaur MES1
Alt = 666,437 ft
= 109.7 nm
Down Range = 166.1 nm
PFJ
Alt = 118.9 nm
Down Range = 182.2 nm
Centaur Disposal Burn
ALT = 454.2 nm@ MES2
T+ 10,685.7 sec
ALT = 495.0 nm @ MECO2
A 2.4 hour post-spacecraft mission extension was added to the DMSP-18 launch to allow for a
number of on-orbit demonstrations.
confirm that the centrifugal settling maintained adequate liquid control and that
disturbances caused by gaseous hydrogen and oxygen venting had no negative mission
impact. Initial flight review indicates that centrifugal settling is promising, but the on-going
detailed data review will be required to determine if it is beneficial for future flights.
3)
Oxygen Venting on-Orbit:
Centaurâs oxygen vent is not balanced so oxygen venting produces a torque on the
vehicle. This is not a problem during current missions since Centaur does not vent on-
orbit, however, longer duration missions in the future may require on-orbit oxygen venting
to control tank pressure. This demonstration was designed to determine both oxygen
and hydrogen liquid propellant management characteristics and vehicle control with the
asymmetric force generated during oxygen venting.
Oxygen venting was conducted during both the low acceleration and centrifugal settling
demonstrations. Results show that the LH2 remained settled during the low-g settling
phase. No adverse propellant motion or vehicle control issues were observed. Following
the GO2 vent during the Solid-Body Rotation settling phase, LH2 sloshing was observed
on the forward bulkhead as predicted. If future missions utilize solid body rotation settling
we may need to inhibit hydrogen venting for a short period of time following oxygen
venting.
4)
LH2 Pulsed Chilldown:
Prior to pumping LO2, the RL10 engines must be chilled. This is typically accomplished
by flowing cryogenic propellants through the engine. Flight demonstrations conducted
during the 1990âs on Atlas and Titan Centaurs demonstrated that pulsing the LO2 flow
significantly reduces the required quantity of propellant. Information gained from the
1990âs demonstrations formed the basis of Centaurâs current LO2 âtrickleâ chilldown
process that substantially reduced the required LO2 consumption.
The demonstration performed on the DMSP-18 mission was designed to provide similar
data for the LH2 pump chilldown. The long coast during which the settling
demonstrations were performed allowed the Centaur feedlines and RL10 engine
hardware to warm to relatively high temperatures. The LH2 flow was then pulsed
multiple times prior to the second main engine start. Each pulse consisted of a period of
liquid flow followed by a period of no flow to allow LH2 to boil and cool the pump. Flight
results showed that the pulsed chilldown did a good job of removing heat from the
feedline and pump housing while reducing propellant usage. This chilldown technique
shows good promise for future long coast missions.
5)
GO2 Venting during Engine Burn:
There are certain situations where it is advantageous to rapidly reduce Centaur LO2 tank
pressure during the burn. The influence of this pressure change on RL10 operation and
Centaur environment was demonstrated on this mission. Extra instrumentation was
mounted on various Centaur aft bulkhead components to determine the vibration
environment and validate that the oxygen vent plume did not create an adverse
environment by interacting with the engine plume. Mission results show that no adverse
environment was observed and RL10 engine operation was unaffected by the pressure
change, thus demonstrating that venting of the oxygen tank is feasible for future
missions.
6)
Modified Minimum Residual Shutdown (MRS):
MRS allows Centaur to continue RL10 operation until liquid pull-through. Centaur utilizes
MRS to maximize performance for missions where precise orbit injection accuracy is not
required. Normal MRS logic commands RL10 shutdown as soon as acceleration starts to
fall off. This demonstration allowed the RL10 to continue operation until thrust fell
substantially.
Good data was obtained. Engine thrust decayed once the LO2 was depleted and vehicle
acceleration dropped precipitously as expected. Following pull-through, the RL10
continued to generate thrust by burning a combination of liquid and gaseous propellants
before the RL10 reached final shut down at a preset time. During this period, Centaur
experienced a few thrust spikes possibly caused by ingestion of trapped liquid. Potential
benefits of utilizing this two-phase engine operation for future missions include increased
engine performance or improved Centaur disposal options.
Useful data has already been obtained from these demonstrations and on-going detailed analysis
by the Air Force and ULA will quantify the potential benefits and impacts of implementing these
techniques. Just as flight demonstrations in the past have led to the high capability of todayâs
Centaur, the results of these demonstrations will further allow ULA to improve on second stage
design and operation to guide development towards more advanced space-based cryogenic
systems.
For further information please contact:
Capt David Ilgenfritz, USAF
Systems Engineering and Analysis Branch Chief, Atlas Engineering Division
email: david.ilgenfritz@losangeles.af.mil
Office: 310-653-3044
Or
Mr. Mark Dornseif
Customer Program Office, United Launch Alliance Atlas AF/EELV Programs
email: mark.j.dornseif@ulalaunch.com
Office: 303-269-5269