The abort control sequence is the software that manages, among
other items, the OMS and aft RCS configuration and thrusting periods
during ascent aborts to improve performance or to consume OMS
and aft RCS propellants for orbiter center-of-gravity control.
Premission-determined parameters are provided for the OMS and
aft RCS thrusting periods during aborts since the propellant loading
and orbiter center of gravity vary with each mission.
The premission-determined parameters for the abort-to-orbit thrusting
period are modified during flight, based on the vehicle velocity
at abort initiation. The premission-determined parameters for
abort once around are grouped with different values for early
or late AOA. The return-to-launch-site parameters are contained
in a single table.
The abort control sequence is available in OPS 1 and 6 and is
initiated at SRB separation if selected before then or at the
time of selection if after SRB separation.
ATO and AOA Aborts
The OMS and aft RCS begin
thrusting as soon as an ATO or AOA is initiated with one main engine
For some aborts, an OMS-to-aft-RCS interconnect is not desired.
A parallel aft RCS plus X thrusting period using aft RCS propellant
and the four aft RCS plus X thrusters will be performed during
the OMS-1 thrusting period to achieve the desired orbit. If a
plus X aft RCS thrusting period is required before main engine
cutoff, the abort control sequence will command the four aft plus
X RCS jets on if vehicle acceleration is greater than 0.8 g and
will monitor the RCS cutoff time to terminate the thrusting period.
If an RCS propellant dump (burn) is required before MECO and vehicle
acceleration is greater than 1.8 g, the abort control sequence
will command an eight-aft-RCS-jet null thrust and monitor the
RCS cutoff time to terminate the thrusting period.
In other abort cases, an OMS-to-aft-RCS interconnect is desired.
This thrusting is performed with the OMS and four aft RCS plus
X thrusters to consume OMS propellant for orbiter center-of-gravity
control. More aft RCS jets can be commanded if needed to increase
OMS propellant usage. For example, for an OMS propellant dump
(burn), 14 aft RCS null jets can be commanded to thrust to improve
orbiter center-of-gravity location.
If the amount of OMS propellant used before MECO leaves less
than 28 percent of OMS propellants, a 15-second aft RCS ullage
thrust is performed after MECO to provide a positive OMS propellant
feed to start the OMS-1 thrusting period.
The OMS-to-aft-RCS interconnect sequence provides for an automatic
interconnect of the OMS propellant to the aft RCS when required
and reconfigures the propellant feed from the OMS and aft RCS
tanks to their normal state after the thrusting periods have ended.
The interconnect sequence is initiated by the abort control sequence.
In order to establish a known configuration of the valves, the
interconnect sequence terminates the GPC commands to the following
valves if they have not been terminated before honoring a request
from the abort control sequence: left and right OMS crossfeed
A and B valves, aft RCS crossfeed valves and aft RCS tank isolation
A request from the abort control sequence for an OMS-to-aft-RCS
interconnect will sequentially configure the OMS/RCS valves as
follows: close the left and right aft RCS propellant tank isolation
valves, open the left and right OMS crossfeed A and B valves,
and open the left and right aft RCS crossfeed valves. The OMS-to-aft-RCS
interconnect complete flag is then set to true.
When the abort control sequence requests a return to normal configuration,
all affected OMS/RCS propellant valve commands are removed to
establish a known condition; and the interconnect sequence will
then sequentially configure the valves as follows: close aft RCS
crossfeed valves, close left and right OMS crossfeed valves and
open aft RCS propellant tank isolation valves. The OMS-to-aft-RCS
reconfiguration complete flag is then set to false, and the sequence
An RTLS abort requires the
dumping of OMS propellant by burning the OMS propellant through
both OMS engines and through the 24 aft RCS thrusters to improve
abort performance and to achieve an acceptable entry orbiter vehicle
weight and center-of-gravity location. The thrusting period is premission-determined
and depends on the OMS propellant load.
The OMS engines start the thrusting sequence; and after the OMS-to-aft-RCS
interconnect is complete, the aft RCS thrusters are commanded
on. The OMS engines and RCS thrusters then continue their burn
for a predetermined period. The interconnect sequence is the same
for ATO and AOA aborts. The OMS and aft RCS will begin thrusting
at SRB staging if the abort is initiated during the first stage
of flight or immediately upon abort initiation during second stage.
A contingency abort is selected
automatically at the loss of a second main engine or manually by
the flight crew using an item entry on the RTLS TRAJ or RTLS TRANS
CRT displays. For the contingency aborts, the OMS-to-aft-RCS interconnect
is performed in a modified manner to allow continuous flow of propellants
to the aft RCS jets for vehicle control and to allow contingency
rapid dump (burning) of OMS and RCS propellants. The abort control
sequence tracks the total time the OMS and aft RCS are on to determine
the amount of propellants used.
The request for an interconnect will cause the interconnect sequence
to configure the valves sequentially as follows: open the aft
RCS crossfeed valves, open the left OMS crossfeed valves A, open
the right OMS crossfeed valves B, close the left and right aft
RCS tank isolation valves, open the left OMS crossfeed valves
B and open the right OMS crossfeed valves A. The OMS-to-aft-RCS
interconnect complete flag will then be set to true.
If the rapid dump is selected before MECO, the OMS-to-aft-RCS
interconnect occurs, and both OMS engines and the 24 aft RCS jets
are commanded to thrust until the desired amount of propellant
has been consumed. The rapid dump will be interrupted during external
tank separation if the thrusting period is not completed before
MECO; otherwise, the thrusting period terminates when thrusting
time equals zero or if the normal acceleration exceeds a threshold
Upon completion of the thrusting period, the OMS-to-aft-RCS configuration
flag will be set to false, and the sequence will be terminated.
A return-to-normal-configuration request by the abort control
sequence will cause the interconnect sequence to configure the
valves sequentially as follows: open aft RCS propellant tank isolation
valves, close the aft RCS crossfeed valves, and close the left
and right OMS crossfeed A and B valves. The OMS-to-aft-RCS interconnect
complete flag will be set to false, and the sequence will be terminated.