Orbital Maneuvering System Dump

When the shuttle is launched, it carries OMS propellant to perform the shuttle mission and then to return to Earth. This load of OMS propellant, which can be as much as 24,000 pounds, is stored in tanks located at the back end of the orbiter. This causes the shuttle center of gravity (e.g.) to be farther aft of the point where the shuttle is designed to fly. For normal end-of-mission entry this situation is not a problem because most of the OMS propellant is consumed. For RTLS, however, this...

GRTLS Guidance

As discussed in the previous section, the one fundamental task of guidance during GRTLS is to arrive at the runway with the right amount of energy to complete a safe landing. To manage the energy state while providing satisfactory flight conditions, GRTLS guidance can be divided into the following sections The first three of these phases are designed to provide the transition from an exoatmospheric ballistic trajectory to hypersonic gliding flight. S-turn and TAEM are where most of the energy...

RTLS Selection

Although it may be apparent that an RTLS is necessary when an SSME fails, the actual time that the RTLS abort is declared is dependent on several factors. RTLS aborts for performance losses are generally performed at T + 2m30s. For example, if an engine fails during first stage, the RTLS abort is delayed until T + 2m30s. The earliest RTLS selection is made at 2m30s because flight control is limiting the vehicle response before 2m30s. Therefore, an abort could be initiated before 2m30s, but the...

Software Major Modes

The space shuttle is controlled by five computers usually referred to as general purpose computers (GPCs). The programs that these computers run are divided up into several sections. Each section is called an operational sequence (OPS) and is tailored to a particular flight phase. For example, OPS 1 is software used during the ascent phase, and OPS 3 is used for entry. The OPS that controls RTLS is called OPS 6. Each OPS is subdivided into smaller sections called major modes (MMs), which are...

Ascent Overview

The space shuttle ascent phase begins at launch with the orbiter, the external tank (ET), and two solid rocket boosters (SRBs). First, the three space shuttle main engines (SSMEs) in the aft end of the orbiter are ignited atT - 6.8 seconds. Ifthe SSMEs have reached 90 percent of their rated thrust, the two SRBs ignite, and liftoff occurs. The early part of the ascent phase in which the main engines and SRBs provide thrust is sometimes referred to as first stage. After about 2 minutes, the SRBs...

Powered Pitchdown

Once the fine countdown phase is initiated, PRTLS guidance has virtually completed its task. The only thing to be done is to pick the MECO time. PEG 5 predicts the time that MECO should occur, which is still many seconds in the future, and then fixes this time. Once set, the MECO time does not change even if subsequent events make this time less than optimum. In order to satisfy the altitude and flightpath angle constraints on the MECO state, the shuttle has a positive angle of attack of about...

Main Engine Cutoff and External Tank Separation

Once the shuttle vehicle has stabilized at -4 angle of attack, the next event is MECO. The time for MECO has already been fixed so the shuttle simply holds alpha -4 until the MECO timer expires. The shuttle then enters a phase called mated coast. Mated coast is the time after MECO but before ET SEP. The shuttle engines can no longer be used for attitude control since they are shut down. The aerodynamic surfaces are not yet effective enough because of the thin atmosphere. This means the entire...

Main Propulsion System Safing and Rtls Init

Some of the information that PEG 5 guidance needs in order to do its job is based on events occurring before the RTLS abort is declared. Specifically, PEG 5 needs to know the shuttle speed at the time of the engine failure. Therefore, a software module called the SSME-out safing task is called whenever an engine fails. This module performs several tasks including saving the shuttle Earth-relative velocity at the time of the engine failure so that it will be available for subsequent use. As soon...

GRTLS Displays

Space Shuttle Horiz Sit

The primary display for the early portion of GRTLS is vertical VERT situation SIT 1, which is the OPS display in MM 602. This display, shown in figure 3-2, is primarily a plot of altitude versus range to go. The three lines , and running from the upper right to the lower left, depict representative trajectories is the nominal trajectory while the leftmost line is the q limit line. Flying to the left of this line will overstress the vehicle. The rightmost line CD isthe maximum L D line....

RTLS Overview

If an engine fails during the first 4 minutes of ascent, the shuttle cannot achieve orbit. For the first 3 minutes or so of ascent, it cannot even reach a TAL. The only runway near enough to be reached is one near the launch site. In order to reach this runway, the shuttle must literally reverse course and fly back the way it came. The turn to reverse course is called powered pitcharound PPA , and the timing of PPA is critically important. Since the orbiter is powerless once the main engines...

Glide Range and Energy

After MECO, the shuttle has a certain amount of energy that determines how far and how long it can glide. This amount of energy is a function of the speed and altitude of the shuttle. The initial energy state, determined by the speed and altitude at the time the SSMEs are shut down, decreases continually during GRTLS, reaching zero at orbiter wheels stop. The horizontal distance or range that the shuttle can glide with a given amount of energy is determined by the vehicle's lift and drag...

Rtls Traj Display

The shuttle cannot fly at all without its computers. This means that if a serious problem disables all the PASS computers, the crew has no choice but to engage the BFS. However, it is possible that some combination of software errors and or hardware failures might cause the otherwise healthy PASS computers to be unable to fly the shuttle successfully. To provide an alternative to engaging the BFS, the capability has been provided for crewmembersto fly the shuttle during ascent. The manual...

Backup Flight System

The BFS is an independent software system which normally resides in the fifth GPC GPC 5 . If any failure or combination of failures renders the PASS computers unable to control the shuttle, the crew engages the BFS. Once engaged, the BFS assumes sole control of the shuttle vehicle and remains in control until after landing or until the problem with the PASS computers is found and fixed on orbit. Both the commander CDR and pilot PLT can engage the BFS by depressing the pushbutton PB on top of...