Saturn V - Apollo   Flight Envelope

Picture illustrating the meaning of the parameters
The Flight path angle and the Velocity are two of the most significant performance parameters to monitor the flight trajectory of a launch vehicle.
Selection of performance parameters to monitor the flight trajectory
For a proper flight control during the ascent phase it is important to be able to detect deviations from the planned nominal flight trajectory in real time. For that purpose a strategic selection of the four most relevant performance parameters has been made. Deviations of these four paramenters also reflect deviations from nominal values of the other six performance parameters mentioned in section "Performance characteristics".
    These four parameters are:
  1. Altitude
  2. Downrange distance
  3. Flight path angle
  4. Space fixed velocity
For a quick and efficient assessment by the ground crew (Mission Control) during flight, the selected four parameters are plotted in:
  1. An Altitude versus Downrange distance chart
  2. A Flight path angle versus Space fixed velocity chart
Flight envelope
These charts contain plots representing the nominal flight trajectory and its envelope reflecting the deviations which are allowed to have an (contingency) orbit attained. During the flight the real time values of the performance parameters are plotted in these two charts on screens or plotboards. If the real time plots cross the envelopes, an orbit cannot be attained and the crew safety is at danger, an abort procedure will therefore be initiated.

The envelopes shown below are the result of simulation studies which were carried out in 1967 and 1968 by the TRW Systems Group by using the TRW Saturn Launch Simulating Program. In these studies careful selected most critical malfunctions, with regard to thrust control, engine failures and guidance, have been simulated. All these malfunctions will result in a slowly developing deviation from the nominal flight trajectory. These simulations have shown to which extend these critical malfunctions are allowed till the ability to attain an orbit has been lost.

A discussion about these simulation studies can be found on this page.

Altitude - Downrange distance plot
This is the first display of two used by Flight Dynamics officers to assess the ascent flight trajectory of the Saturn V launch vehicle. It enables them to react in time on impending abort situation and inform the flight crew accordingly.

Flight path angle - Velocity plot
The chart at the top is the second display of two used by Flight Dynamics officers to assess the ascent flight trajectory of the Saturn V launch vehicle. It enables them to react in time on impending abort situation and inform the flight crew accordingly.

The plot starts at a point in which the inertial flight path angle is zero and the velocity is non zero. That is because space has been chosen as the frame of reference, that implies that Earth's rotation is taken into account. So the launch vehicle has already a horizontal space fixed velocity of about 410 meters per second due East when it is sitting on the launch pad in Florida. When the launch vehicle is ascending the vertical component of the velocity is increasing and therefore the inertial flight path angle is increasing. But at the same time the launch vehicle is starting to pitch over to gradually maneuver into an orbit in which it is aligned along the local horizon (flight path angle is zero.). So the flight path angle increases at lift-off, will reach a maximum at which the vertical component of the velocity will reach a maximum and will then taper off to zero when an orbit has been obtained.

In this picture is also shown which abort procedures are available in time during the ascent. Four abort modes are distinguished each corresponding to the three flight regimes: atmospheric, transitional or sub orbital and space. In mode I three sub modes are distinguished reflecting the three modes of operations of the Launch Escape System depending on the altitude and velocity of the launch vehicle.

In the last minutes of the ascent, near orbit insertion, the abort mode decision process is somewhat complicated. If the actual trajectory is then deviating too much from the nominal trajectory, the decision whether to start a reentry or to attain a contingency orbit are depending on rapidly changing abort cue conditions. In general, abort mode IV, the contingency orbit, is preferred. It provides the capability to meet some of the mission objectives and gives ground control and the flight crew time to select proper landing areas and make preparations for reentry.

Abort mode III, in contrary, is a rather hectic and most complicated procedure. Its objective is to have a landing in the Atlantic Ocean. But in this phase of the ascent the spacecraft has attained near orbital altitude and near orbital velocity and it is approaching the African coast rapidly.
In contrary to the abort modes I and II the spacecraft has to be actively guided and contolled to meet the proper reentry trajectory conditions. Without guidance the landing footprint is unacceptably large and the spacecraft might follow a high G-load and high heat load reentry trajectory which will endanger the crew.

If it is to land in the Atlantic Ocean to avoid a high risk land landing on African soil, then in a very short period of time flight data has to be uploaded from Misson Control via ground based or sea based tracking stations into the onboard computer of the spacecraft as input for its reentry software. These flight data basically contain data on the spacecraft's postion, velocity, acceleration and the location of the selected landing area. In the same short period of time, after the reentry data has been uploaded, the flight crew has to initiate a propulsion maneuver with the Service Propulsion System to decrease the velocity, has to separate the command module from the service module and has to initiate the reentry procedure.

The reentry software is controlling the roll angle of the spacecraft during its reentry. The Apollo command module is deliberately designed to have a center of gravity which doesn't co´ncide with its geometric center. By controlling the roll angle the geometric center can be put above or below the center of gravity which results in changing pitch angles. Changing the pitch angel implies changing the vertical angle between the surface of the heat shield and the drag force because of which an upwards or a downwards lift can be created. This mechanism enables the software by employing the Reaction Control System (RCS) to control the descent trajectory to have a controlled landing in a pre-selected landing area.

This ability to create upward and downward lift by controlling the spacecraft's roll angle is also used during a mode II abort. But in that phase of the ascent the landing footprint and its location is not much of an issue, so attitude control is not used for trajectory control but just to minimize the G-load on the spacecraft during descent. Therefore the spacecraft is fixed in a "maximum upward lift" attitude because of which reentry software doesn't require flight data from ground based stations like in abort mode III.

In the bottom chart is shown the altitude, the downrange distance and the 3 different flight regimes (atmospheric flight, transitional flight, space flight) which have been distinguished in the ascent phase.




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Copyright 2013 by   Sander Panhuyzen
Comments and questions are welcome. All pictures and drawings contained on and through these pages are the author's, unless otherwise noted. No unauthorized reproduction without permission.