View source for Launch Vehicle Digital Computer

[[Image:Coord.jpg|thumb|180px|right|Coordinates of the Iterative Guidance Mode]]
The LVDC++ emulates the function of the Saturn V and Saturn IB Launch Vehicle Digital Computer using C. Project Apollo NASSP uses this emulation to allow the Saturn to be flown with the most accurate guidance model and procedures possible. To activate the LVDC++, the USE_LVDC PLEASE line must be placed under the vehicles parameters in the Orbiter scenario file.  The LVDC++ generates a lvlog.txt file in the Orbiter main directory, that contains the calculations of each major cycle and can be used to as a debugging tool.

== Components ==


'''LVDC'''

The Launch Vehicle Digital Computer (LVDC) is a digital computer mounted on the Saturn rocket’s Instrument Unit (IU). The IU is a ring of instruments located below the SLA, used for Guidance, Navigation and Control of the Saturn Launch Vehicles. It was the LVDC that guided Saturn rockets into and out of Earth orbit, not the AGC. The real LVDC was programmed by IBM, and Project Apollo NASSP’s LVDC++ emulates the function of this computer using the same guidance equations and logic for timing events. The LVDC++ interacts with the Launch Vehicle’s independent IMU (the STM-124-M) to receive velocity and attitude data, computes an optimal trajectory according to its guidance mode, and then issues data in the form of attitude errors to the Flight Control Computer (or FCC). The AGC is sent data as to the LVDC’s commanded attitude and minimal information as to the present phase of flight, but the crew has no means of interfacing with it.

'''Inertial Measuring Unit'''

The ST-124-M is the launch vehicle inertial measurement unit. Much like the IMU of the Command and Lunar Module, the Launch Vehicle’s IMU contains 3 gimbals which measure the rate of attitude change and drive the platform to maintain a stable point of reference. On the gimbals are mounted 3 accelerometers capable of measuring acceleration in the x, y, and z axis. This information is sent to the LVDC++ to enable it to maintain a state vector of the Launch Vehicle’s position. On the ground, it is caged to a theodolite viewed through a small window in the S-IVB. When the IU receives the Guidance Reference Release Interrupt, the IMU is released and begins measuring attitude rate and acceleration.

'''Flight Control Computer'''

The Flight Control Computer (FCC) receives attitude error commands from the LVDC and converts them into control signals to place the vehicle on the proper trajectory. In reality, the FCC was an analog computer and needed a data adapter to convert the LVDC’s digital commands into analog commands. The LVDC++ does not model this stage. The control commands issued to the Saturn are drive commands for the Saturn gimbal actuators and firing commands for the S-IVB’s Auxiliary Propulsion System thrusters. By commanding the outboard engines in unison the stack will pitch and yaw, and opposing sets of outboard engines allows the vehicle to roll. With the single engine S-IVB, an APS is required to control roll rates. In addition to turning the LVDC’s guidance commands into vehicle control commands during powered flight, the FCC utilizes the APS to keep the S-IVB in a heads down LVLH attitude during Orbital Guidance and position the S-IVB to attitude require for different maneuvers. 

== Guidance Modes ==

'''Tilt Sequence'''

The Tilt Sequence is the first mode of guidance that the Saturn family of rockets uses after liftoff. The Saturn follows a preset pattern on three different maneuvers. During the tilt sequence, the LVDC uses the IMU to update the state vector, but it does not factor this state vector into any guidance calculations. The LVDC will blindly fly the Saturn until a few moments after the LET jettison. The reason for the tilt sequence lies in the importance of angle attack while the Saturn V is still in the denser parts of Earth’s atmosphere. Both Saturn vehicles are aerodynamic in exactly the vertical direction. While not modeled, as the Saturns went through Max-Q, too high of an angle of attack (a) could stress the vehicle’s frame and cause a catastrophic breakup. The function of the tilt sequence is to get the Saturn booster out of the densest, and therefore most dangerous, parts of the atmosphere as quickly as possible while minimizing a and building downrange velocity.

The tilt sequence accomplishes its objectives using 3 maneuvers. The first is the Yaw Maneuver between 1 and 8.75 seconds of Timebase 1. The LVDC issues a brief yaw command to prevent a collision by deflecting 1.25 degrees away from the launch tower, before returning to vertical. The second maneuver is the roll program, which is preprogrammed to bring the vehicle’s heading from its pad heading of 90 degrees to its orbital heading, or azimuth. The LVDC understands the launch azimuth as 0 degrees roll and simply commands a roll maneuver to 0 degrees. The third and longest maneuver is the pitch program. The pitch program is defined by the pitch polynomial variable F[x][y] in the scenario table. This four segment polynomial allows the LVDC to generate a commanded pitch attitude based only upon the time since launch. Prior to staging the tilt sequence enters a “tilt arrest” mode. This mode terminates the pitch program to prevent any complications from attitude rates during staging.

The tilt sequence can be monitored a number of ways. First, when the debug line is enabled attitude commands are displayed in the ERR fields. The three numbers represent commanded roll, pitch, and yaw attitudes respectively. The numbers will increase when the LVDC issues a new target, and decrease as the vehicle moves to null the error. Second, the CMC will command the FDAI error and rate needles to allow the astronaut to view the commanded attitude and rate if the ATT SOURCE switch on panel 1 is set to CMC. The launch REFFSMAT will place the launch azimuth at 90 degrees so a completed roll maneuver will place the fight bath along the FDAI centerline. Third, when the a/Pc indicator is set to a, the a indicator will display the Launch vehicle’s angle of attack. Alpha should remain near zero, rising slightly as the Saturn pitches over. Finally, Program 11 displays Speed, H, and H-dot on R1, R2, R3.

{|class="wikitable"
!width="100"|Debug line|| Function
|-
|T1||Time in seconds left in Stage One IGM Guidance
|-
|T2||Time in seconds left in Stage Two IGM Guidance
|-
|T3||Time in seconds left in Stage Tgree IGM Guidance
|-
|ERR||Attitude error in R, P, Y
|-
|eps||Attitude commands to APS in R, P, Y
|-
|V||Current total velocity
|-
|R||Current radius from center of Earth
|-
|ATT||Current orbital attitude
|-
|CMD||Commanded orbital attitude
|}

'''Iterative Guidance Mode'''
[[Image:igm_flow.jpg|thumb|180px|right|Flow Chart for the IGM Loop]]
The Iterative Guidance Mode uses a set of equations to find the cheapest path from the rocket’s present position, as measured by the IMU, to an orbit of 100nm x 100nm. After the LET jettison, the Saturn has cleared the thickest parts of the atmosphere and it is free to safely steer. The IGM consists of 2 stages on the S-IB and 5 on the Saturn V. Stage 1 ends at PU-Shift (explained below), which the LVDC refers to as MRS or Mixture Ratio Shift. Stage 2 ends with SECO on the S-IB, and S-II cutoff on the Saturn V. The Saturn V’s SECO ends Stage 3 IGM Guidance. Stage 4 and 5 control TLI before and after the S-IVB MRS. At the end of Stage 2 on the Saturn V, the LVDC freezes the IGM’s attitude to allow for a safe staging, much like the tilt sequence’s tilt arrest. 

At tower jettison, the LVDC begins a closed guidance loop that calculates a desired velocity vector (xi dot, mu dot,  and zeta dot) by multiplying the desired terminal velocity (V sub t) by the sign or cosign of the terminal flight path angle (gamma sub t). This terminal velocity vector is designated in three axis (xi, mu, zeta). The IGM then compares the state vector’s present velocity vector to the desired velocity, including variables such as thrust and gravitational forces. From this number the LVDC generates a velocity to be gained (delta xi dot, delta mu dot, and delta zeta dot). A pitch steering angle (chi-tilde) can be commanded with the inverse tangent of delta xi dot divided by delta zeta dot. Chi tilde produces a velocity vector as to allow SECO to occur at V sub t and a terminal radius from the Earth (R sub t), placing the S-IVB in a stable parking orbit or lunar transfer orbit. This is referred to as the chi-tilde logic. A more complete explanation of the IGM can be found in the NASA document DESCRIPTION AND PERFORMANCE OF THE SATURN LAUNCH VEHICLE'S NAVIGATION, GUIDANCE, AND CONTROL SYSTEM.[https://www.ibiblio.org/apollo/Documents/tn_d-5869_1970023342.pdf]

The IGM can be monitored most directly through the LVDC++ debug line. T1 indicates the time in seconds from the beginning of the IGM loop through to MRS. T2 is the time in seconds from MRS to S-II staging on the Saturn V, and the time from S-IVB ignition to SECO on the S-IB. T3 indicates time in seconds from S-II staging to S-IVB cutoff. Tt lists the total time in seconds left until S-IVB cutoff. The ERR values serve the same function as during the tilt sequence. When the S-IVB’s APS is active, the eps values display attitude commands sent to the S-IVB’s APS. Finally V and R display the current velocity and radius values, respectively.  The FDAI and P11 serve the same function as in the tilt sequence.

'''Orbital Guidance'''

The Orbital Guidance uses the APS to maneuver the S-IVB to a commanded attitude when LV GUID on panel 2 is set to IU. The S-IVB APS is used during unpowered flight to conserve propellant from the 4 Service Module RCS quads. The orbital guidance defaults to orbital rate while in Earth Orbit, with the command module facing heads down and docking probe forward. This mode can also be programmed to perform specific maneuvers, like the maneuver to separation attitude, depending on mission requirements. Like all other modes, the Orbital Guidance cannot receive input from the Command Module. Orbital guidance can be monitored by the attitude and commanded attitude fields, as well as the three eps commands on the LVDC++ debug line. The rate commanded by the LVDC will be evident both visually and on the FDAI rate needles. To take manual control from the LVDC the LV GUID switch on panel 2 is positioned to CMC and SC CONT on panel 1 is positioned to CMC.

'''Out of Orbit Guidance'''
[[Image:oob_flow.jpg|thumb|180px|right|Flow Chart for TLI Guidance]]
The Out of Orbit Guidance computes a first and second opportunity elliptical transfer orbit to the moon. The LVDC will aim to change the booster’s orbit from a 100nm x 100nm circle to an ellipse that crosses in front of the leading edge of the moon. The LVDC++ is capable of calculating a terminal velocity, terminal radius, and time for S-IVB reignition for a free return trajectory independently of any simulated input from the ground through MFD’s or NASSP’s MCC function. The LVDC then uses the IGM loop to steer the S-IVB through the TLI maneuver. Alternatively the terminal velocity, terminal radius and TIG can be received by the IU from the ground when the UPLK TLM (IU) switch on panel 2 is set to ACCEPT. For NASSP this would mean accepting it from an MFD used to calculate the TLI maneuver.

At 9 minutes 38 seconds before the start of TLI, Timebase 6 begins. This is indicated through the SII SEP lamp illuminating on the LV ENGINES display. At TB6+560 (TIG-18), the LVDC++ will check to see the position of the XLUNAR INJECT switch on Panel 2. When set to inhibit, the sequence is terminated. At TB6+570 (TIG-8), the S-IVB ullage motors fire. At TB6+577 (TIG-1) the engine 1 light will illuminate, and at TB6+578, it will extinguish signaling TLI. V and R can be monitored through the debug line. In reality, astronauts used the EMS dV counter and CMC Program 47 to monitor delta-V.


== Saturn V Sequence of Events ==

The flight of the Saturn rockets required some events to happen in a timed sequence. However, not everything can accurately be timed from engine ignition. If an event was sensed by faulty equipment it could result in a catastrophic vehicle failure, such as the S-II igniting shortly after liftoff because it “thought” enough time had passed from an S-IC separation that never actually occurred. To alleviate these issues the LVDC operates in a series of Timebases. Each one is triggered by the previous Timebase and allows certain events to be armed and executed in reference to the current Timebase. The Saturn V and Saturn I-B have slight variations in their respective flight plans, and the Saturn IB variations are indicated below.

'''PTL/GRR Interrupt'''
[[Image:SaturnV_on_pad.jpg|thumb|180px|right|The Saturn V awaiting PTL Interrupt]]
While the Saturn is sitting on the pad the launch vehicle is oriented so that the hatch window is pointing directly east, with the launch vehicle IMU stabilized to a theodolite. At T-20 minutes, the LVDC receives the Prepare to Launch interrupt (PTL interrupt). After this point it begins monitoring the caged IMU and PIPAs for any deviations. The LVDC waits for the Guidance Reference Release interrupt (GRR interrupt) to release the IMU. At T-17 seconds the GRR interrupt is issued placing the vehicle on its own internal guidance and the IMU begins measuring the vehicle’s rate and acceleration in preparation for ignition. This begins Timebase 0. Timebase 0 controls the ignition sequence of the launch vehicle. A TB0+7.1 the center engine (or engine cluster of the S-IB) ignites. A quarter second after, two diagonally opposed outboard engines ignite, with the remaining two igniting a quarter second later. This prevents unnecessary stress from all engines starting simultaneously. 

'''Timebase 1'''

Timebase 1 begins at T-0 and includes launch and the tilt sequence guidance mode. The LVDC++ simulates the “soft release” of the Saturn V by imparting extra drag the first second of TB1. In reality, the soft release was controlled by pins on the bottom of the S-IC that slowed the vehicle’s ability to accelerate for the first second of flight. This prevented the Saturn from launching without healthy engines. The LVDC begins the tilt sequence immediately after liftoff. At 4g acceleration, TB1 commands center engine cutoff (CECO), which signals the beginning of Timebase 2. Because the Saturn is difficult to control during this stage of flight, time acceleration is not permitted by the LVDC++.

'''Timebase 2'''

Timebase 2 begins when the center engine (or engine cluster of the S-IB) is shut down by TB1. Timebase 2 provides the tilt arrest in preparation for a safe staging and monitors vehicle acceleration to determine when the outboard engines should be cutoff. At 4g acceleration, the LVDC commands outboard engine cutoff. This signals the end of Timebase 2.

'''Timebase 3'''
[[Image:Screenshot_SaturnV_First_Staging_1.jpg|thumb|180px|right|Beginning of Timebase 3]]
Timebase 3 begins after first stage outboard engine cutoff and controls the burning of the S-II on the Saturn V and the S-IVB on the Saturn I-B. Timebase 3 triggers first stage separation and fires the second stage ullage motors. A 3 second delay allows for the second stage to gain sufficient distance from the spent first stage before ignition between TB3+2.5 and TB3+4.5. On the Saturn V, the SII SEP light will illuminate to signify the beginning of TB3. Also specific to the Saturn V, the interstage plane or “skirt” will separate at TB3+30. This will cause the SII SEP light to extinguish. For both Saturn vehicles, Launch Escape Tower jettison will occur at TB3+34. After LET jettison, the LVDC++ begins Iterative Guidance Mode.

T1 and T2 are timing variable used to distinguish the multiple IGM guidance stages of Timebase 3. At T1, the first stage of the Iterative Guidance mode ends and commands a shift to take place in the fuel/oxidizer ratio of the burning engines. This shift is referred to as the Mixture Ratio Shift (MRS) or Propellant Utilization Shift (PU-Shift). It was designed to ensure that both the fuel and oxidizer tanks were emptied simultaneously, preventing any propellant from going to waste. The PU-Shift can be detected by a slight drop in the Accel G meter on Panel 1, along with the IGM maneuvering to compensate for the change in the thrust produced by the new mixture. A second measure used by the S-II to ensure complete depletion of propellants was the propellant level sensors arming (level sense arm). The level sensors are emulated by Orbiter reporting the fuel quantity to the LVDC++. A quantity of 15% will trigger center engine cutoff, and 0% will trigger outboard engine cutoff and the beginning of Timebase 4. At T2-11 (or chi-freeze gate), the IGM’s chi-tilde logic is frozen briefly to allow for staging. Timebase 4 occurs in unison with the T2 variable, ending the second stage of IGM Guidance.

On the S-IB, TW will trigger SECO and Timebase 4. TB4 operates similarly to the Saturn V’s Timebase 5. The orbital guidance of the S-IB does not include a post-insertion ullage burn. After about 1 orbit, Apollo 7’s S-IVB vents it’s remaining Liquid Oxygen.

'''Timebase 4'''

Time base 4 begins with S-II outboard engine cutoff, which triggers the separation of the S-II stage and fires the S-IVB ullage motors. At TB4+2, the S-IVB has gained sufficient distance from the S-II and ignites. After ignition the chi-freeze is disabled and IGM steering resumes. At TB4+8, the LVDC begins issuing commands to the S-IVB’s APS for attitude control. At T3, the LVDC shuts off the S-IVB engine, beginning Timebase 5.

'''Timebase 5'''

Timebase 5 begins at T3 with S-IVB cutoff when the Saturn is in a stable orbit. Immediately following cutoff the S-IVB ullage motors are burned for 1 minute and 27 seconds until TB5+87. At TB5+100 the LVDC changes to orbital guidance mode and uses the APS to hold the S-IVB in an orbital rate attitude of with heads down, and docking probe forward. The 100nm x 100nm parking orbit is still in the upper edge of the Earth’s atmosphere, and placing the S-IVB with its most aerodynamic edge forwards reduces micro drag and increases the stability of the S-IVB’s orbit. The liquid oxygen and liquid hydrogen used as propellant for the S-IVB is heated by sunlight hitting the spacecraft’s exterior, causing it to “boil off.” The LVDC++ simulates this at 10 second intervals. Timebase 5 ends at TLI-9:38.

'''Timebase 6'''

Timebase 6 begins at 9 minutes 38 seconds prior to TLI ignition. The SII SEP light is briefly illuminated to announce the beginning of TB6. The SII SEP light extinguishes at TLI-9 minutes, to allow the crew to coordinate the use of the Digital Event Timer with the LVDC. At TB6+560, (TLI-18s) the IU checks the position of the XLUNAR INJ switch on Panel 2. If it is set to INHBT the TLI maneuver is aborted. At TB6+570 (TLI-8s), the ullage thrusters on the S-IVB fire to settle the propellants at the end of the booster. LV ENGINE lt 1 is illuminated at TB6+577, to announce the engine will fire in 1 second. TLI engine ignition and IGM fourth stage guidance begin at TB6+578. At TB6+588 (TLI+10s) the LVDC commands another MRS for the S-IVB, beginning stage five IGM guidance. When the IGM’s targeted velocity vector has been achieved, the S-IVB engine is shutdown and Timebase 7 is triggered. Timebase 7 will maneuver the booster to CSM/LV separation attitude and execute the evasive maneuver both according to the mission Flight Plan.