Thor Missile Guidance: What, Where and How

[P Bellamy]

I've briefly mentioned the various constructions around each Thor site relating to aiming the missile, so now it's time to bring it all together and show how they were used.

As a reminder, the main points on the ground are as follows:

Two concrete trig-points surveyed accurately off the OS Grid, to provide a known baseline for each Squadron site from which the positions of the next items were plotted.

Theodolite Reference Target Pillars, placed in various locations within each site.


(Photo: REF)

Launch Mount Bolt, indicating the LE centreline and the missile centreline when in the raised position.



SRT Platform Bolt, indicating the centre of the SRT platform rail, a known offset from the LE centreline and level with the missile guidance pack along it when in the lowered position.



LRT Platform, on the LE centreline.

[P Bellamy]

Some knowledge of the Thor missile guidance system is needed to understand how these structures on the ground were used, so here's the relevant section from the Bomber Command Strategic Missile School training notes. Sorry, it's the simplest version I've found:

Inertial Guidance System

Introduction

1. The Inertial Guidance System is the "Navigator" of the Thor, as it has the ability to correct errors in position by comparing a set of pre-calculated figures with others freshly measured during flight. It sends the required corrections in the form of steering signals to the Flight Control System, which is simulating the functions of a "Pilot".

2. The Inertial Guidance System is entirely self contained when in flight, and so interference with its functions, in the form of attempted "jamming", is impossible.

3. After 108.5 seconds of flight the Guidance system takes over control of the missile and when it has calculated that the missile has achieved a velocity which will enable the nose cone to follow a ballistic trajectory to the target, the following signals are produced, in sequence:-
(a) The Pitch Steering signal.
(b) The Yaw Steering signal.
(c) Main Engine Cut-off (and Nose Cone seperation enable)
(d) Vernier Engine Cut-off.
(e) Pre-arm of Warhead in Nose Cone.

4. Missile Axes
(a) The Missile Rolls about its X axis (Longitudinal axis)
(b) The Missile Yaws about its Y axis (Vertical axis)
(c) The Missile Pitches about its Z axis (Horizontal axis)

5. The Gyro Stabilised Platform

The Stable platform is held space referenced during flight by the action of three stabilisation gyros, and an associated servo system. Mounted on it are three accelerometers, which measure individual accelerations along three mutually perpendicular axes.
The X and Y axes of the stable platform are orientated differently to the gimbal axes by 52 degrees, measured anti-clockwise about the pitch gimbal axis. This ensures that the accuracy of the accelerometer is greatest when close to the mean value of the angle of pitch of the missile (Between 40°and 70°).

6. Guidance System Alignment Modes

A mode is a condition of the guidance system. The platform is aligned by each mode in sequence to a progressively increased degree of accuracy.
The mode sequence is:-

(a) Off
The only power entering the guidance system is 115V, 400c/s for the gyro heaters. This will maintain the flourolube at its correct viscosity.

(b) Standby
Filament power is applied to all amplifier circuits.
Gyro heater power continues to be applied during this mode, and it is required to be provided until lift-off.
The system will normally remain in in this mode at all times on the Launch Emplacement, but should it be switched "off" for servicing, it must not be advanced into "power on" mode until it has been at least 30 minutes in "standby". If calibration or targeting are required after servicing, the guidance system must remain in "standby" for at least two hours before advancing to "power on".
The missile may be raised or lowered with the guidance system in "standby" condition, provided that no gyro wheel power has been applied in the fifteen minutes immediately preceding movement of the missile.

(c) Power On
An HT supply is applied to all amplifiers, which are now "live" and are available for use.
The Guidance Generator (PU-411) and the Airborne Amplidyne (PU-424) are started from a low voltage DC supply, and these will supply reference power and gimbal torque motor power, respectively.
All voltages may be checked on the digital voltmeter of the MGAC, by turning the selector switch, and the nominal 400c/s of the PU-411, may be checked on the EPUT meter (EPUT = Events Per Unit Time).
Stabilisation gyro float nulling commences, and this can be monitored on the stabilisation gyro float nulling meter, by using the selector switch.
In the "normal" mode of operation, the gimbal position readouts may be "slewed". This gives a check on the operation of the readouts and the associated circuitry.
In the "manual" sub-mode of operation, the repeaters assume the position of their respective gimbals.

(d) Align One
In the "manual" mode, the gimbals may be set to their required positions by manual operation of the slew switches, whilst observing the gimbal position readouts and the gimbal position nullmeters.
The Roll and Pitch gimbals should be set up first, as only azimuth movement of the yaw gimbal is required to "capture" the Short Range Theodolite. Once a signal from the SRT has been obtained, ensure that the gimbal is adjusted to coincide with the "True Null" signal from the SRT. This may be done by checking that the gimbal position nullmeter deflects to the right and left of a centre null, when the manual gimbal handset control is turned very slowly in the same direction.
In "normal" mode, power is applied to the gyro spin motors (wheel power).
In addition, the full facilities of the "manual" sub-mode are still available, but no attempt should be made to slew the gimbals, as there is now a risk of damage being done to the stabilisation gyros.

(e) Align Two
The normal mode provides a coarse automatic alignment of the gimbals. This is achieved by the electrical output signals from the SRT and the vertical sensing element (VSE), which are amplified and can then control the position of the gimbals. As long as the gimbal remains in its correct alignment, no electrical error signals are produced, and no corrective signals are sent to the gimbal torque motors.
The VSE is a form of electrical "plumb line", and its output signals will control the roll and pitch gimbals during horizontal alignment.

Align Two manual mode = Align One normal mode.

(f) Align Three
This mode provides fine electrical alignment of the gimbal positions. The stabilisation gyros enter the gimbal alignment circuit in this mode, and this introduces two major errors:-
(i) Apparent Gyro Precession due to Earth's rotation.
(ii) A large difference in relative axes between the X and Y stabilisation gyros on the stable platform and the gimbals.

To cover error (i) The erection integrators are so arranged as to compensate the position of the gyro stabilised platform for errors due to Earth's rotation. This it does by eliminating all constant errors from the gimbal control circuit, including those due to inaccuracy in manufacture.
Error (ii) is nulled by the resolvers which are referenced to the pitch axis and correct for the relative angular displacement of the gyro axes (and gimbal axes), about the pitch axis.

(g) Complete
Before manually advancing the guidance system to this mode, the following must be checked:-
(i) Gimbal position nullmeters at centre of green band.
(ii) Stabilisation gyro nullmeters at centre of the green band.
(iii) Erector integrator potentiometer readouts steady.

The stabilisation gyros are in control of the platform gimbals, and the VSE and SRT signals go to the nullmeters for monitoring purposes.
The missile can be erected or lowered when guidance is in the "Complete Horizontal" mode.
The "Guidance Complete" signal is also produced in this mode, and this is normally required for the countdown to complete Phase One.

(h) In Flight
After lift-off, the airborne stabilisation circuits form closed servo circuits, and are entirely independent of any external control.
The stabilisation gyro compensation voltage is applied in order to correct errors due to manufacturing inaccuracies.

7. Preparation for a Countdown

If the missile is in the "ready" condition, the following will have already been checked, and all that is required is for the LCO to turn the launch sequence start key.

Long Range Theodolite Preparation
(a) Switch "On".
(b) Centre assembly on LRT bench mark.
(c) Check azimuth and elevation readings with required values.
(d) remove lens covers, and open LRT Hut windows.

Short Range Theodolite Preparation
(a) Check that the correct azimuth angle is set into each SRT, as specified in the target data sheet.
(b) Check that the visual optical system is aligned with the collimator, and locked.
(c) Check that the SRTs are mechanically level (Bubble).
(d) Check cables are generally secure.

[P Bellamy]

To actually set the bearing of the intended target into the guidance system, the Electrotheodolites were used.
(Warning: Geometry and Maths ahead...! )

The Short and Long Range Theodolites

One factor in the orientation of the gyro stabilised platform normally changes for individual targets, the alignment of the platform X axis with the required "aim point".
With the missile horizontal, X axis alignment will be determined by the azimuth setting of the yaw gimbal.
However, with the missile vertical, X axis alignment relies on roll gimbal alignment.
The SRT has the job of monitoring the azimuth alignment of the yaw gimbal when the missile is horizontal, and the LRT monitors the azimuth alignment of the roll gimbal when the missile is vertical.

The Short Range Theodolite - Operation

The monitoring optical system consists of two glow tubes which send out parallel beams of light toward a mirror which is mounted on the yaw gimbal.
If the yaw gimbal is in the correct position, the light beams will be reflected straight back to the glow tubes. This condition in known as "True Null".

Positioned behind a narrow slit in a silvered prism is a photo-electric cell, a device which, if it detects light, will produce an electrical error signal.
This photocell will receive light if the yaw gimbal is not in the correct position, because one or the other of the two light beams will be deflected by the yaw gimbal mirror, and will pass through the slit in the prism.
The more light that falls on the photocell, the greater the electrical error output and so the electrical error signal in proportion to the amount of error existing. This circuit can also tell in which direction the error lies.
This error signal can be used to indicate the amount and direction of error in alignment on the yaw gimbal nullmeter and, in addition, to actually drive the yaw gimbal back to its correct alignment angle, should an error occur.

The Short Range Theodolite - Physical Characteristics

The SRT consists of two assemblies:-

(a) The monitoring optical system contains the glow tubes and the photo-electric cell, with their associated circuitry. A bubble level enables the SRT to be mechanically levelled prior to operation.
(b) The visual optical system, a standard telescope which has been modified so that it can project a beam of orange light, if required.

Both systems can be rotated independently about a vertical axis, and an illuminated scale indicates the angular difference between their azimuths.



The SRT is mounted at a pre-determined position on a rail which extends parallel to the missile and about ten feet from the stable platform assembly. At each end of the rail there is a collimator, which supplies a source of green light, upon which a set of crosshairs is superimposed.

An angle X is calculated, and is set in between the optical monitoring and the visual optical systems. Prior to initial calculation, the SRT "Zero angle" must be determined, by use of the zeroing mirror. The "Zero angle" is the angle indicated on the SRT scale when there is no azimuth error between the two optical systems.
It is of different value for each SRT.

The whole assembly is then rotated until the visual optical system is accurately aligned with the collimator and is therefore parallel with the missile.
The light beams from the monitoring optical system are therefore being emitted towards the gyro stabilised platform, and the yaw gimbal should now be moved in azimuth until the SRT is "captured" (i.e. a condition of zero error exists).

SRT ... Calculation of Angle X

1. Angle A is given. This is the angle between true north and a line extending to the aim point.
2. Angle B is given. This is the angle between true north and a surveyed reference point.
3. Angle C must be measured. This is the angle between the reference point and the collimator, measured in a clockwise direction.
4. The equation for calculating the angle is:- Angle X = Angle A + 270 degrees - (Angle B + Angle C)

The illustration below shows the SRT Geometry for a zero-deflection shoot:

1. The angle between the light beam and the Aim Point is always 90°
2. Angle A is given on the Target Data Sheet.
3. Angle B is given on the Target Data Sheet.
4. Angle C is measured.
5. Angle D equals Angle B plus Angle C.
6. Angle E is computed.
7. Angle E equals Angle A plus 270° minus Angle D.



Using the southern LE at Harrington as an example, and a fictional target pillar, the angles are:
A = 75°
B = 90°
C = 165°
D = B+C = 255°
E = A+270-D = 90°

[P Bellamy]

The next illustration shows the geometry for an aiming point to the left of the LE centreline, the (somewhat exagerrated) missile centreline offset indicates where the guidance system would want to move it to in flight.
To ensure the angle between the SRT and the guidance system remains constant, the SRT has to be slid along the rail downrange.
The further the aiming point is to the left of the centreline, the further the SRT would have to be moved, and angle E will decrease accordingly.


Each RAF Thor had two pre-determined targets.
To facilitate this two SRTs with their own collimator were installed on each SRT Platform.

[P Bellamy]

The Long Range Theodolite

Purpose To monitor the alignment of the roll gimbal, when the missile is vertical, and to provide correcting signals if an error develops in gimbal alignment.

Operation
(a) The Shutter Circuit
This circuit monitors the alignment of the porro prism, and will produce electrical error signals in order to correct any deviation.
(b) The Translation Circuit
This circuit cancels out the effect of missile sway, by producing a lateral movement of the LRT Assembly, as required.

1. A equals angle between true north and platform azimuth (given)
2. B equals angle between geodetic azimuth or reference point and true north (given)
3. C equals angle between geodetic azimuth of reference line and the edge of mirror (measured)
4. E equals mirror angle



SRT and LRT Summary

1. There are two SRTs, one for each target.
2. The LRT emits its light beams in the same direction for all targets on any given launch pad. Variations in target azimuth are represented by a porro prism, mounted on the roll gimbal.
3. The Launch Countdown can proceed without an SRT, by selection of "Auxiliary Azimuth" operation. Any errors on the yaw gimbal will be corrected by the LRT when the missile assumes a vertical attitude.
4. If the LRT is unserviceable, a vertical hold of the guidance system may still be effected by use of the "Airframe Azimuth Reference" circuit. However, use of this circuit requires that the SRT was fully serviceable prior to the erection of the missile.
5. Pressing the "Airframe Az. Ref." button locks the roll repeater, and effectively holds the roll gimbal in the setting given to the yaw gimbal by the SRT.

Well, that's pretty much it as regards how to point a nuclear missile in the general direction of the USSR.
Apologies for length, but I hope it gives the reader a better understanding of what some of these rusty bolts and crumbling concrete slabs at the remaining Thor sites were originally there for.

All the best,
PB

[PETERTHEEATER]

Very comprehensive Paul and very well illustrated; you have been busy.

I did have some idea of the guidance system and methods of alignment but did not know that the missile was part of the 'set-up' but then on-board computers with target programming were in their infancy.

Do you know which tradesmen (RAF) were responsible for 'targetting'?

Presumably, the RAF was obliged to follow USAF Technical Orders to the letter use the same checklists?

[Ossington_2008]

Excellent stuff Paul, on the limits of my comprehension, but excellent. Could it have been launched in very high winds?

[YellowPinkie]

Bloody hell. Some good research there, Paul. When is the practical demonstration?

[PNK]

I need to print that lot out and read it in a quite room.I also echo Denis' comment on another thread - where do you find the time? I have only got one project on the go and finding it difficult to assign any reasonable amount of time to it.

Perhaps this could be part of a future AiX/ARG book on the Cold War era?

[Carnaby]

I need to print that lot out and read it in a quite room

My thoughts too. Great stuff. Often wondered about all this. Now I need some time to digest.