NASA's Shuttle and Orbiter - Background
The Shuttle, officially a Space Transportation System, (STS), is composed of a large
external fuel tank to which Orbiter and two solid fuel boosters are attached with
pyrotechnic fastenings. See image that follows:
At lift-off it is powered by the two boosters and by the three main engines of Orbiter.
A cut-away view of Orbiter showing only two of the three engines is shown next:
The nozzles of the three main engines and those of the two boosters are adjustable
and are controlled by the onboard computer to adjust the flight path.
There are many links on the net from which specifications and other information
for the Shuttle and its components can be obtained. Example links are:
Space Station - ISS
The altitude of the ISS ranges from a minimum of about
278 km to a maximum of about 460 km. The normal maximum is about 425 km. Atmospheric
resistance causes it to drop at a rate of ~ 2.5 km per month. Shuttle missions
are believed to often nudge it back up into orbit a bit when docked and when there
is OMS fuel to spare.
Our Choice of an ISS Orbit
We require a numerical description
of the orbit that is to be achieved. We chose an orbit with an apogee of ~304 km, a perigee ~300 km and an inclination of ~51.6 degrees.
Keplerian Orbital Elements
Keplerian orbital elements were explained in Topic
1 of Chapter 7. They can be used, when examining the trajectory of Orbiter,
to characterize its motion in terms of instantaneous Keplerian orbital
The term instantaneous is used because the object is often
under powered flight and the orbital elements are continually changing. Adjusting
the direction, duration and, at times the magnitude, of the thrust during powered flight is used
to arrive at the desired orbit with the required orbital parameters.
Our database for Orbiter's parameters
at MECO was described in the final topic of Chapter 8. There the Latitude
is given as 37.173, Longitude as -68.991, Altitude as 104,653.08 the Velocity as
7869.02 and the Azimuth of the velocity as 51.22.
The Elevation of the Velocity was unknown but likely to be near zero. We chose 0.1
This Elevation was chosen simply because it was unlikely to be exactly zero.
The mass of an Orbiter varies from mission to mission.
We chose 104,328 kg.
Not being aware of Orbiter's drag we guessed 1.0 as the value of the drag coefficient
and estimated Orbiter's sphere equivalent radius as 3.5 metres.
A step size of 0.01 seconds was chosen
for the integration steps. Order 2 integration was employed.
The foregoing parameters employed in the atmospheric model spreadsheet led to the
altitude behaviour of Orbiter from MECO onward, in the absence
of further thrusting, as is seen next:
Without further thrusting Orbiter would crash to Earth.
After reaching MECO and dropping the external fuel tank, Orbiter is left with two other sets
of engines, the first referred to as OMS, the Orbital Maneuvering System consisting
of two aft engines, the second as RCS for the Reaction Control System consisting of 44
One of the two OMS engines can be seen in the second image of this topic. The
are used for the heavy work of getting into the desired orbit, meeting up with the
ISS and for de-orbiting.
The RCS is used
for more delicate maneuvers such as are needed in changing Orbiter's
orientation and in docking.
An Astronaut's Navigation Interface
An astronaut in earth orbit can look out the widow and see the earth. The visible
size of the earth and its position in the window provides clues to the altitude
of the orbit and to the orientation of his vehicle with respect to the tangent plane.
He may see sufficient surface features to identify a northerly direction and roughly
the azimuth of his direction of his travel. If the earth appeared to receding
over time then he would deduce that his velocity had an elevation component.
The foregoing considerations strongly suggest the view that the writers have taken,
that commands to the Course Computer for control of the engines should call for
thrusts directed at azimuth and elevation angles that are with respect to the current
Modeling the OMS Engines and
When an Input Table is added to the atmospheric model spreadsheet, its object is
acted on further by the parameters of that table. The table could be thought
of as the equivalent of Shuttle's on-board computer in providing direction to its thrust engines.
Although an example application of the table is shown following, more is provided
in this and the next topic about determining the values for its entries.
See a few rows of such a table next:
Row two contains the initial values for t=0, Cd as 1.0,
radius as 3.5 and mass as 104,328.
In columns C and D, starting in Row three, there are Elevation
and Azimuth values. These do not refer to the angles of a rocket's thrusters
relative to its orientation but rather they are course or heading controls where the
angles are with respect to the tangent plane and are taken as the heading in which
the corresponding force is to be applied.
We imagine that our rocket is
equipped with a "State Computer" that employs an inertial guidance system plus a GPS-like system, that may include wireless signals from ground stations and from the ISS,
so as to have good
estimates of its Cartesian position and velocities at all times.
The State Computer provides the evolving
current state to the Course Computer, which
in turn controls the angles of the rocket's thrusters so as to maintain a desired course.
Force and heading instructions remain in effect until changed
by a subsequent line of the input table. Heading is not controlled in an interval
in which no force is applied. The angles of the
thrusters can be changed when there
is no thrust in anticipation of future thrusting.
Input values can be placed in columns E, G and I and will override initial values
if that is desired.
The time values in column H are the cumulative times that
the actions of the rows are in effect.
Column I shows the calculated mass of the object in accord with its initial mass,
its rate of mass loss per integration step as shown in column F and the number of integration
steps as given in column
The value 53,400 Newtons for the thrusts
in column B is for the two engines firing at the same time. This thrust value, a
value 0.000326 for MassDot and a total burn time of 625 seconds were obtained
This Input Table indicates that there are 30 seconds of coasting before any thrust is applied. Force is then applied
for 70.32 seconds. This is followed
by 2451.68 seconds of coasting until again force
is applied for 110.65 seconds
A total of 180.97 seconds of the available
625 seconds of thrust were employed in this Input Table.
The initial 30-second coast allowed for
the time taken to separate from the main fuel tank.
The first burn was used to increase the radial velocity such
that Orbiter would
coast to ISS altitude.
Elevation was not employed because we presumed that thrusting at
zero elevation would make the most efficient use of thrust.
The second coast and burn served to circularize into ISS orbit and to arrive closely behind the
The values in the table serve in
to reach the desired result. (As is shown in the third topic of this
It Works This Way
We think of the navigation process in terms of the functions of three computers,
the first two of which have already been mentioned.
There is a Course Computer that
given a burn time, and a heading for the thrust relative to the tangent plane, controls
the attitude of the gimbaled thrusters so as to maintain that heading over a given
burn time. To carry out this function, the Course Computer must be
informed regularly of Orbiter's ever-changing position, velocity and orientation. The
State Computer provides that needed information.
The State Computer is an adaptation
of the Earthly GPS system, which is augmented by a regularly updated inertial system
and by wireless data from Earth, satellites, and the ISS. This computer provides
continuously updated position, velocity and orientation data regarding both Orbiter
and the ISS.
The Model Computer maintains regularly
updated models of the atmosphere and Orbiter and its Engines. These models
are employed to predict the path that will be taken by Orbiter
resulting from a sequence of directed thrusts. This predicted path is compared with
the path information received from the state computer to provide error values.
The differences between the state values predicted for Orbiter from the models
of the atmosphere and engines, and the achieved
positions are regularly employed to update the models, which results in adjustments
being made to values equivalent to those in
rows 4:6 of our spreadsheet. (The actual duration and heading
of a burn may change before that burn completes.) The total process is
known as Adaptive
Conditional Feedback Control.
It is Conditional because the Adaptation of the models only occurs to the extent
that achieved values depart from values predicted by the models.
Although the spreadsheet model employs a constant burn force, we
expect that in a real situation the force depicted would increase
as the density of the atmosphere
diminishes and would likely experience other changes
during the ascent from MECO.
Similarly, the rate of mass loss could change.
More about the OMS Engines and the RCS
The two OMS thrust engines can be operated individually or both at the same time.
They have minimum burn times of two seconds. Their rate of swiveling is limited
as is their range of swiveling.
Should the OMS swiveling rate be insufficient or the range limits be reached, the
Reaction Control System is brought into play to assist.
At times the RCS may be employed solely to align the gimbaled thrusts with Orbiter's
centre of gravity in readiness for a thrust command.
When the RCS is required, the Model Computer will provide the Course Computer with
a vector of thrust values and their durations to be applied to a selection of the
44 RCS nozzles that surround the Orbiter. Such a vector can provide translation
or rotation or both.
The translation component will contribute to the path followed by Orbiter and that
path is compared with the actual path as determined by information from the State
Computer. The comparison allows for corrections to the vector of thrusts or may
lead to further RCS commands.
In some cases the rotation will need to be cancelled by a further command to the
RCS nozzles. In other cases it may be maintained during a one-engine or two-engine
OMS thrust as a measure to counter imbalanced engine thrust.
Navigation Computers, Modeling and updating the models. The design
and update of the sequence of Course Commands. Some navigation techniques.