Apollo 1999

Gemini

version 1a

Marco Di Mauro and the Apollo development team.
(C) 1999, 2000 DMG.

I. PURPOSE

The Apollo 1999 project is aimed at developing a PC based spaceflight simulator. The purpose of the project is to provide people interested in space flight a basic understanding of the principles behind the 20th century rocket technology. It may be regarded as a simple tool which can be exploited in simulating the effects of maneouvers and techniques on the trajectory of a rocket or a satellite, from launch up to orbit and down to descent and landing.
Apollo may also help understanding trade offs and how these have an impact on the performance of the space vehicles. Clearly Apollo 1999 scope is educational and it may be used by high school students as well as university students entering aeronautics and aerospace studies similarly to what a flight simulator can do for trainee pilots.
By using the Apollo simulator, you'll be able to launch a space vehicle, guide it through the atmosphere during the ascent phase, achieve parking orbit, perform orbital maneouvers and rendez-vous with other orbiting objects, plan and execute the reentry and approach and land on the Earth using retro-thrust.

Although the Gemini module part of the Apollo 1999 project is still quite simple in term of graphics and simulation design, future releases are meant to provide more functions and features and an enhanced interface and graphics design, while still retaining the concept of keeping the simulation 'playable' and 'fun'.

II. OVERVIEW

Gemini is a space launch simulator allowing the control of a launch vehicle from countdown to orbit acquisition. The scenario is a bi-dimensional non rotating Earth. This is to avoid excessive three-D complications in terms of both graphical interface and maneouver degree of freedom.

With Gemini your task is to launch a rocket and guide it through the atmosphere by accelerating in the proper way until a safe and stable parking orbit is achieved. To do this you can act on different controls in order to 'steer' the launcher. But beware, the rocket is not a Star Wars fighter and you have several limits. You can't just do whatever you want, but you need to take into account the changing weight of the rocket, the gravity effect on the trajectory, the limited fuel and the aerodinamic drag effected by the atmosphere while trying to get out into space.

Gemini is still a beta version and so there are potential bugs. I did not have time to fix all the problems and yet the interface has to be improved a lot. For example, the Quit in the File Menu does not work! To exit the program close it by clicking onto the 'close' in the upper right of the window.

III. SCENARIOS

Gemini has four predefined scenarios:

- Launch Pad

In this scenario your task is to launch your rocket and achieve a stable Earth Orbit. You have a limited amount of fuel so the task is not so trivial as it may seem. After you are able to reach orbit, you can test further your space flight skills trying, for example, to place what's left of the rocket into an 'Escape Orbit'. The Escape orbit (or interplanetary orbit) is a trajectory which causes the spacecraft to exit the Earth's gravitational field and never come back again. This means that the trajectory is not anymore a closed trajectory (orbit) around the Earth, rather it is an open hyperbola. These are the kind of trajectories which are needed to get to the Moon and also those performed by any interplanetary spacecrafts to the planets of the solar system. Details on orbits are provided in the relevant 'Orbit' section.

If you have still fuel in the tanks after reaching a stable orbit, you can try as well to return to Earth by doing a re-entry followed by an approach to the launch pad from where you left. This is possible because the simulator will allow you to touch down on the launch pad, although this is increadibly difficult. The programmer made it after several attempts and a number of different factors are affecting the successful end of this manoeuver. To perform this maneouver successfully you will need to use a combination of aerobraking (use the atmosphere to slow down) and retro-rocket thrust to decelerate from the 7.8 km/second of low Earth orbit to the 3 m/sec of the final approach. More details on how to land are provided in the relevant section below.

- Space Shuttle

In this scenario you have to launch and perform a 'rendez-vous' with the Space Shuttle, already orbiting the Earth at an altitude of about 250 km. So, where is the problem? You'll say. Try and you'll realize that to reach the Shuttle you have to perform the necessary maneouvers very carefully and timely. You can't just direct to the Shuttle and fire the engines. Those with an orbital mechanics background will be advantaged.

- Low Earth Orbit

In this scenario you are already in a circular stable orbit. You can try to do a reentry planning adequately the exact moment when to initiate the decelaration. Or you can see how the orbit shape and characteristics change by firing the engines at different moments and directions. This scenario is good to test and study how you can control your orbit by using the engines.

- Meteor

In the 'Meteor' scenario you have to intercept a giant meteor bound for Earth before it reaches the planet. For this purpose, the payload or your rocket has been equipped with a 10 megaton nuclear warhead which will (hopefully) blast off the meteor or kick off it to a different course.

The Meteor is coming at a high speed from the right. You have to launch and place your rocket onto a collision course with the Meteor. But this is not easy as there is gravity both affecting your trajectory and also the Meteor trajectory. Try to find out which is the correct sequence of maneouvers to reach the meteor. For example, better to launch directly to an escape trajectory, or it may give more chances a first launch into a parking orbit, and then a second 'burn' for final intercept after a 'coasting' phase? (A coasting phase is a phase during which the engines are not running and the spacecraft's trajectory is only affected by gravity, as in a non-perturbed orbit).

The default scenario is Launch Pad, so when you first run the program you'll see your rocket sitting on the launch pad and waiting for go for launch.

IV. QUICK LAUNCH

You may like to see how it works and do a launch quickly. This is how to do.

Run the program. The rocket is on the launch pad and the countdown is going, starting from -60 seconds. The countdown is on the upper left side. On top you have the main flight instruments:

- Vertical speed: is the vertical speed in meter per seconds. Note that this speed is measured in the direction center of the Earth-rocket.

- Horizontal speed: is the speed measured in the direction of the Earth surface.

- Speed: is the absolute speed of the rocket.

- Altitude: is the distance from the Earth surface, in meters. After a given altitude, it switches to kilometers.

- Load Factor: is the ratio of all thrust and aerodynamics forces to the gravity force. This will be explained more in details later. For the moment, just know that the LF can be used to indicate how the rocket is accelerating. If LF is greater than one, the rocket is accelerating, if it's less than one, the rocket is decelerating. Note however that this applies for ascent trajectories.

As the countdown goes on, the events window below the countdown counter indicates the various pre flight events and final checks.

At T minus 6 seconds, the main engine starts. Final checks are made on all engine parameters (in the reality, not in the simulator! although this feature can be implemented in future versions) and then the two solid rocket boosters (SRB) are fired. With the addirional SRB thrust, the vehicle is now able to lift off. The main engine is a LOX-LH2 engine and its thrust can be controlled using keys 0 (increase) and 9 (decrease). The T bar indicator on the right of the main screen is showing the engine thrust. At launch, engine thrust is at 100 %. While the Main Engine (ME) can be controlled, the SRB's thrust can not. So, once they are fired, the only way to stop them is by waiting until all fuel is burned or by jettisoning them using the 'JETTISON' button (in case of emergency reentry).

With SRB firings, liftoff will occur. The initial direction is vertical, thus initial acceleration will cause the vertical velocity to increase. As soon as you are free of the launch pad, try to steer the rocket using keys 1 (rotate anticlockwise) and 2 (rotate clockwise). Keys A (fast rotate anticlockwise) and Z (fast rotate clockwise) can also be used for a faster rotation.

Initiate as soon as free of the launch pad a gentle turn to the right (clockwise rotation). To reach orbit, you need to accelerate up to a velocity of about 7.8 km per second. But the velocity direction can't be whatever you want. You need to have a velocity direction which is parallel to the surface, i.e. almost all of your velocity should be horizontal. Why? Because when you are in orbit with no engines or wings counteracting gravity, it's like you are falling (free fall), but you never reach the Earth since you fall at the same rate of curvature of the Earth, so the distance from the surface is constant and you go parallel with respect to the ground, which makes your velocity direction to be constantly horizontal (to the local horizontal of course, since you follow the curvature of Earth).

You may think also in such a way. Imagine to be on top of a mountain and you launch a tennis ball in the horizontal direction. As the ball leaves your hand, gravity will start to act and bring the ball down. So you have that, while the horizontal velocity you gave the ball is quite constant, the vertical velocity will build up quickly (in the 'down' direction) and the ball will fall. The resulting trajectory is a parabola. Now, if you increase the horizontal velocity, the ball will fall, but will touch the ground farther from the top of the mountain. The parabola is more flat. Increase the velocity and maybe the ball will fall apart 500 m (this in reality will not happen since the atmosphere will cause drag on the ball and the horizontal speed would rapidly go to zero, but for the sake of argument let's suppose here we do not have atmosphere, i.e. the mountain is very very high, maybe 50 km!).
Then, continuing to increase the horizontal velocity with which we launch the ball, we get to a point where, even if the ball is falling, it is going so fast horizontally that it will actually never reach the ground because the Earth surface below is curving. The ball is in orbit.

V. FLIGHT DISPLAY

The Flight Control Display allows you to receive real time information on the status and progress of the flight. All necessary information are available for interpretation of flight progress, vehicle performance, trajectory, etc.

The Flight Control Display is broken down in the following parts.

V.1 Cameras.

Cameras allow to monitor the rocket trajectory, position, attitude from a variety of view points, graphically representing the view of the vehicle(s). Different camera views are possible. A main camera screen (in the center) is complemented by three additional views on the right side. Each view can be tuned to a different camera view, allowing to monitor simultaneously the flight from different view points. There are a total of 10 camera views which can be tuned on any of the four screens. The selection of the cameras is done by clicking on the small arrows on the lower right part of each screen.

The 10 cameras views are the following:

CAMERA 1: TRACAM, tracks continuously the main vehicle.

CAMERA 2: LAUNCH PAD, shows a fixed view from the top of the launch pad, use this only during launch.

CAMERA 3: FISHEYE, this view is very useful as it allows to keep track of the vehicle and the surrounding environment (launch pad and/or Earth surface) by automatically panning/zooming in order to keep in the same view the different objects.

CAMERA 4: LAUNCH SITE, this view shows the launch site from a nearby location, and it allows to monitor the very initial phase after liftoff such as initial trajectory.

CAMERA 5: MAIN ENGINE, monitor the engine nozzles.

CAMERA 6: not allocated yet.

CAMERA 7: CUSTOM VIEW, is similar to TRACAM, but can be zoomed in or out using the ZOOM (Z+ and Z- keys) on the upper right side of the main screen. This is especially useful when needing to show on the same screen the vehicle and the chase target for a rendez-vous for example.

CAMERA 8-9: LEFT / RIGHT BOOSTER TRACAM, track the boosters.

CAMERA 10: TARGET ONE, tracks the target, if present.

V.2 Flight Instruments Panel

The Flight Instrument Panel is shown on top of the main screen. It includes:

- Vertical speed indicator: is the vertical speed in meter per seconds. Note that this speed is measured in the direction center of the Earth-rocket.

- Horizontal speed indicator: is the speed measured in the direction parallel to that of the Earth surface.

- Velocity indicator : is the absolute speed of the rocket.

- Altitude: is the distance from the Earth surface, in meters. After a given altitude, it switches to kilometers.

- Load Factor: is the ratio of all thrust and aerodynamics forces to the gravity force. The LF is expressed in unit of the Earth's gravity acceleration (g). This means that a LF of 1 g is indicating that the thrust plus all aerodynamics forces are equal to the gravity force exerted on the mass of the vehicle. Thus, if the thrust direction is opposite to the gravity force, a LF of 1 g indicates that the weight of the rocket (gravity force) is balanced by the thrust so to keep the vehicle at a constant speed (or at rest).
If, on the other hand, the thrust direction is the same as the gravity force direction, a LF of 1 g means that the thrust will add to the gravity force causing a double g acceleration towards the ground. Note also the difference between force and acceleration. The gravity force is given by Fg=mg where m is the vehicle mass and g is the local gravity acceleration. Thrust and aerodynamic will add up to the gravity force to give the total force. The total force applied drives the rocket acceleration, according to Newton law F=ma where F is the total force, m is the mass and a is the resulting acceleration. Note also that, when in orbit outside the atmosphere and with engines off, LF will be zero, indicating a zero gravity condition. This means that the rocket is 'weightless', i.e. is in a free fall. This does not mean (of course) neither that there is no gravity nor that the rocket is weighting zero. It simply means that the only acting force is gravity and thus there are no structural loads on the vehicle. Infact there is no aerodynamic nor thrust forces so the ratio of these forces to the gravity is always zero. The vehicle is of course accelerating (at 1 g) but with no loads on its structure, so its LF is 0 g. A high LF on the other hand indicates that there is a large force different from gravity (may be thrust and/or drag) acting on the vehicle. This not only affect acceleration, but also stress within the rocket because the load distribution within the vehicle is not zero. During ascent, you'll notice that the LF slightly increase up to a maximum of about 3 g for a default launch. This is caused by the fact that, while the thrust is almost constant, the gravity force decreases mainly because the mass of the vehicle decreases (the rocket burns fuel at an incredible rate). Thus the ratio thrust/gravity increases.

V.3 The Systems Status Display

The System Status Display is on the left part and includes system lights indicating the status for launch. All system should be green (GO) to launch. During descent and landing, the system light will change to show the status with respect to vertical speed, horizontal speed, angle and touchdown zone limits for landing. Moreover, the atmosphere light will go off when atmospheric effects on the trajectory are negligible (altitude > 80 km).

V.4 The HOLD and MASTER CAUTION lights

The HOLD button can be pressed to put the launch on hold. For T less that 20 seconds, the hold will place the countdown back at T minus 20 seconds.

The MASTER CAUTION light is used to indicate any dangerous situation such as low altitude or excessive vertical speed during landing. To inhibit the MASTER CAUTION alert, press the button.

V.5 The Flight Data Computer (FDC)

The Flight Data Computer panel is shown in the bottom part of the screen. The FDC includes the following subsystems:

V.5.1 Head-Up Display (HUD).

The HUD activates a rectangle on screen highlighting the position of the selected vehicle (normally rhe main rocket but may be also one of the two SRB after separation). Altitude and speed are shown on screen

V.5.2. The Electronic Flight Situation Indicator (EFSI)

The EFSI provides all navigational indication for the flight such as relative position with respect to targets as well as launch pad, trajectory, range information on the main screen, attitude and velocity information. The EFSI includes:

V.5.2.1 Attitude and Velocity Situation Indicator (AVSI).

Toggling the AVSI after having activated the EFSI, you'll see a blue line (always active when the main EFSI button is toggled) indicating the local horizon, and a magenta vector indicating the velocity direction. The velocity direction is useful in different situations such as when circularizing the orbit or when on final approach.

V.5.2.2 The FMC.

FMC highlights the vehicle position on screen.

V.5.2.3 The Navigation Display (NAV).

Toggling the NAV button, a series of circles will be shown on screen indicating ranges in km to provide a scale to what shown in the camera view. Moreover, an orange segment will show the direction to the target selected in the TGT window below. By pressing TGT, the different possible targets will be selected (main rocket, boosters, launch pad, shuttle, meteor). The distance to the target in kilometers is also indicated to the left window. If the target is within the view range, it will be highlighted by an orange box and an orange vector indicating the direction.

V.5.2.4 The Trajectory Display (TRAJ).

Toggling the TRAJ button, the vehicle trajectory will be shown as a series of cyan points. The Trajectory Display is useful in the FISHEYE camera view to monitor the ascent profile, and also during descent and landing.

V.5.2.5 The RNV button.
It is yet not operational.

V.5.3 The Space Flight Data Computer (SFDC)

The SFDC on the right of the FDC panel, enables the Flight Management Computer Interface (FMC) on the left of the screen. The FMC provides detailed numerical information on a number of flight data such as total mass, fuel mass, fuel flow, thrust, acceleration, air density and drag, load factor and orbital information. There are 5 pages on the FMC and all of them can be accessed by pressing the (Page +) and (Page -) buttons or by using the quick buttons on top of the FMC.
Real Time Fuel and Thrust is also shown separately on the right side of the main screen.

V.5.3.1 Global

The Global page provides information on thrust, mass, fuel, fuel flow and load factor.

V.5.3.2 Vehicle

The Vehicle page provides the same info as the Global, but detailed for the vehicle selected (booster for example).

V.5.3.3 Atmospheric

The Atmospheric page provides information on the local air density and drag. Air density affects drag which in turn affects the rocket performance. The drag acceleration is the drag contribution to the acceleration (being this contribution always a 'deceleration' since drag is always opposite in direction to the velocity).
Atmospheric effects become normally negligible for altitudes over 80 km. The altitudes ramging between 80 km and 100 km constitutes the 'reentry interface'. In this layer, atmospheric effects are not negligible and will cause the orbit to decay rapidly as altitude decreases.

V.5.3.4 Orbital

The Orbital page includes orbital information such as the orbit semiaxis (a), the perigee (ap, the point of the orbit closer to the Earth), the apogee (aa, the farthest point of the orbit from the Earth), the Eccentricity (e) , the Orbital Period (T) and the local velocity for circular orbit and escape trajectory (Vc and Ve).

The following relationships apply:

Perigee altitude=Semiaxis x (1-Eccentricity)-Earth radius

Perigee altitude=Semiaxis x (1+Eccentricity)-Earth radius

Period=2 x pi x sqr [semiaxis^3 / (G x M)] where G is the gravitational constant and M is the Earth Mass

Vc=sqr[(G x M) / a]= circa 7.81 km/sec at 200 kilometers

Ve=1.41 x Vc for a given altitude (a) = circa 11 km/sec at 200 kilometers

Note that:

e=0 circular orbit with ap=aa

e<1 elliptic orbit with aa>ap

e=1 parabolic orbit

e>1 interplanetary trajectory

When the Orbital page of the FMC is loaded, the cyan flight annunciator below the main flight instruments will indicate the type of orbit. The possible types are:

- Suborbital, the perigee is below the ground.

- Parking Unstable, the perigee is low and the orbit may decay rapidly due to atmospheric drag.

- Elliptic, e<1 and orbit is stable in the short term.

- Interplanetary, e>1 and vehicle will escape the Earth gravity field.

V.5.3.5 Fuel and Thrust data

Real time fuel and thrust data are shown on the right part of the main screen. Two sliding bars show visually the Fuel Mass (F) and the Thrust ratio (T).

VI. FLIGHT CONTROLS

The main controls used to steer the rockets are:

A - Rotate Anticlockwise
Z - Rotate Clockwise
1 - Fine Rotate Anticlockwise
2 - Fine Rotate Clockwise
0 - Increase Engine Thrust
9 - Decrease Engine Thrust

Moreover, the JETTISON button can be used to jettison the SRB. This should be normally done after the SRB fuel is burned. A warning beep will advise of SRB fuel almost empty. Shortly afterwards the termination of the warning beep, the SRB thrust will go to zero.

VI.1 Zoom

Zoom on the main window can be performed by clicking on the Z+ and Z- buttons on the upper right part of the main screen.

VII. LAUNCH

During Launch you have to perform and monitor the lift off and the initial phases of the climb out. Depending on the Mission selected and the Propulsion system used (Shuttle type reflects the Shuttle performance, with Main Rocket providing the Shuttle thrust and SRB providing the Shuttle SRB thrust), you also may choose to hold the launch.
For example, if in the rendez vous, you have to wait the right moment to launch. Try to launch several times at different instant to assess when is the right time, depending on the way you want to approach the orbiting target

VIII. ASCENT

In the Ascent phase, the objective is to accelerate through the atmosphere in the optimal way. You have to find a compromise between a rapid vertical ascent (which will allow you to leave quickly behind the densest atmosphere layers causing most of the drag and preventing acceleration) and a more inclined flight profile allowing to use the thrust to accelerate horizontally most effectively. Do not forget in fact that to reach orbit you need to have a high horizontal velocity, but keep in mind that you first have to get clear of the atmosphere before turning your velocity vector horizontally. Moreover, with the vehicle fully loaded of fuel, the initial acceleration will be slow. You risk to burn off huge amount of fuel while accelerating very slowly. If you turn too early, your trajectory will be too 'flat' and you'll stay too much time at low altitude without accelerating. If you turn too late, you will not be able to accelerate horizontally enough to gain the minimum orbital velocity of 7.8 km/s.

A good profile will result in a curving trajectory, with a top altitude of 100 km. In this initial phase the thrust angle should be quite close to the vertical (45-55 deg, with 0 deg horizontal dir. and 90 deg vertical up direction). Afterwards there will be a gentle descent to a lower altitude (not too low). You can accept a slight loss of altitude for a big increase of the horizontal velocity during this phase, and you obtain this by rotating the rocket thrust angle to 0-45 deg.
Finally, when the velocity is high enough, with a small thrust angle (0-20 deg) you will be able to accelerate quickly without losing too much altitude.

Use the velocity vector and the artificial horizon (AVSI) to change your flight profile.
You'll see how the velocity vector changes when changing the rocket thrust angle (rocket direction) and the thrust.
If the velocity vector falls below the horizon, it means that you are losing altitude, i.e. the vertical velocity is negative.
The following guidelines are applicable (Thrust and Velocity direction measured with respect to local horizontal: horizontal right = 0 deg, vertical up = 90 deg):

ALTITUDE (km) | EVENT | SPEED (km/sec) | THRUST DIRECTION | SPEED DIRECTION
0 - 0.5 | Liftoff | <1 | 90 | 90
0.5-1 | Gravity Turn | <1 | 90-70 | 90-->70
1 - 40 | Atmospheric Ascent | 1-2 | 60-45 | 50-->30
40-120 | Atmospheric Exit | 2-3 | 45 | 45-->20
120 | SubOrbital Acceleration | 3-7.8 | 0-30 | 20-->0
110- | Main Engine Cutoff (MECO) | 7.8 | - | 0 (circular orbit)

IX. ORBIT

The end of the ascent phase is the injection into a parking orbit. You can check wheather you are in a stable parking orbit or not by looking at the Orbital FMC page. The events window (cyan) below the countdown counter will indicate the orbit type. If the orbit is stable, it will be indicated as 'elliptic'. The FMC page will show the orbit characteristics.
See also (V).e.3.4.

The apogee altitude indicates the maximum altitude for the actual orbit.
The perigee altitude indicates the minimum altitude for the actual orbit
A perigee below zero (negative) will indicate that the trajectory is suborbital, i.e. that you will impact the ground as the perigee is below the terrain.
A perigee greater than zero but less than 100 km will result in an "unstable orbit" since at altitude below 100 km the atmospheric effects are not negligible and this means that, in the long term (more than one orbit around the Earth) the orbit will decay and turn into a sub orbital trajectory (the perigee altitude will decrease).
Note that in an unperturbed orbit, the perigee, apogee, eccentricity and semiaxis will remain constant in time.

We recall here the relationship between the various orbital parameters:

The following relationships apply:

Perigee altitude=Semiaxis x (1-Eccentricity)-Earth radius

Perigee altitude=Semiaxis x (1+Eccentricity)-Earth radius

Period=2 x pi x sqr [semiaxis^3 / (G x M)] where G is the gravitational constant and M is the Earth Mass

Vc=sqr[(G x M) / a]= circa 7.81 km/sec at 200 kilometers

Ve=1.41 x Vc for a given altitude (a) = circa 11 km/sec at 200 kilometers

Note that:

e=0 circular orbit with ap=aa

e<1 elliptic orbit with aa>ap

e=1 parabolic orbit

e>1 interplanetary trajectory

When the Orbital page of the FMC is loaded, the cyan flight annunciator below the main flight instruments will indicate the type of orbit. The possible types are:

- Suborbital, the perigee is below the ground.

- Parking Unstable, the perigee is low and the orbit may decay rapidly due to atmospheric drag.

- Elliptic, e<1 and orbit is stable in the short term.

- Interplanetary, e>1 and vehicle will escape the Earth gravity field.

IX.1 Orbit Transfers

When you reach a stable orbit, you can turn off the engines. You'll notice that the orbital parameters will remain the same, while during the burn phase (engines running), the orbital parameters were changing. In fact, the vehicle in orbit is affected only by the gravitational force. If any other force (such as atmospheric perturbation or engines) are present, the resulting trajectory is an unperturbed orbit. If the orbit is stable, the vehicle will continue to travel around the Earth with no end, and at each orbit it will reach a maximum altitude from the surface (at the apogee) corresponding to the apogee altitude and a minimum altitude (at the perigee) corresponding to the perigee altitude. The point along the trajectory where perigee is found, is opposite to the point where the apogee is placed. Along the orbit, thus, apogee and perigee are opposite each other (180 degrees).
If you want to reach an orbiting body having a higher or lower altitude, you need to perform a so called "orbit transfer".
This maneouver is far from being an easy and straightforward one. In fact, for a rendez vous with an other vehicle, you need not only to change orbit and make sure that your final orbit is very close to that of the chased body, but also you will need to be at the right place at the right time, i.e. you must be in the same part of the orbit as the chased object. If you are on the same orbit, but 180 degrees separated (i.e. on the opposite side of the orbit), you need to perform a series of maneouvers to "re-phase". One thing to bear in mind is that you can't slow down in an orbit. The velocity along the orbit is linked directly to the orbit and the position of the vehicle along the orbit. This means that, for a given orbit, the velocity at perigee is defined as well as the velocity at apogee. If you try to slow down (using the engines), you'll change orbit. (different perigee and apogee and eccentricity).
Now, it happens that, due to orbital mechanics consideration, the velocity at the perigee is greater than that at the apogee. This is because at the perigee the vehicle is closer to the Earth, thus the gravitational force is greater and this results in a greater acceleration (centripetal acceleration). A higher centripetal accelaration will result in a higher speed.

Coming back to the orbit transfer, in order to change orbit you have to do the following:

- if you want to have a higher apogee, you have to increase the velocity (horizontal) at the perigee: so, fire the engines at or close to the perigee, for some time, in the direction of motion. You will accelerate locally, but you'll also notice that the apogee altitude will increase. You'll raise the apogee.

- if you want to have a lower apogee, you have to decrease the velocity (horizontal) at the perigee: so, fire the engines at or close to the perigee, for some time, in the direction opposite to motion. You will decelerate locally, but you'll notice that the apogee altitude will decrease. You'll lower the apogee.

The same applies for the perigee. So to raise or lower the perigee, fire the engines in or opposite to the direction of motion at or close to the apogee.

- To reach a higher circular (or near circular) orbit from a lower circular (near circular) orbit: first increase the apogee altitude to the altitude of the target orbit. You'll be placed onto a transfer orbit with perigee at the altitude of the lower orbit and apogee at the altitude of the higher orbit. Then, when at the apogee of the transfer orbit (altitude of the target orbit), fire and increase the perigee altitude up to the altitude of the apogee. This is a Hohmann transfer.

X. DESCENT

The descent phase allows the reentry of the vehicle into the atmosphere.
To reentry you need to have fuel in the tanks after reaching a stable orbit.
To perform this maneouver successfully you will need to use a combination of aerobraking (use the atmosphere to slow down) and retro-rocket thrust to decelerate from the 7.8 km/second of low Earth orbit to the 3 m/sec of the final approach.
The descent phase starts when the vehicle is placed from a stable orbit onto a renetry trajectory, normally consisting initially of a unstable orbit.
In fact, you need first to calculate exaclty when to start descent in order to reach the launch pad. This is not easy. If you reduce fire the engines in order to lower the orbit, you need to do this in that part of the orbit opposite to the part where you want to be the perigee. So, firing where you are "at the bottom" of the Earth, and decelerating will cause to lower the orbit exactly "on the upper part" of the orbit (with respect to the Earth). If the perigee altitude is below 100 km, you'll have that the atmosphere will produce the effect of decreasing your velocity when you are close to the perigee. If you are not too low, you will exit the atmosphere but the orbit would be once more changed (lower apogee). The atmosphere acts like an engine firing in the direction opposite to the motion.

If the perigee is too low, the atmospheric effect (which increases rapidly as the altitude decreases) can cause the orbit to decay into a sub orbital trajectory. You have to make this happen, but you should make sure that the suborbital trajectory brings you very close to the landing area at an altitude of 50 km or so. Below 80 km, the deceleration is very rapid. The Load Factor can increase significantly as the atmospheric drag quickly surpasses and overcomes the gravity. Until the speed has decreased so that drag (which depends on the speed) is counteracted by gravity, the vehicle will decelerate. You'll notice that, by changing the attitude and placing the vehicle so that its section area with respect to the direction of motion is maximised, you decelerate more quickly. If you point the rocket in the direction of motion, drag will be minimised. By doing this you have the possibility of affecting the trajectory. If needing to slow down and go down more quickly, point the rocket 90 degrees with respect to the direction of motion.

During the descent you should not be using the engines. Engines are effective only when, still out of the atmosphere, you change the trajectory by lowering or raising the perigee altitude. You can use the engines to perform slight changes during the reentry interface, but once you are in the densest atmosphere, engines are only partially effective in changing the trajectory. The risk is that of wasting a lot of fuel , which will be badly needed during approach and landing phases.

XI. APPROACH

At the end of the descent phase you should be onto a vertical or near vertical trajectory, at an altitude of less than 50 km and close to the landing pad. Now you need to use the engines to steer the vehicle.

The descent phase finishes when the velocity has fallen to a few hundred of meters per seconds, and is constant. The trajectory will be a vertical trajectory. You need to have still fuel and your task now is to steer the rocket towards the landing area. use the NAV and AVSI displays to perform the approach. Select the Landing Pad in the TGT window of the EFSI and activate the NAV and AVSI functions of the EFSI. Also use the custom view to control the rocket attitude while at the same time zooming oout and in to have the overall perspective of where is the rocket with respect to the landing pad.

The AVSI will show the rocket direction and the NAV orange vector will indicate the landing area direction.
Initially you can perform an instrument approach. Use the engines so that the magenta velocity vector is on or above the orange "glide slope" vector. The trajectory should be inclined around 45 / 60 degrees with respect to the horizontal (velocity vector -60 / -45 deg). If the trajectory is too "flat" (-20 / -10 deg) and you are too low, you may not be able to reach the landing area, because the engine thrust and fuel are not enough to perform a flat approach. Bear in mind also that, as your speed increases, the drag will slow you down. The result is that you waste a huge amount of fuel to travel horizontally, at speed of a few hundred meters per seconds.

XII. LANDING

If you arrive at 1 km or so from the landing pad, then you can initiate the landing phase. Place the rocket in a near vertical trajectory in a landing attitude (90 degrees, or nose pointed upwards). Use the engines to slow down the speed and make slight corrections to place the landing pad at the center of your landing trajectory. The landing vertical speed should be no greater than 3 m/s. Also the landing angle should be 90 degrees. The horizontal speed as well cannot exceed a certain threshold. Landing is quite difficult, and you probably would need a lot of efforts before you make it.

XIII. PERFORMANCE AND OPTIMIZATION

Gemini was designed to be run on PII processor class PC. Gemini was developed on a PII 266 Mhz machine and was optimized for this type of computer. This means that, running on slower machines will cause the prgram to be less smooth. On the other hand, running it on faster machine can cause the program to be more smooth, even too smooth and fast. An automatic machine speed adjustement is not available at the moment (it will be implemented in futire versions) so, in order to improve performance you have the following options:

- In the secondary views (on the right side), deselect cameras (click on the left button until no active camera is selected). This will increase speed.

- If willing to run the sim at very high speed (because for example you want to coast or wait until next orbit), deselect all views, also in the main screen (click on CAM 0). You'll see all indicators changing very fast. All graphic computation would be disabled.

In order to slow down the simulation (for faster machines):

- Select cameras in all views. This will increase the processor workload, thus slow down the simulation.

XIV. KNOWN BUGS AND LAST HOUR INFO

v10a

- Quit under the File Menu does not work.
- Jettison button is inhibited after first mission. Reset mission does not reactivate the button.
- After explosion , clicking on OK does re-explode the rocket.
- Old trajectory remains after reset.

XV. COST

Gemini is free. The source code is free. Selling it is not allowed.

XVI. DISTRIBUTION INTEGRITY

This chapter applies to the official Gemini distribution.
Gemini must be distributed only in the original archives. You are not allowed to distribute a modified version, nor to remove and/or add files to the archive.

XVII. REUSE OF SOURCE CODE

The source code cannot be used in a commercial product without a written authorization of the authors.

XVIII. DERIVATIVE WORK

Derivative work and cooperation in the project is welcomed. Gemini is an open project and improvements are foreseen, including:

- Sound and graphics
- supporting more customised mission with a greater range of vehicles
- Improving the general interface and layout
- Upgrade the graphics to 3D and make full use of 3D graphic boards
- Improved model of atmosphere and reentry heat effects
- Improved cockpit and mission control design

Anyone interested in joining the development team can freely contact it at this email:

Mdimauro@hotmail.com