TMac3000
Evil Republican
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For some reason Orbit Hangar would not let me post this, so I'll do it the less formal way
By request, here is a quick and dirty description of what a direct ascent is and how to do it.
HOW TO DOCK IN A HURRY
by TMac3000
Welcome to my first tutorial! Today you will learn how to perform rendezvous and docking Tmac style Well, okay, I didn't invent this method, but you get the idea
Contrary to what you might think, this is not an advanced skill. It only seems that way because it is neglected in many Orbiter tutorials. Even the wonderful “Go Play In Space”, possibly the most revered and lauded tutorial in Orbiter history, teaches only the “traditional” approach, where you have to align, synch, and wait about 8 hours to catch up to your target. But in fact, any Orbinaut can learn it...and do it well!
Now, every Orbinaut who has been in the game for more than a few weeks has noticed that a ship's orbit does not trace the same path over the ground in two laps. This is because the Earth is rotating under the orbiting ship. Since the Earth rotates east, the path of an object in orbit gradually moves west over a period of hours. The Map MFD in Orbiter 2006 did not show this, but the one in Orbiter 2010 does: it predicts future orbital tracks and displays them as a line darker than the current orbit.
At that rate, unless the object's inclination is lower than our latitude when we are sitting on the ground, it will eventually pass right over our heads. This is what enables us to launch and reach the object in what is known as a direct ascent.
The trickiest part of a direct ascent is timing. We have to know exactly when to take off, so that the target will be near us when we circularize our orbit. The moment of takeoff depends on the target's altitude (which determines its speed) and how long it takes our ship to reach space. If the target is higher or our ascent takes less time, we will take off later. If the target is lower or our ascent takes longer, we will take off sooner.
Of course we first want to wait for the target's ground track to pass over us, so we save ourselves an alignment burn: that is basic Go Play in Space. The second thing we want to wait for is for the target to be a certain distance “behind” us. This is the tricky part, because the distance we want depends on the ship we are flying, but unless the target has an unusual orbit, the distance should fall somewhere between 3,000 and 4,000 km.
Once you know the distance is right, take off! This is just a normal ascent. You want an apoapsis that matches your target. But do not circularize after MECO. This is where it gets a little different. Pull up IMFD on one screen and tune your radios to the target on the other. You should already be in range! If you are more than 1,000 km away, you should probably start over and try again.
Once your radios are tuned, pull up the docking MFD and watch the value CVEL: it should be counting down. When this value reaches zero, you have reached your closest approach. (If it's counting up, don't worry. Just follow the next step at a distance of about 100 km.) Now we need match velocities with the target. Be very careful! You are probably going to be burning toward the Earth, and your periapsis will get very low.
The neat thing about a trajectory like this is that you spend most of the trip suborbital. You should not even reach a full orbit until you are close to the target
Once velocity is zeroed out, quickly turn toward the station and burn until your apoapsis matches the target's. Steadily pare down your closure velocity until you are close enough to the station to start your approach. Then it's all routine from there! If your closest approach isn't close enough, you may have to repeat the last two steps. Happy docking
By request, here is a quick and dirty description of what a direct ascent is and how to do it.
HOW TO DOCK IN A HURRY
by TMac3000
Welcome to my first tutorial! Today you will learn how to perform rendezvous and docking Tmac style
Well, okay, I didn't invent this method, but you get the ideaContrary to what you might think, this is not an advanced skill. It only seems that way because it is neglected in many Orbiter tutorials. Even the wonderful “Go Play In Space”, possibly the most revered and lauded tutorial in Orbiter history, teaches only the “traditional” approach, where you have to align, synch, and wait about 8 hours to catch up to your target. But in fact, any Orbinaut can learn it...and do it well!
Now, every Orbinaut who has been in the game for more than a few weeks has noticed that a ship's orbit does not trace the same path over the ground in two laps. This is because the Earth is rotating under the orbiting ship. Since the Earth rotates east, the path of an object in orbit gradually moves west over a period of hours. The Map MFD in Orbiter 2006 did not show this, but the one in Orbiter 2010 does: it predicts future orbital tracks and displays them as a line darker than the current orbit.
At that rate, unless the object's inclination is lower than our latitude when we are sitting on the ground, it will eventually pass right over our heads. This is what enables us to launch and reach the object in what is known as a direct ascent.
The trickiest part of a direct ascent is timing. We have to know exactly when to take off, so that the target will be near us when we circularize our orbit. The moment of takeoff depends on the target's altitude (which determines its speed) and how long it takes our ship to reach space. If the target is higher or our ascent takes less time, we will take off later. If the target is lower or our ascent takes longer, we will take off sooner.
Of course we first want to wait for the target's ground track to pass over us, so we save ourselves an alignment burn: that is basic Go Play in Space. The second thing we want to wait for is for the target to be a certain distance “behind” us. This is the tricky part, because the distance we want depends on the ship we are flying, but unless the target has an unusual orbit, the distance should fall somewhere between 3,000 and 4,000 km.
Once you know the distance is right, take off! This is just a normal ascent. You want an apoapsis that matches your target. But do not circularize after MECO. This is where it gets a little different. Pull up IMFD on one screen and tune your radios to the target on the other. You should already be in range! If you are more than 1,000 km away, you should probably start over and try again.
Once your radios are tuned, pull up the docking MFD and watch the value CVEL: it should be counting down. When this value reaches zero, you have reached your closest approach. (If it's counting up, don't worry. Just follow the next step at a distance of about 100 km.) Now we need match velocities with the target. Be very careful! You are probably going to be burning toward the Earth, and your periapsis will get very low.
The neat thing about a trajectory like this is that you spend most of the trip suborbital. You should not even reach a full orbit until you are close to the target
Once velocity is zeroed out, quickly turn toward the station and burn until your apoapsis matches the target's. Steadily pare down your closure velocity until you are close enough to the station to start your approach. Then it's all routine from there! If your closest approach isn't close enough, you may have to repeat the last two steps. Happy docking
