So, You Want to Be an Astronaut? An Orbiter AAR

Rambler

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Feeling adventurous and ready to take a trip to the International Space Station? Good, because that's where we're headed!

Here is the ship that will be taking us there; the Delta Glider Mk 4:



Glider is a bit of a misnomer. She's a rocket powered spaceplane that can do powered flight in the Earth's atmosphere as well as in space. She has a wingspan of 17.86 m and weighs 11,000 kg empty (max take-off weight is 24,900 kg). Thrust is provided by two main engines rated at 160,000 N each. She is also equipped with two retro rockets in the leading edges of the wings and three hover engines located on the underside of the ship. Finally, she has a crew compliment of five; one pilot and up to four passengers.

We'll have to wait for an hour or so until the ISS is in the ideal position for us to start our ascent. In the meantime, I'll have to do some calculations to figure out what azimuth we need after takeoff in order to make sure our orbit will have the same inclination as the ISS's. What's inclination? Well, the simple definition is that it's the angular difference between an orbital plane and a reference plane, usually the Earth's equator for things orbiting Earth. In our case, the ISS has an inclination of 51.6 degrees to the Earth's equatorial plane. That's what our orbital inclination will have to be too. If our inclination is off by a large number of degrees when we reach orbit, we will have to perform some very expensive burns in order to align our orbital plane with the ISS's. Plane changes eat fuel like crazy! Ideally, we will want our relative inclination (The difference between our orbital plane and our target's orbital plane) to the ISS to be less than five degrees. After running the numbers (Yeah, I actually worked this out with pen and paper), we will have to adjust our heading to 137 degrees azimuth once we are in the air. We are currently sitting on Runway 33 at the Kennedy Space Center, meaning our bearing is 330 degrees azimuth. We'll almost be doing a 180 after takeoff.

OK, looks like it's time to climb in and get situated for our journey. Systems checks are complete, and the Delta Glider is in good shape and ready for takeoff. Everybody buckled in nice and tight? Good, because it's about to get intense!

3...2...1...

The main engines roar to life as the throttles are maxed out, and our Delta Glider begins to rapidly gather speed down the runway. At around 150 m/s, the Delta Glider rotates for takeoff and breaks its bond with Terra Firma. Time to raise the gears and begin our turn to the correct heading:



As we make the turn we throttle back and keep pitch at about 10 degrees above the horizon. It's not time to start the ascent yet! Take in some of the sites around Cape Canaveral in the meantime. In the lower left, you'll see Launch Complexes 39A and 39B. Those are the pads where the Saturn rockets and the space shuttles launched from. In the upper left, you'll see missile row at Cape Canaveral Air Force Station. Finally, the Vehicle Assembly Building (VAB) can be seen in the upper right. This building was built for the final assembly of the Saturn Vs, the rockets that launched man to the moon. Due to its enormous volume, the building has to be constantly climate controlled or else it can actually rain inside of it!:



We've finally turned to our correct azimuth, and now we're ready to rocket out of Earth's atmosphere and start our ascent into orbit. The throttles are again pushed forward to 100% thrust, and the Delta Glider's pitch is set to 30 degrees above the horizon. Altitude and speed accumulate at a frightening rate. Florida quickly falls away below us:



As we continue to climb, we slowly begin to shallow out our ascent in order to gain more tangential velocity. This will help our orbit become more circular as we approach orbital velocity, which is around 7,700 m/s. At an altitude of ~20 km, our pitch is ~20 degrees and our speed 600+ m/s. At 10 degrees pitch, we are traveling at 1,000+ m/s and at an altitude of ~40 km. Now that we are in the upper reaches of Earth's atmosphere, the Delta Glider's atmospheric flight control surfaces are no longer effective. There's too little air for the elevons (control surfaces that combine ailerons and elevators) and rudders to work well. The control mode is switched to RCS (Reaction Control System). These are small thrusters that will allow us to control our flight in the vacuum of space. There are two modes for the RCS, rotation and translation. Rotation does what the word means. Our craft will rotate around its central axes when the thrusters are applied. The second mode is traslation, or linear thrust. This controls our craft in the x, y and z directions; left/right, forward/back, up/down. After turning on the RCS, we let the nose continue to drop until our altitude is ~70 km and our nose is slightly above the horizon. We'll hold it here as we continue to build speed while slowly gaining altitude:



While we wait for main engine cutoff, let's go over some of the basic orbital mechanics we'll be using to rendezvous with the ISS. Here are a couple diagrams to help you visualize some of the concepts I'll be talking about:



Let's start with Figure 1. We've already covered inclination, so we'll begin with with apoapsis and periapsis. Apoapsis is the point that is furthest from the body you are orbiting, and periapsis is the closest point in your orbit. Looking at the diagram, you can see that the apoapsis is further from the earth than the periapsis. When you do burns to manipulate your orbit, you usually do them from either apoapsis or periapsis. Next up are the ascending node and descending node. These are the points where your orbit intersects your plane of reference, which in this case is the Earth's equator. The ascending node is the node where you intersect the plane of reference going north, and the descending node is where you intersect it going south. Why are these important? Well, to change the inclination of your orbit, you perform your burns at these nodes. Keep these terms in the back of your mind as we go on to Figure 2.

Figure 2 shows a spacecraft in orbit around the Earth. When our ship is pointed in the direction of our motion, this is called prograde. When it's pointed opposite to it, the ship is retrograde. Burning in either orientation will affect our orbital parameters. Prograde and retrograde can also refer to how the planets rotate on their axis and orbit around the Sun. Earth has a prograde rotation, meaning it rotates west to east in the same manner as the Sun. The majority of the planets in our solar system also have prograde rotations, except for Venus and Uranus. Both have retrograde rotations. We also have a prograde orbit around the Sun, meaning we orbit it in the same direction that the Sun rotates, west to east. In terms of spacecraft, nearly all orbits will be prograde, or in the same direction as the rotation of the planet they're orbiting. This is much more fuel efficient since you use the planet's rotational velocity to help you when launching into orbit. We'll be going into a prograde orbit, so for our purposes we'll be using prograde and retrograde to refer to how our ship is oriented in regards to our velocity vector. On to Normal and Antinormal. Normal is the positive vector perpendicular to our orbital plane, and, you guessed it, antinormal is the negative vector perpendicular to our orbital plane. In order to change our inclination, we'll have to burn from either a normal or antinormal orientation depending on which node we're at. We'll do this by rotating the ship up +90 or down -90 degrees.

Now that we've gone over these terms, let's apply them to spaceflight. First, let's talk about speed. The further away a spacecraft is from the Earth, the slower it goes. Earth's gravitational pull on the ship becomes weaker. The closer a ship gets to Earth the faster it goes since Earth's pull is much stronger. Remember apoapsis and periapsis, the highest and lowest points of an orbit? These are the points we control to determine how fast we want go around the Earth. For example, if we want to have a slower orbital period, or the time it takes to do one orbit, we will first raise our apoapsis to a much higher point. To do that, we will burn prograde at our periapsis. Why at the periapsis? Any changes you make to your orbit affects the opposite side of the orbit. Burning prograde at the periapsis will raise the apoapsis, and burning retrograde at the periapsis will lower the apoapsis (Keep in mind that if you lower your apoapsis enough it will become the new periapsis). Likewise, burning prograde or retrograde at the apoapsis will raise or lower your periapsis (You can also raise your periapsis to the point it becomes the new apoapsis). Once we reach the new higher apoapsis point, we would burn prograde to raise the periapsis to an altitude equal with the apoapsis. This now circularizes the orbit at a much higher altitude, and we would be traveling slower with a longer orbital period than at our lower orbit.

That takes care of how we control our orbital speed and period. Now on to plane changes. As I've already alluded to, in order to control your inclination you do burns at either the ascending or descending nodes of the orbit. To change the inclination at the ascending node, you rotate your ship to burn antinormal. The easy way to remember this is AntiNormal at Ascending Node. Both can be abbreviated AN. If you do your plane change burn at the descending node, you rotate the ship normal.

Hopefully you now have a better grasp on the mechanics we'll be using to rendezvous with the ISS. Now let's get back to getting this Delta Glider in orbit!

We haven't quite reached orbital velocity yet. We're still on a ballistic trajectory, meaning if we cut the engines now we'll fall back to Earth because, technically, our periapsis is still inside of the Earth. We need to get enough speed up so that the periapsis clears the Earth's surface, and we'll continue burning so that it becomes our new apoapsis. The ISS orbits at about 360 km, so we'll push the apoapsis out to just above that. I like to get it just above because that will give us two orbital intersection points to meet the ISS once our planes are aligned, and it also gives us some slush due to orbit perturbations. Many other outside forces act on our ship that affect our orbit, not just the Earth!

We're just about there...and main engine cutoff!:



Not bad! Looking at the left MFD, which displays our orbit information (green) along with the ISS's (yellow), our apoapsis (ApA) is at 372 km. Our periapsis (PeA) is at 85 km. Once we reach apoapsis, we'll do a quick prograde burn to raise the periapsis up to about 220 km. At 85 km, we will still experience the effects from Earth's atmosphere. We want our orbit to be above 150 km in order to avoid any atmospheric effects. We are also trailing the ISS, which means we will need to have a faster orbit to catch it. After our burn at apoapsis, our periapsis will still be inside of the ISS's orbit, so we will be traveling faster than the ISS overall.

The other good news is that our relative inclination (RInc) with the ISS's orbit is only 1.24 degrees. You can see that in the right MFD. We'll be able to take care of that with a 10 second burn at the descending node (TthD). The goal is to get the RInc to zero. However, because our orbit will perturb, we will have to do periodic mid-course corrections to keep RInc that low.

Time to switch our HUD into orbit mode, and engage the autopilot to orient our ship prograde. We'll also deploy our radiator. This will keep our computers and other ship systems from overheating, since the heat will be radiated into space. Best to keep those systems cool. It would be very bad to lose them.

Now that we're in orbit, feel free to unbuckle your safety restraints and float about the cabin. Don't hog the windows either. Everyone will have plenty of time to view Earth from orbit before we reach the ISS. You're in store for some spectacular views.

 
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Rambler

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Everyone getting used to a weightless environment? If you're having trouble, you'll find some of those handy dandy throw up bags in the backs of the seats. Space sickness ain't fun.

Alright, we've made our nodal burns to align our orbit with the ISS's orbit, and we've also done our prograde burn to raise the periapsis. Our orbit is squared up enough to begin syncing our orbit with the ISS:



If you look at the right MFD, you'll see our RInc is now .04 degrees. This is good enough for now, as anything less than a RInc of .05 degrees is acceptable. We'll tighten that up the closer we get to rendezvousing with the ISS. Now, turn your attention to the left MFD. This is where we will sync our orbit with the ISS. This means that we want to pass the same point in space at the same time that the ISS does. Since our orbit goes slightly above the ISS's orbit, we have two intersection points that are candidates for rendezvous. We just need to find out which point is the most acceptable. The parameter we want to keep our eye on is DTmin. DTmin is the difference in time when the ISS and our ship pass the same reference point. We want it as low as possible, and we'll eventually make it 0.00 with some burns.

We're going to the ISS the fuel efficient way, meaning lots of orbits. Let's expand the orbit list to 16 orbits and see if there are any good DTmin times. The best ones will be highlighted in yellow. Also, Sh-ToR is Ship Time of Reference, meaning at what time we pass the reference point, and Tg-ToR is Target Time of Reference. Our target is the ISS, so it's the time that the ISS reaches the reference point. All numbers in the list are in seconds. Yep, it looks like for Intersect 1 orbit 12 provides the closest time for rendezvous with a DTmin of 29.64 seconds. We can easily get that down to zero with some small burns. I didn't show it, but Intersect 2 had a better DTmin in 13 orbits, and that's the one I went with.

After some short burns to get it roughly down to zero, it's time to wait 19.5 hours (thank goodness for time acceleration) until we intercept the ISS. Again, we'll have to do some mid course corrections to the DTmin due to perturbations as we go along.

Settle in, read a book, get some sleep, look out the windows, eat some wonderful freeze dried space food; it's going to be awhile before we rendezvous with the ISS.

Here we are over the Bahamas:



Another orbit takes us over New England. You can see Cape Cod, Boston, Long Island, and New York City.



Finally! We have one orbit to go until we rendezvous with the ISS! Let's do one last check to make sure everything is in order:





DTmin looks good at .08 seconds. We'll kill the last remaining eight hundredths of a second this orbit. Looks like we'll also reach our rendezvous point in about an hour and 15 minutes. We've also set up our Com/Nav receivers and tuned them to the appropriate frequencies. We're actually starting to pick up the ISS's signal, although it is faint at this point. We've also put in the NAVAID frequency for the ISS dock we'll be docking at (NAV 2). Tuning to this frequency and slaving it to our HUD will display a series of boxes on the HUD we'll have to line up with and fly through in order to dock. Very helpful. Finally, a glance at our planar alignment shows a RInc of .01 degress. I'm very happy with that, and we won't try to get it down to zero.

We're go for rendezvous as we head into the night portion of our orbit:



Just as the sun is peeking over the horizon, we get our first glimpse of the ISS over the Canadian Rockies. There she is! That little speck 17 km away:



However, we're not ready to dock yet. We're moving a little faster than the ISS, and our relative velocity is 81 m/s. We're going to have to kill that when we reach the intersect point to set up in ISS's orbit and prepare for docking. Only 3.5 minutes left...

3...2...1...

Throttles are maxed, and our relative velocity quickly decreases. Engines are cut with a final relative velocity of .2 m/s. We are now 2 km from the station and, for all intents and purposes, traveling at the same speed. Let's rotate the ship around and point it at the ISS. Man, that's a sight!:



Now it's time to start maneuvering toward the station and get lined up for final approach. The docking port we are going to be docking at is Dock 1, and it is on the far side of the station from where we are. We're going to have to fly past the station, and then a little below. From here on out we'll be using translational thrust, since using our main engines near the station can severely damage it. Forward translational thrust is implemented, and we begin moving toward the station at about 10 m/s:



We're aiming for the end of the docking gates. Once we reach those, we'll kill our forward speed and orient our ship for final approach. Easier said than done, since the ISS is slowly rotating itself. By the time we reach the gates where they are displayed now, the docking port will be pointing more toward Earth, forcing us to fly further down to hit the gates. Here we are getting into position for final approach over the Amazon River's mouth:



Success! Somewhere over the coast of South America, we get properly aligned for final approach:



Now comes the most nerve-wracking part, final approach and docking. Easy right? Just fly it straight to the dock. If only it were so easy. Remember I mentioned those other forces that perturb our orbit, like gravity gradient torques, the moon, etc.? Well, they're still affecting our ship as we try to dock. Not to mention we also have to deal with ISS's rotation. Imagine trying to make love to a woman in the back of a rally car going 80 mph cross country in the dead of night. That's pretty close to what docking is like. It's going to be a constant ballet act involving translation and rotation thrust. We're only 350 m from the docking port, but it's going to be a LONG 350 m. Here we are all set to start our run:



The docking MFD on the right is what we'll be religiously monitoring. The goal is to keep the white X, which is your pitch and yaw, in the center. The yellow cross also needs to be kept in the center. This represents the dock, and we'll be using translational thrust to try keep it there. The final indicator we need to watch is the white arrow at the top. This represents your roll, and you need to keep it pointed at the top of the circle. All of the indicators will turn red when you're out of parameters. Before we apply forward thrust, we'll need to open our nosecone to expose our docking mechanism.

Ready? Let's go. Burst of forward thrust that accelerates us up to 5 m/s. Rotation. Burst. Translation. Burst. Rotation. Burst. Burst. Burst. Translation. Burst. Burst. Rotation. Burst. Burst. Translation. Burst. Burst. Burst. Burst. Rotation. Burst. Burst. Translation. Burst...Over halfway there:



Rotation. Burst. Burst. Translation. Burst. Burst. Burst. Burst. Burst. Rotation. Burst. Burst. Burst. Translation. Burst. Time to start slowing our rate of closure to about .01 m/s, the speed we'll dock at.

Translation. Burst. Burst. Rotation. Burst. Burst. Burst. Burst. Translation. Burst. Rotation. Burst. Is this over yet? Argh! Translation. Burst. Burst. Rotation. Burst. Burst. Almost there!

Rotation. Burst. Translation. Burst.

Capture. Hard Dock. Sounds of the capture mechanism anchoring the Delta Glider to the ISS fill the cabin. We've finally arrived at the ISS!



We docked just in time too, as we are fast approaching nightfall. We now open the outer and inner doors of our airlock to allow passage into the ISS. Now that that is complete, it's time to power down the Delta Glider's systems. RCS Off. MFDs Off. Retro doors closed. Nav lights Off. With everything powered down, it's time to make our way into the station. And what a sight to welcome us; an orbital sunset:



Coming back into daylight finds us over Yellowstone National Park:



And later, over Houston, TX (on the left). Thankfully, Houston, we have not had a problem:



Enjoy your time on the ISS!
 
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Rambler

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Thanks! Yeah, it's hard to do an AAR about spaceflight without getting somewhat technical simply because it is technical. I tried to simplify it as much as possible, which is why I drew those diagrams and had a section on the basics so when I threw terms out there people weren't going, "wtf is he talking about?" :D.
 
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