Space Shuttle Abort Modes quickly explained

There are 2 categories of abort scenarios in the beginning of a shuttle flight, a pad abort, and an ascent abort.

PAD ABORT

A pad abort is precisely that, a launch abort that occurs while the shuttle is still firmly bolted down to the pad and SRB ignition hasn’t taken place. In any point of the countdown, an abort may occur, be it because of weather conditions or systems malfunction scrubbing the launch, or because the GLS (during the last 9 minutes of the count up until 31 seconds remaining) detected a fault and paused the countdown (or was commanded by a flight controller to do so), or because the RSLS gave a cutoff to the launch in between T-31 seconds and T-0.001 seconds (not kidding. These computers are precise!). Many missions have been scrubbed in many points in the countdown, particularly when tanking is about to occur, or during the Launch Status Check (go/nogo for launch) during the final 9-minute hold. But only 5 missions have had these aborts, and 4 of those have taken the crown for being the most stressful of all aborts: right between main engine start (T-6.6 seconds) and SRB ignition (T-0). Below is a video of all 5.

In most cases, the Redundant Set Launch Sequencer program on the shuttle’s onboard GPCs, gave the cutoff because one or two of the three main engines did not ignite. In other cases, it might have detected a leak within the engines, or a fault on a critical system. When the RSLS gives the cutoff, it is known as an RSLS abort. This abort is most stressful when the engines actually fired, be it one or 2 or all three and then it gets cut off. The shuttle starts to sway with the sudden thrust, and keeps swaying for a couple of minutes until all motion stops. The astronauts onboard know it is only 8 explosive bolts holding down all 4,400,000 pounds of the shuttle in its upright position via its SRBs. It is not a pretty scenario for them when suddenly the shuttle sways forward and back for a few minutes, straining against those bolts. Luckily, no one was hurt in these 5 aborts. Well, the first 4. The fifth never even got to fire the engines (cutoff was at T-7.5 seconds), so no swaying occurred. These missions were attempted later, and succeeded.

ASCENT ABORTS

From T-0 to MECO, there are different abort scenarios that can occur depending on the kind of failure and the altitude/speed/position of the shuttle. In this category, there are 2 subcategories, intact aborts and contingency aborts. In order to select an abort, when called to do so, the commander selects the abort mode in a rotary switch and presses the ABORT button to execute the selected abort. The shuttle then does a pre-programmed set of commands that will put it in the right path and configuration to that specific abort (accelerating remaining engines to max, dumping unnecessary fuel, reorienting the shuttle, releasing the external tank, etc.). This usually happens when an engine, or 2, or all 3 are lost, or a critical system malfunction occurs (life support, APUs, Fuel Cells, etc) during ascent. I will name them from most desirable to least desirable.

ATO: Abort To Orbit. This mode isn’t a real abort, as it actually makes the shuttle reach a lower than planned, but safe orbit, thus requiring just an OMS burn or 2 to raise the orbit to the right one so the mission can start. The moment when this abort becomes available is when CapCom says “Press to ATO”. (no, not the company that makes Street Fighter or Mega Man. This is the CAPsule COMmunicator, the guy whose job is to relay orders and info to the astronauts and receive replies by them. He is the only one who ever talks to the shuttle, and anything that must be told to the astronauts has to go through him. It’s usually an astronaut, because they believe an astronaut would convey info/orders in a way that the crew would understand, being an astronaut himself.)

AOA: Abort Once Around. This abort mode makes the shuttle make one complete orbit before reentering the atmosphere. There’s a short margin in which this abort is possible, right before ATO is possible. However, a (not so critical but meaningful) emergency might cause them to execute this abort anyways. This abort is available also after MECO (Main Engine Cut Off) when needed. It can land on Kennedy Space Center, Florida, Edwards Air Force Base, California or White Sands Missile Range, New Mexico (not preferred since the sand can damage the orbiter, as seen on STS-3, however, the facility’s been upgraded to permit a landing with the least amount of complications should the need for it arise.). Theoretically it can land on any runway, considering it’s long and wide enough, should the emergency arise. This applies to all intact abort modes, except RTLS.

TAL: Trans-Atlantic Abort. This abort mode is selected when ATO is unreachable, and RTLS is either not needed or already unavailable (“Negative Return”). The shuttle has a ballistic trajectory that has it crossing the Atlantic Ocean for a landing in Europe/Africa friendly bases.The ones being selected for TAL sites right now are Moron and Zaragoza Air Force Bases in Spain and Istres Air Force Base in France.

RTLS: Return To Launch Site. This is the fastest, and most dangerous intact abort mode. If an engine failure before TAL capability is reached, or an extreme emergency requiring immediate landing occurs (such as cabin leak), RTLS is activated. If the SRBs are still latched onto the shuttle, it waits till the SRBs are detached and clear, then it executes the abort. If the SRBs are already clear, it immediately executes it. It pitches the shuttle around to aim back at Kennedy Space Center and burns the remaining engines, OMS engines, and RCS thrusters into the flight path until downrange speed is killed. It then accelerates back to Kennedy, releases the tank so it still falls into the ocean, and glides back to Kennedy Space Center as if it were a normal descent and landing. The danger comes with the pitch around of the shuttle while still burning its main engines, resulting in a lot of gravitational stress to the crew AND orbiter, not to mention aerodynamic forces still present at those altitudes, although NASA says it’s already at a sufficient altitude for those aerodynamic stresses to be a non-issue.

The aforementioned aborts are the “intact” abort scenarios, where both orbiter and crew are safely recovered, well, intact. However, if a catastrophic malfunction occurs (all-engine malfunction, multiple APU/fuel cell malfunction, etc), which leaves the orbiter incapable of reaching a suitable landing site, a contingency abort is made. It simply consists of attempting to put the shuttle into a stable glide, aiming at open ocean, until it reaches the survivable altitude and speed limit. There, the crew pops the side hatch, slide a pole out, and bails out of the orbiter, the pole clearing them from the shuttle (they have parachutes latched onto their orange launch and entry suits. Their suits are called the Advanced Crew Escape Suit). There, they parachute down to the ocean. Their backpack also contains survival gear, including a personal life raft. There they wait until search and recovery forces pull them out of the water. As for the orbiter, it is ditched into the ocean.

The same thing can happen if this failure occurs during/after entry interface. Of course, the same survivable speed and altitude limit criteria must be met before bailout is possible, unlike STS-107 Columbia’s case, where the speed and altitude were too great for bailout to be survivable (and, consequently, had their unfortunate deaths for granted, as they could only sit and wait until the crew module broke up) when the orbiter breakup began. Popping that hatch open would have instantly killed the astronauts because of the plasma generated by the high speed reentry into the atmosphere, EVEN with their suits on and sealed. In fact, they found melted pieces of the suits. PIECES. And the body parts supposed to be inside the pieces miles away from said suit pieces, and in several degrees of heat/dismembering damage (a woman heard a noise on that day and went out to find her dog chewing happily on a charred body part. This is no joke. Look it up.) A suit designed to keep your body at a stable temperature even under severe temperatures, MELTED. And they were still being protected by a significant portion of the crew cabin as it broke up. Think about how bad the day was for the person INSIDE the suit when the heat went through the suit. Even worse when the crew cabin broke up and what was left was exposed to the limb-severing, flesh-ripping airstream of 12,500 MPH. And its associated flash-burning due to the friction. IF somehow the suit would have held together, it still didn’t solve the fact that they were still flying at over 200,000 feet, and because of their immensely reduced weight (230,000 pounds of the orbiter to a measly 200 pound guy), their dropping speed would be at an all-time low, and their oxygen reserve would run out before they reach breathable air. May I remind you that the guys who climbed all 29,029 feet of Mount Everest, Earth’s tallest mountain, needed OXYGEN MASKS/TANKS to stay there. 200,000 feet is like vacuum in comparison. May I also remind you that altitudes in excess of 26,250 feet is called “The Death Zone”, since a person slowly suffocates to death, no matter how used he or she is to low air pressure. The body has its limits on how far it can adapt before it gives up, as stated in this article: http://en.wikipedia.org/wiki/Death_zone, and I quote, “Finally, in the "death zone" at 7,000 to 8,000 m (23,000 to 26,200 ft) and higher, no human body can acclimatize. The body uses up its store of oxygen faster than it can be replenished. An extended stay in the zone without supplementary oxygen will result in deterioration of body functions, loss of consciousness and, ultimately, death.” That’s at 26,250 feet. 200,000 feet is like holding your breath and never breathing again until your brain stops. Or being hanged by the neck. Same result. Death by suffocation. (And somehow, there is still a sufficient airstream to rip you apart at 12,500MPH. Nature is clinically bat-shit insane. But so are we for messin’ with it xD)

So, popping the hatch and bailing out at insane speeds and altitude is a big no-no.

And on that note, for my next blog… …REENTRY. …and landing.

…fricken tiles.

Orbital Mechanics Extremely Simplified

After reading this, you’ll understand how is it that something stays up there and doesn’t come crashing down unexpectedly.

An orbit is, as taken from a dictionary, “the path of a celestial body or an artificial satellite as it revolves around another body”. IT is exactly what it means. Something just going in circles around something. No, not your kids when they attempt to burn you at a makeshift stake. A CELESTIAL body or an artificial satellite. An example of a celestial body is… our very own planet Earth!

Why? Because it’s going in circles around the sun. It’s a 365.25 day long circle, but a circle nonetheless. An easier example is the moon, going around the Earth.

An example of an artificial satellite is, well, one of many satellites you might be using to watch your favorite TV channel. When a shuttle launches and performs its mission in orbit, it is, well, IN orbit, so it’s considered an artificial satellite at that point. The International Space Station, or ISS, is also a perfect example of an artificial satellite. The ISS typically has a roughly 90-minute orbit, meaning it can do 16 laps around the Earth in a day. They have 16 sunrises and 16 sunsets per day. Talk about a ‘fast-paced environment’!

So how does it stay up there? Is it because there’s no gravity?

NO. Even at 200 miles above the Earth, there’s 80% gravity. So, how come the space shuttle, satellites and ISS stay up there without using any engines to stay up?

Two words. Permanent freefall.

Grab a transparent plastic cup. Put 2 or 3 colored paper clips. Cover the cup. Now throw it up, but not too hard. You’ll notice that when it reaches the top part of the climb, when it drops down again, the clips inside seem to float before flying towards the top of the cup. It all happens extremely fast, but you should notice that. In that micro-instant, the clips were floating as their falling speed matched the cup’s. That instant is known as free-fall, or as others would recognize the term, the feeling of weightlessness.

When astronauts go up to space, they have already experienced weightlessness so they know what they’re dealing with, but how do you recreate that feeling on Earth? Simple. Grab a plane, go up real high, then let the plane drop. Anyone inside the plane will immediately feel weightless, and everyone inside will float… until the plane runs out of altitude to keep dropping from, so it’ll have to pull up to avoid crashing into the ground, and immediately, everyone will feel heavy and drop down. NASA has a contract with a company called Zero Gravity Corp that does exactly this. They also give shorter rides to paying customers for around $5000 a ride (go http://www.gozerog.com if interested). They’re shorter because the sensation of going weightless and going back to weighting a lot again will cause nausea to people with queasy stomachs (hence, the airplane’s name, the Vomit Comet). Astronauts are trained to bear this so their training rides are longer.

Now, we threw the cup straight up. The same can be achieved if you throw it up at an angle, which would be similar to a shuttle blasting off into orbit. The harder you throw it, the more time gravity takes to pull it down to the ground. Even harder, and gravity takes even longer. Do you notice a pattern?

…eventually, throw an object hard enough, and gravity will take from almost forever to forever to pull it down, and this object is considered being in orbit around the Earth. This is how the shuttle achieves orbit. It accelerates to insane speeds, speeds where gravity will never be able to pull it down, but still has a sufficient hold on it to keep it going around the Earth. Thanks to the upwards of 17,000 MPH speeds acting against the 80% pull of gravity, the shuttle remains in orbit, and doesn’t slingshot away from the Earth. Higher speeds will result in higher, longer orbits, until you reach what is called “escape velocity”, in which you’re going so fast, the Earth’s gravity can no longer pull you back and you just keep going into space. We don’t want that to happen, so they stay within 17,250~17,500 MPH.

A quick note. If the shuttle can stay up without engines if travelling fast enough, why does it have to be at such a high altitude?

Atmospheric drag and friction.

Run a car to 65MPH. Crank the window down and put your hand out, the palm of your hand aiming at the airstream. You will feel that the air is pulling your hand backwards. The air, when one is going fast enough, actually acts as a braking force. At higher Mach (faster than sound) speeds, the air then causes heating due to friction of the air particles running through the object. The shuttle would be a fireball if it were travelling at such speeds at a low altitude. Not good. That’s why it must ascend (climb) to where there are no air particles whatsoever: above 400,000 feet. THEN it can accelerate undeterred AND stay in orbit.

Alright, so now we know how the shuttle stays up there. Quick notes on 2 orbit terms. There are usually some variations in the orbit altitude, and these are identified as “apogee” and “perigee”. Very simple terms. The apogee is the point in orbit where the altitude is highest. The perigee, always on the opposite side of the orbit, is the lowest point. Quick example, an orbiter reaches its apogee at 150 miles above the Earth. It then slowly decreases altitude until it reaches perigee on the other side of the planet, at around 135 miles above the planet. It then increases altitude until it again reached apogee, again on the other side, once again at 150 miles above the Earth, and the cycle repeats.

So, how does it change its orbit? Just accelerate and that’s it?

No. A simple rule applies. If you want to change an orbit, you must increase or decrease your speed precisely at either the apogee and/or perigee. Changing your speed in perigee will affect the apogee’s altitude. The other way around is also true. Changing speeds at apogee will change the perigee altitude. Increasing speeds will increase the other point’s altitude, and decreasing speed decreases the altitude at the other point. There might be irregular burns done at a point other than perigee or apogee, but these are rare, thanks to NASA programming their launches in a way that taking off at one point will identically match the orbit they’re trying to get to, meaning they only need to make certain adjustments to altitude in order to catch up to what they’re trying to reach (thus, saving fuel). The changes in speeds are measured in feet per second, since the changes in speed are subtle, but it affects the altitude greatly.

There’s another rule applied here when trying to catch an object in orbit. It’s called Kepler’s Law, actually, the third law. Basically, and as an example, an object travelling at 17,250 miles per hour at an altitude of 125~150 will actually travel faster than an object going at 17,300 MPH, but at an altitude of 225~230 miles. In other words, higher orbit means longer orbit, lower orbit means faster orbit.

To view this, it’s simple. Draw a small circle, and draw a bigger circle, and then an even bigger circle. You’ll end up with 3 circles, one inside the other. The smallest one is the Earth, and the bigger one is orbit 1, and the largest one, orbit 2. Then, trace with your finger at a slow speed, so you take 10 seconds to make a lap on orbit 1. Now, using that same speed, trace your finger around orbit 2. You’ll notice it took you longer than 10 seconds to make a lap.

That’s Kepler’s Law in effect. Assuming both objects on both orbits are going at roughly the same speed, give or take a few hundred miles, the wider the circle (the higher the orbit), the more time it will take to go around the Earth.

A ground example of this is a race. There are 5 tracks on a racetrack, and the racetrack has curves. However, the ones running on the inner tracks have a starting position that is behind the ones who are on the outer tracks. This is done because the ones on the inner track have less of a curve to go around than the outer track, so they clear these curves faster. The race coordinators calculated how far back each of the inner track racers need to start so if they all run at the same speed, when they come out of the final curve, everyone will be evenly matched.

This is used as an advantage to NASA to catch up to satellites or the ISS. The ISS is in an orbit of roughly 225 miles, give or take a few miles. When the launch window opens for the shuttle, it takes off, MATCHING the ISS’s ground track. Therefore, it only needs to go a bit faster to catch up to the ISS if it’s too far ahead. So the shuttle remains at a lower orbit, going slightly “faster” than the ISS in relation to the Earth, until it catches up. At this point, the shuttle changes its altitude, in a series of burns designed to roughly match the shuttle’s altitude with the ISS, precisely at the point where the ISS is very close to the shuttle, around 40,000 feet away. The shuttle then uses its RCS thrusters to close the distance and fine-tune and match its speed with the ISS, before it docks with it.

In orbit, the shuttle has several dimensions of movement that are unavailable/impossible for us who are at gravity’s mercy. It can roll, it can pitch, and it can yaw, and it can translate up, down, left, right, forward or back. Those movements are divided into 2 categories, Rotation and Translation.

For rotation, grab a tennis ball. Roll it forward or backwards. Now imagine it doing that while staying at the same place. Now imagine the shuttle doing that. That’s the pitch up and pitch down. Now, same exercise, but make the ball roll left or right. That is the roll of the shuttle. Once more, but this time, SPIN the ball to the left or right. This is the yaw of the shuttle. The rotation controls of the shuttle affect the attitude of the shuttle.

For translation, use the same tennis ball, only now, imagine the shuttle remaining in the same direction it’s pointing at, and that it’s simply drifting to the directions given. The shuttle can move forward, it can back up, or move backwards, just like a car. The difference is that it can also move left and right, unlike a car which can only go forward or back. And of course, it can move upwards and downwards like an elevator. The translation controls affect the position of the shuttle.

Rotation and Translation are combined to affect the attitude and position of the orbiter, and it can get to places like that. Usually, when docking to the ISS, it must maintain a precise attitude so the docking adapter can latch into the mating adapter of the ISS precisely and without any angles. And of course, it must be positioned exactly so that the mating adapter is centered with the orbiter’s docking adapter, before slowly closing in to dock.

Well, I guess that’s it. For my next blog, I’ll explain the abort modes available during ascent of the shuttle, and after that, descent and landing. I need to finish this before STS-129 Atlantis takes off in November…