The phrase orbital mechanics– like nuclear particle physics or the theory of relativity– is something that makes many individualss eyes glaze over. The typical person believes that subjects like these are far too intricate to comprehend. However if you remove away all the math and merely try to comprehend what is going on, they are actually not that hard to grasp. As for orbital mechanics, it is the physics of spaceflight– it describes how objects move relative to one another in space. Its easy to make a spacecraft do what you want it to do when you comprehend the mechanics of how worlds and spacecraft communicate with gravity.
If youre a significant league pitcher, your throw will probably go further than mine since it releases off your hand faster. Now, lets construct a maker to toss the ball much faster– it will go farther, of course … but the ball will constantly curve down and hit the dirt. With every boost in speed, the ball goes further.
Naval weapons can fire projectiles so far that they review the horizon before falling back to the earth (hopefully on their targets). That phrase, over the horizon, is crucial to understanding orbital mechanics. The spherical shape of Earth means that as you take a trip horizontally, the surface area is constantly curving down. If you can throw the item quickly enough that its drop, its ballistic course, is equivalent to Earths curvature, then it will never hit the world– and voilà– it has gone into orbit!
It takes a tremendous amount of speed to reach that point where your item is not going to strike the earth– in the neighborhood of 25,000 feet per 2nd (thats over 17,000 miles per hour) if youre speaking about flying in the region called low-Earth orbit (LEO). Lets call that anywhere from about 100 miles above the surface to about 400 miles– take or provide. Thats where the Space Shuttle did all its work. Eventually it will go so quickly that it never ever falls back to Earth if we take our fictional baseball or projectile and keep upping the speed. The speed at which that occurs is referred to as escape speed. We never fretted about escape speed with the Shuttle– it didnt have the capability to go that quickly. Unlike the Apollo spacecraft that took males to the moon, the entire Shuttle spacecraft was constantly coming back to Earth. It would have taken roughly its own weight in fuel to move the Orbiter to the moon. (I asked my navigators to figure that out one night …).
So the first aspect of orbital mechanics isnt that hard– make a things relocation quickly enough relative to the planet, and it will never strike the world. It will simply keep going around and around and around– until, naturally, something slows it down to where it begins dropping towards the surface area. What might slow it down? Well, something may be facing the thinnest wisp of the atmosphere, the widely spaced particles of gas that grab hundreds of miles into area. The environment doesnt simply end suddenly, it slowly gets thinner and thinner as you move away from the worlds surface. It never ever really disappears completely, it just ultimately fades in density till it no longer has any effect. Just about 100 miles above Earths surface area, there suffice air particles that if a spacecraft faces them, an infinitesimal quantity of energy is lost with every crash. You cant determine the energy from any one accident, but if you build up enough of them you can eventually discover that you have lost some speed. Which speed loss builds up.
The slower you go, the more you fall back to Earth. If you dont add some velocity with a rocket motor burn every when in a while, you will not remain in orbit. It does not take a lot– just a couple of feet per second every day– but if you do not account for it, your objective isnt going to last really long.
The faster you go, the greater you go. The slower you go, the lower you go (till you fall out of orbit and are captured by the planets atmosphere).
To get the Shuttle into orbit, you have to do two things: get it out of the environment and accelerate it to orbital velocity. Each SRB put out about 3 million pounds of thrust, for a combined overall of 6 million pounds. By contrast, the three Shuttle main engines contributed another half-million pounds of thrust each, for an overall of 1.5 million pounds– a much smaller part of the overall thrust readily available at liftoff.
When the SRBs dropped off, the lorry was up where the air was not a real aspect, however it was just going a couple thousand feet per second horizontally. It was now the task of the primary engines to accelerate the ever-lightening stack (the Orbiter and External Tank) to that magic number of 25,000 feet per 2nd to get it to stay in orbit. Thats a consequence of Newtons standard law of motion– if the force remains the same and the mass reduces, the velocity goes up!
In the most basic terms, when you reached the wanted velocity to make orbit, you shut down the engines and drifted into orbit. It sounds simple– however it isnt. Lets stretch our knowledge of orbital mechanics a bit. Lets presume that you are in a circular orbit– the same altitude above Earth at all points in the orbit. If you decide that you wish to go higher, then you have to increase your speed. This is finished with a thrust event, referred to as “doing a burn,” due to the fact that we thrust by shooting– burning– an engine. If you squint and allow yourself to approximate, doing a burn to increase your speed by 1 foot per second will increase your altitude by about a half mile. Thats not an instantaneous gain– what you are in fact doing is driving yourself uphill up until you reach that brand-new elevation, which you will reach when you are midway around Earth. However you will not stay there. Consider the ballistic path that a ball takes when you toss it– it very first boosts in height, then gravity pulls it back down, and so it returns down again. The same thing takes place when you increase the Orbiters velocity– it will go uphill to the new elevation, but it ultimately returns to where it began … right to the altitude where you increased the speed. It so occurs that youll reach your new elevation midway around the world, and after that youll be back where you started when you finish the orbit. It will continue in this elliptical course for as long as you let it..
However, if we wish to raise the orbit all the method around, we can merely thrust again by the same amount when we reach our new height (referred to as the apogee). We will have raised our altitude at the beginning point by a half mile as well, suggesting that we will be in a new circular orbit a half-mile higher than when we began the set of burns. We will have also increased our speed by 2 feet per 2nd total.
The mathematics is really practical if youre attempting to do it in your head– if you wish to raise the orbit by 10 miles, you merely burn 20 feet per 2nd (fps) initially, then another 20 fps at the brand-new apogee, and voilà– youre in a brand-new circular orbit 10 miles higher, and it cost you a total speed modification (referred to as delta V) of 40 fps. Raising and decreasing the orbit is how you execute a rendezvous. For now, its important that we get into, and then understand how to get out of, orbit.
Lets take a look at the very end of the initial launch. There we are, thrusting all 3 main engines, speeding up at 100 feet per 2nd every second. Understanding what we now learn about orbital mechanics, we understand that for every single second we burn the main engines at this point, we are raising the orbital altitude by 50 miles when we navigate the world. We need about 100 miles of altitude to reliably avoid of the atmosphere– consider it the minimum safe elevation we desire to wind up in. The International Space Station is at an elevation of about 200 miles, the Hubble Space Telescope is about 350. The Orbiter resided in the altitude band between 100 miles and 350 miles– a distinction of simply 250 miles. In terms of orbital insertion speed, that is simply 5 seconds of burn time.
A full ascent, from the launch pad to Main Engine Cutoff (MECO) was about 8 and a half minutes, or about 510 seconds. The orbital altitude series of the lorry suggested that cutoff would be plus or minus 5 seconds, which is an extremely small portion of that overall burn time. Miss it by 1 percent and we were either not in orbit or we were going way expensive, without enough thrusting ability to circularize the orbit– or to get home. So MECO was critical– you had to time it exactly right in order to get precisely into the orbit you wanted, and we considered precise to be within a couple of miles.
It doesnt go from complete thrust to zero in an immediate– it tapers off. When you look at the hundreds of variables involved, you rapidly realize that its going to take a lot of wise people to figure this out. In the Apollo and Shuttle eras, we required to have precise control of where we were going to end up– and the accumulation of rocket flight experience made that possible.
Now those who are following carefully have actually already found out that spaceflight is a lot more complex than this. Recall the essentials of orbital mechanics, bearing in mind that what increases must boil down. If we have actually thrusted ourselves from the ground up to an orbital altitude, state 200 miles, we are just at our apogee. Like tossing a ball directly into the air, were going to be coming back down to our beginning elevation. This will take place when we get all the method around Earth. The nitty gritty, of course, is that we require to do some shaping of the trajectory to make sure we arent going to come all the method back down to the ground. Remember we require to travel horizontally at a high enough speed so that we do not fall back to the surface. We need to do a burn about midway around the planet if we actually want to end up in a circular orbit (and not an ellipse). Because we go around the world in ninety minutes, it means that forty-five minutes after launch we require to do that burn– and we cant utilize the main engines to do it. For that, we change to our Orbital Maneuvering System (OMS) engines.
You can burn them together, or separately, depending on how much thrust you need and how finely you want to handle the final velocity. These engines burn propellant saved in their pods, the very same kind of fuel and oxidizer used for the attitude control jets– in truth, the tanks for the OMS and Reaction Control System (RCS) jets can be shared (or interconnected) in between the 2 systems, if requirement be. Because these engines are so much smaller than the primary engines in terms of thrust, you have to burn them longer to get the same quantity of velocity change– in reality, the velocity available from the OMS engines is hardly visible inside the vehicle.
Getting back to circularizing the orbit. In the earliest Shuttle flights, we were pleased to see that it worked to keep us in orbit– later on, we had discovered enough about trajectory shaping and burn times that we were quite unhappy if we missed our orbital criteria by more than a couple miles.
When you made it to orbit, changing that orbit was merely a matter of adding or deducting speed by adding speed with a burn or taking it away (you did that by thrusting backwards). By now, nevertheless, you can see that to bring the Orbiter home from a 200-mile orbit, you need to drop the perigee (the low point of the orbit) by about 120 miles (200 minus 80), and to do that, you require to slow it down by about 240 feet per second. If you filled the Orbiters tanks before launch, then you had roughly 600 feet per second overall orbital maneuvering ability (the overall delta V) that could be utilized throughout the objective– to raise and to decrease the orbit.
It takes a remarkable amount of speed to reach that point where your object is not going to hit the earth– in the area of 25,000 feet per second (thats over 17,000 miles per hour) if youre talking about flying in the area known as low-Earth orbit (LEO). For now, lets remember that if we go fast sufficient horizontally, we end up orbiting Earth rather than falling back to the surface area. The faster you go, the greater you go. The slower you go, the lower you go (until you fall out of orbit and are caught by the planets environment). Miss it by 1 percent and we were either not in orbit or we were going way too high, without enough thrusting ability to circularize the orbit– or to get home.
