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Thrust

A couple of posts ago, we discussed the idea of Newton’s Third Law: for every action, there is an equal and opposite reaction.  Let’s figure out how to mathematically apply that to getting a rocket off the ground and investigating how airplanes fly!

The main idea with thrust is that if you throw enough stuff out the back at a high enough velocity, it will propel you forward. It is therefore likely that the thrust that is experienced is somehow related to the velocity at which the material is expelled and the mass of the expelled material. This is almost exactly right, except that when we are talking about a rocket, or even an airplane, the stuff that is coming out is a stream of material, and so the mass is not really just a mass, but a mass flow rate (i.e., the amount of stuff per second). In other words, thrust is equal to the mass flow rate times the exhaust velocity, or:

thrust_equation

For a normal, everyday rocket, the exhaust velocity is roughly equal to about 3,000 m/s (meters per second, or about 6,700 MPH).  This exhaust velocity doesn’t vary very much, and it could be as low at 2,000 m/s or as high as about 4,000 m/s, but those are about the limits for a “normal” rocket engine. I will talk about why in further posts, but it mostly has to do with chemistry.

On the other hand, the mass flow rate can vary a large amount from rocket to rocket. That is because the only knob the rocket manufacturers have (if the exhaust velocity is about fixed) is the mass flow rate.  A smaller engine will have a smaller mass flow rate, and there won’t be able to lift large things off the ground.  A large rocket, like the Saturn 5, the largest successfully flown rocket ever, has a huge mass flow rate, and was able to lift a gigantic amount of stuff off the ground (elephants in space?).

My favorite example of a rocket is the V2, which was the first really “useful” liquid propulsion rocket ever made.   This was the rocket build by the Germans in WWII to bomb the Allies.  I will discuss these in more detail in another post also.  But for now, I would like to use this as an example to look at some aspects of thrust and getting a rocket off the ground. So, let’s get the stats on a V2 rocket:

  • With no fuel and no warhead, it had a mass of about 12,500 kg;
  • The warhead was about 1,000 kg;
  • It carried about 8,800 kg of fuel;
  • The mass flow rate about 110 kg/s; and
  • The exhaust velocity of the engine was about 2,400 m/s.
V2
A V2 Rocket. It was extremely advanced from about 1944 to 1957.

With this information, we can calculate some things about the rocket.  For example, the most important thing to calculate is whether the rocket can actually lift itself off the ground. To do that we calculate:

  1. The total mass of the rocket at launch, which is equal to the mass of the rocket, the warhead, and the fuel, which is 22,300 kg.
  2. The weight of the rocket, which is just the mass times the acceleration due to gravity (-9.8 m/s²), which would then be -218,540 N.  It is negative, since gravity is pulling down on it. In order to lift off the ground, the rocket engine has to produce more than 218,540 N of thrust.  That will just overcome gravity, and cause the rocket to lift into the air.
  3. The thrust of the rocket, which is given by the formula above.  We know that the exhaust velocity is 2,400 m/s, and the mass flow rate is 110 kg/s. The thrust is therefore 2400*110 = 264,000 N, which is larger than 218,540, so the rocket will definitely move upwards.

In order to actually calculate that rate of acceleration, we can do a bit more math, using Newton’s second law, which is F = m * a, or force equals mass times acceleration, but we can rearrange it to calculate the acceleration (a = F/m).  The force is the total of the weight of the rocket (-218,540N) plus the thrust of the rocket (264,000N), which is 45,460N.  The total mass of the rocket is 22,300 kg, so the acceleration is 45460/22300 = 2 m/s² upwards.

Now, the cool thing about this has to do with the mass flow rate.  The mass flow rate literately means that the rocket is becoming less massive every second. For the V2 rocket, it is becoming 110 kg less massive every second.  So, after one second, the rocket has a mass of 22,190 kg.  After two seconds, it has a mass of 22,080 kg. And so on, for about 80 seconds, until the rocket has a final mass of 13,500 kg. During the final second of thrust, the acceleration will be quite different than the first second.  In the last second, the rocket weighs about -132,300N, while the thrust is exactly the same as before (264,000N), so the difference is 131,700.  The mass of the empty rocket is 13,500 kg, so that the acceleration just before the thrust cuts off is about 9.8 m/s² upwards, about 5 times larger than the starting acceleration!  This increasing acceleration effect happens with all rockets that have a constant mass flow rate – as they use more fuel up, the accelerate upwards faster and faster.  It is no real surprise then that when rockets are sitting on the launch pad and fire their engines, they look like they are just crawling upwards – because they are!  As the rocket loses more and more fuel, it gains speed rapidly.

mass_flow
An illustration of a rocket shooting mass (80 kg – should be 110 kg for the V2 – oops!) at a given velocity (2200 m/s) every second. In 80s, the rocket is empty, but in that last second, it is accelerating upward fastest.

For manned missions, it is important that the acceleration not become too large, since people can’t withstand huge accelerations.  People tend to pass out when they are accelerated at rates that are about 5 times Earth’s gravity.  And the point of maximum acceleration doesn’t happen at the beginning, but near the end of the thrust.

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Ballistic Motion

Ballistic motion is an important concept in our path to understanding how rockets get up into orbit. Really, there are a few types of rockets: (1) rockets that just go up and then come right back down, otherwise known as ballistic missiles; (2) rockets that put something into orbit; and (3) rockets that take something away from the Earth and put it on a trajectory to somewhere else.  Each of these requires more energy than the last one, with the ballistic missile requiring the least amount of energy.

But, what is ballistic motion?  It is the motion that something feels when the “only” force acting on it is gravity. (I say “only” because often atmospheric drag is acting on it also, but we will ignore this for now.)

Let’s take a person throwing a baseball as an example.  Figure 1 illustrates a person getting ready to throw a ball.  (My son Alan drew most of the images again!)

Ball 1
A person getting ready to throw a baseball.

The person then moves their arm forward, accelerating the ball up to some speed.  Typically, this speed is roughly parallel to the ground. Figure 2 illustrates the person’s hand accelerating the ball (wow, that is a beautiful hand!)  When the ball leaves the person’s hand, it is moving with a velocity of Vx parallel to the ground.  In addition, gravity is acting on the ball, so it starts, immediately, to accelerate towards the ground at a rate of 32 feet/sec per second.  The ball will follow an arched trajectory, with the velocity towards the ground growing and growing all of the time, but with the velocity parallel to the ground (Vx) staying the same all of the time.

 

Ball 2
A person throwing a baseball. The person accelerates the ball up to some speed, then lets it go. At that point, it starts falling towards the Earth, but still moves with a speed of Vx parallel to the ground.

Because I am in America, where we are lovers of guns (although I am not), we should use a gun example! Imagine a person shooting a bullet towards a target.  If the person is far enough away from the target, or the bullet is slow, gravity will have enough time to pull the bullet down, and the person could miss the target.  A person far away from a target with a low-muzzle-velocity gun, has to aim upwards to compensate for gravity.

Target
A person shooting a gun directly towards a target will miss the mark, because gravity pulls the bullet down.

A much better example, in my opinion, is a catapult, which has an extremely slow speed, so that all objects need to have a very large upward velocity, in order to actually get the object to where you want it to go.

Cow
A catapult is a perfect example of a machine that relies on ballistic motion to crush enemies. With cows.

Ballistic missiles (or InterContinental Ballistic Missiles, ICBMs) operate on exactly the same principle as the baseball, catapult, or bullet.  Each goes through an acceleration phase, in which something is giving it an initial velocity (the rocket engine, which thrusts for a short amount of time). Then, the force cuts out, and the “only” force left is gravity. Gravity acts to decrease the upwards velocity down to zero, then causes the object to fall at faster and faster speeds. Just like the catapult.

Free Fall 1
A ballistic missile goes through an acceleration phase, then a free fall phase, where gravity is the only force acting on it.
Free Fall 2
A ballistic missile does not actually thrust through the vast majority of its flight!
Free Fall 3
When it lands, the ballistic missile is moving quickly, and typically causes quite a bang.

ICBMs are not the only types of ballistic missiles being developed right now.  There are many companies that want to take “space tourists” on a very fast ride (like 5 minutes).  These companies are creating reusable rockets that have a ballistic trajectory, taking the tourists to about 60 miles into the air, and bringing them back down safe and sound.

Ballistic Motion - Page 8
After the rocket engines turn off, the rocket is traveling under ballistic motion, so it is in free fall, and the people inside will be weightless.  That will continue until the rocket re-enters the atmosphere and the rocket is slowed down by atmospheric drag.  It is at this point, in which the people will experience the most acceleration! (This picture was drawn by me, and not by Alan. Notice the difference in quality. Which is vast.)

The rockets work exactly the same as ICBMs, in that they accelerate for a short amount of time (maybe 100 seconds), and then go into a free fall phase, where the only force acting on the rocket is gravity.

In reality, what happens next is that the rocket, which is well above the breathable part of the atmosphere, keeps going up for a while, reaches its maximum altitude, comes down, and re-enters the atmosphere.  At this point, the rocket is moving at very fast speeds, and starts to feel an incredible drag force.  The people inside the rocket actually have to lay down, since the forces acting on their bodies become so large.  The rocket is slowing down at such a fast rate that the people weigh about 3 times their normal weight.  Gravity is still acting to pull them down towards the ground, but the drag force is rapidly slowing them down.

The space tourists get a large force on them as they take off, and an even larger force on them when they re-enter the atmosphere.  It is truly a wild ride!