The Whys and Whats of Rocket Science
We’ve all heard of the common phrase: “It’s not Rocket Science!”. It’s usually introduced when you don’t think something is as difficult or complex as, well, Rocket Science. Things seem clear and simple when juxtaposed with Rocket Science, though most people aren’t aware of its own simplicities and benefits to our society. I’m here to teach you both!
The whys and whats of Rocket Science are presented below and can be digested separately, though it’s recommended that both are read sequentially.
Why rockets?
The history of rocketry dates back to the 13th century, where most developments were for military purposes, such as the Chinese “arrows of flying fire” in the battle of Kai-Keng. It wasn’t until 1903 that the use of rockets for space exploration was conceptualized by Konstantin Tsiolkovsky, a Soviet rocket scientist.
The access to and exploration of space is the reason why rocket science is important. Let me explain below.
Since that turning point, there has been a wave of interest from government agencies and aerospace conglomerates to put things (called payloads) in space and improve one thing: the cost of putting one pound of payload into orbit. These payloads can be in the form of satellites, more rockets, human-residing spacecraft, etc. Although there are various orbit types, we will focus on Low Earth Orbit (LEO), which is the most common orbit reached. Currently, the Falcon 9 can deliver a payload to LEO with a cost-to-weight ratio of about $1235/pound. By improving this ratio through the development of spaceflight technology, many of the following benefits and dreams can be realized:
Profitable endeavors
- Resource mining — “trillion dollar industry”
- Telecommunications — GPS, internet for all, cell service, etc
- Suborbital transportation — anywhere on earth in less than an hour
- Agriculture — Earth-imaging and crop state monitoring
Entertainment
- Space tourism — get to space for $200k-$300k or a slingshot around the moon
- Human exploration — manned spaceflight past the moon.
- Planet/Moon colonization — Moon / Mars base
Science
- Satellite exploration — survey probe sent to planets
- GIS — social sciences and geosciences (like meteorology)
- Research in space
What about rockets?
Now, it’s time to get into some basic concepts (with some equations) that constitute the fundamentals of Rocket Science.
All forms of propulsion, and thus rocketry, starts with Newton’s Third Law: For every action, there is an equal and opposite reaction. For example, if I throw a ball with some mass at some acceleration, it simultaneously applies a force on me, causing me to move backward. We’ll come back to this.
Thrust Equation
If you took an introductory physics course, I’m sure you’d remember Newton’s second law:
The product of mass and its acceleration is the force required to move that mass at its acceleration. The units of this equation are Newtons or kg*m/s². As long as the resulting units are consistent with Newtons, then any combination of variables are mathematically correct. However, the combination must also be physically coherent to be useful. For example, it’s hard to imagine the physics of two variables of kg*m and 1/s², respectively.
There are different types of rockets, but the most common is a chemical rocket engine. In this type of engine, propellants are combusted through a high energy exothermic reaction in a combustion chamber; this means that heat and pressure in the chamber are increased as a result of the reaction. Next, the hot gases that make up the combustion byproducts are accelerated through a nozzle. Let’s try to apply Newton’s second law to this system.
To start, imagine a plane located at the nozzle exit and perpendicular to the combustion chamber’s walls. Let’s introduce a new variable called mass flow rate, in units of kg/s. Conceptually, it is the amount of mass that travels through this plane on a per second basis.
Some may wonder what the speed of this mass is as it travels through this plane. We have a variable for that too! It is called exit velocity, in units of m/s, which is the velocity of the combustion byproducts at the exit plane.
What if we multiply these two terms? The mass flow rate times the exit velocity of the hot gases. The product of the units is kg/s * m/s = Newtons or the units of force. The result of this evaluation is a simplified version of the Thrust equation.
Back to Newton’s Third law: For every action, there is an equal and opposite reaction. Therefore, if we point the rocket nozzle and thus force in one direction, then an equal and opposite force will be applied, causing the rocket to move, upwards, and hopefully into orbit.
Final Note
The industry is concerned with the following two objectives: 1) reliably producing a lower cost, lower weight rocket that 2) maximizes the product of these terms. Sounds simple, but it’s kind of hard. NASA has conducted and will continue to explore experiments which investigate ways to optimize the latter objective. With the advancement in production, private companies can now stand on the shoulders of NASA’s accomplishments and focus on the former objective.
With companies like SpaceX, Blue Origin, and Virgin Galactic, the next race to low-cost spaceflight can come to fruition as we look to the stars for our future.
Thanks to Tekil Persaud and Royce Prescott for feedback on this post.
