What Makes a Rocket Soar?

Since the launch of Sputnik 1 by the Soviet Union on 4 October 1957, the world has been entranced by the idea of traveling in deep space. For the past six decades, humanity has watched all manner of rocket and shuttle be launched into space for missions in low earth orbit, to the very edge of our solar system. However, despite the grand ambitions of generations past and present, wide scale space travel and exploration is still not commercially viable, instead relying primarily on government funded space agencies, like the US’s own NASA. Part of the problem is the complicated, not to mention expensive, process of creating and launching a rocket into outer space. This blog post will cover the chemical basis of that exact process, describing in detail the various fuels that make a rocket zoom.

Rocket technology does not consist of one piece, and neither does a rocket’s propulsion system. To get a rocket into orbit, or out of Earth’s influence altogether, requires a combination of different fuels and propellants to best suit the situation. In the following section, I will cover some of the most common fuel sources, as well as some interesting experimental ones.

Liquid-Based

Liquid Oxygen and Liquid Hydrogen

One of the most popular, and simple, of the conventional fuels is a combination of liquid hydrogen and liquid oxygen, which are kept compressed and cooled in storage buildings on-site until needed at the time of launch, as seen in the white dome in the leftmost edge of the image on this page, where NASA describes in detail some of its transfer and storage methods for this cryogenic fuel. An interesting point to note is that, at least in the case of the liquid hydrogen, no active pump in necessary to transport the fuel. A small amount is simply allowed to vaporize, creating a pressure differential that pushed the liquid fuel out of the tank, through the piping, and into the waiting rocket.

The liquid oxygen and hydrogen are then combined in the rocket. From there, it is a simple matter of igniting the two, setting off a reduction-oxidation (redox) reaction. As might be expected, the oxygen is the oxidizer, and the hydrogen is the reducing agent. Written in expanded form, the reaction proceeds as such.

Hydrogen is oxidized (gives up electrons)

 H2 → 2H+ + 2e

Oxygen is reduced (gains electrons)

O2 + 4e → 2O2-

Combined, they yield a net reaction of:

2H2 + O2 → 2H2O

This reaction, however, is very spontaneous under elevated temperatures. As anyone who has ever ignited hydrogen will tell you, given the right conditions, it more explodes (or pops) than burns. It is this controlled explosion that powers the first state of the rocket’s flight. From then on, however, different means of propulsion come into play, one of the most prominent of which is solid fuel rocket boosters.

Other forms of liquid propellant include using kerosene instead of hydrogen and using a combination of nitrogen tetroxide and hydrazine as the fuel, but they all rely on the same fundamental process of a redox reaction.

Solid-Based

Solid-fuel based rockets come in a wide variety of configurations, depending on the scale, cost, safety, and performance needed for a given operation. They are useful in providing boosts to the main liquid-based propulsion systems of some rockets (i.e. the Space Shuttle), hence the name of solid rocket boosters. While the exact fuel varies, one of the most common general fuel types for large scale rockets (the kind needed to launch spacecraft) is a powdered metal, usually magnesium or aluminum, that is oxidized by a strong oxidizing agent, such as ammonium nitrate or ammonium perchlorate. These are called composite propellants. An example reaction goes as follows:

10Al + 6NH4ClO4 → 4Al2O3 + 2AlCl3 +                                                 12H2O + 3N2

The aluminum is oxidized, yielding the high energy output of the reaction, and the ammonium chlorate acts as the oxidizer, easily accepting the aluminum’s electrons.

Other forms of solid rocket propellants include conventional black powder, adding high explosives to composite propellants, a powdered zinc-sulfur mixture, and using sugar as the oxidized material, amongst others.

 Other Forms of Propulsion/The Future:

One promising form of rocket propulsion lies in ion-based systems, pursued by space agencies up to and including NASA. In an ion-based system, the careful position of electrodes is used to accelerate charged particles (the ions) to speeds that can reach a significant fraction of the speed of light. Theoretically, given enough time, the rocket and/or vehicle can accelerate with an upper bound of the speed of its ion exhaust, but other factors (i.e. time, particles in space) make the top functional speed of the rocket significantly slower, but still faster, depending on scale, than the current batch of propellant technologies. Already, ion propulsion is used to make minute adjustments in a rocket’s (or other space vehicle’s) trajectory and/or orientation, but the time of ion propulsion as the primary source of acceleration is still a ways away.

This system relies on a constant source of electrical power, usually solar or nuclear, given the constraints of a system with limited resources in outer space. It also requires a computer and a source of particles to ionize to operate, conditions unique to this system of propulsion.

The future is promising for ion propulsion technology, given the high theoretical velocity vehicles using it can reach and its relatively fuel-cheap design. If may well be that ion propulsion systems come to dominate the industry for space vehicle propulsion, but for now, we just have to rely on conventional fuels, be they cryogenic liquids or metallic powders.

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Nuclear Fusion in Stars

In the previous article, we discussed what nuclear fusion is and learned that this complicated chemical process can only occur under a very extreme set of conditions. To the average person, it may seem that nuclear fusion can only occur in a laboratory setting, where ideal conditions are created and maintained by scientists. However, that is not the case. Believe it or not, nuclear fusion is one of the more common chemical reactions, as it occurs naturally in stars throughout the universe.

 

Our sun, like all stars, fuses elements together to create denser elements, in doing so creating enough energy to maintain the star’s high temperature. If a certain temperature (100 million Kelvins) is not maintained, the sun will collapse in on itself. Fusion is possible in the sun (and any star for that matter) only because of the star’s immense mass, leading to an astronomically large gravitational field. In our solar system, the gravitational field is responsible for the aligned orbits of the planets. The size of the star causes extremely high pressure and temperature within its core, where large amounts of hydrogen are present (http://www.sciencealert.com.au/features/20130711-24990.html).  In these conditions, hydrogen atoms can overcome the electromagnetic repulsion between themselves, and undergo nuclear fusion. The magnitude of the repelling force increases with the electrical charges on the two nuclei. To keep this force small therefore, the interacting nuclei should have the lowest possible charge(or atomic number).

Fusion primarily takes place during the main sequence of a star. The main sequence is the time in which the star turns all of its available hydrogen into helium, which in turn produces light and heat energy. The exact length of the main-sequence stage depends on the star’s mass: the lower the mass of the star, the longer it takes to burn up all its hydrogen, chiefly due to the relationship between mass and gravity. The main sequence length depends on the mass of the star in question. For example, our sun is about halfway through its 10 billion year main sequence. (http://www.astronomynotes.com/evolutn/s4.htm). During this stage, the star is in hydrostatic equilibrium. This means that the gravity of the star and the opposing thermal pressure created by fusion are pushing against each other equally, creating stability within the star. Hydrostatic equilibrium is the pressure gradient force that also prevents gravity from collapsing theEarth’s atmosphere into a thin, dense shell, while gravity prevents the pressure gradient force from diffusing the atmosphere into space. 

Now that we have talked about what fusion is along with where and why it occurs naturally in the universe, in our next post, we will discuss how this complicated chemical process has already been used (on Earth), so keep on reading!

(pictures from http://www.buzzle.com/articles/nuclear-fusion-in-the-sun.html)

Semiconductors and Their Relevance to Solar Panels

Let’s backtrack for a moment and discuss something mentioned in our earlier post: semiconductors.

What Are Semiconductors & How Do They Work?

semiconductor is a material that has a conductivity between that of insulators (i.e. plastics, rubber, Styrofoam, etc.) and conductors (copper, iron, gold, etc.). These semiconductors exhibit the photoelectric effect. This effect causes the semiconductors to absorb photons of light, and the energy from the photons is then transferred to electrons. With an electron’s new gain in energy, it can escape from its normal position in the atom and integrate into the electrical current, which can be used to produce electricity. However, when the electron escapes from its normal position (the valence band), and moves into the conduction band, it leaves a “hole” in the semiconductor (as depicted in picture to the right).

Semiconductors and Solar Panels: What’s the correlation?

 Due to the holes that are formed within the  semiconductors, they are ideal to use within  photovoltaic cells. “Why?” you may ask. In  order to induce an electric field inside a photovoltaic cell, two semiconductors are put together,  one P-type semiconductor and one N-type semiconductor. The P and N correspond to positive  and negative respectively. In a P-type semiconductor, there is an abundance of holes while an  N-type semiconductor has an excess of electrons. By putting a P and N-type semiconductor  together, a P-N junction is formed where they meet.

The excess electrons from the N-type semiconductor flow to the P-type semiconductor to fill  the holes. Now, holes are present in the N-type semiconductor and electrons from the P-type  semiconductor move to the N-type material. This process/motion of electrons between the two  types of semiconductors occurs back and forth, and as a result of this back and forth motion of  electrons, an electric field is created at the junction. This electric field causes the electrons to  jump out from the semiconductor to the surface of the material, and from there, they can  move to the electrical current. Other electrons fill the vacated holes and then move out to the circuit, and in this way, the current continues. No electrons are technically “used up” throughout this process. The photons merely give their energy to create electron-hole pairs. The electrons transfer this energy to the electrical circuit in the form of electrical power, and then they return to the cell to recombine with the positive holes. Simply put, this process conveys the idea of the conservation of energy.

Silicon & What Makes It a Great Material for Solar Panels                  

                        

In the earliest successful solar panels, silicon was the semiconductor material that was used, and no surprise, it is still a commonly used material for solar panels today.

Silicon is a perfect example of a semiconductor. On an atomic level, silicon has four valence electrons and has the ability to form four covalent bonds. When it does so with four neighboring silicon atoms, the resulting crystalline silicon solid is implemented into solar panels.

However, pure crystalline silicon is a metalloid that looks shiny, but is brittle and a fairly poor conductor since its electrons are not free to move around. In order to solve this issue, the silicon that is found in photovoltaic cells is not just silicon alone–it has impurities. Usually when one hears of “impurities,” it has a negative connotation, but that is not the case here. In a process calleddoping, impurities (other atoms) are purposely mixed with silicon atoms. Typically, silicon is mixed with phosphorus and boron. For approximately every 1 million silicon atoms, there is 1 phosphorus atom or 1 boron atom. Why phosphorus and boron?

Let’s start with phosphorus. Phosphorus has five valence electrons instead of four. It can still bond with neighboring silicon atoms, but what makes it different is that one of its electrons does not bond with another atom. In comparison to if it was pure silicon, it takes much less energy to cause this extra electron to break free. With several phosphorus atoms mixed with the silicon atoms, several free electrons, or free carriers, become available, and the silicon becomes an N-type material. N-type doped silicon serves as a good conductor.

Mixing silicon with boron forms P-type doped silicon. Boron has three valence electrons, and only forms three bonds with nearby silicon atoms, leaving one “incomplete bond.” This incomplete bond is capable of capturing one of the free electrons from the phosphorus. The capture of a free electron leaves a “hole,” so another electron moves to fill this “hole.” This causes the aforementioned motion of electrons, the creation of the electric field, and the flow of electric current.

In the end, if doping is not done, silicon photovoltaic cells would not work.

A visual representation of how silicon P-N junctions work in solar panels can be found below. .

Let’s backtrack for a moment and discuss something mentioned in our earlier post: semiconductors.

What Are Semiconductors & How Do They Work?

semiconductor is a material that has a conductivity between that of insulators (i.e. plastics, rubber, Styrofoam, etc.) and conductors (copper, iron, gold, etc.). These semiconductors exhibit the photoelectric effect. This effect causes the semiconductors to absorb photons of light, and the energy from the photons is then transferred to electrons. With an electron’s new gain in energy, it can escape from its normal position in the atom and integrate into the electrical current, which can be used to produce electricity. However, when the electron escapes from its normal position (the valence band), and moves into the conduction band, it leaves a “hole” in the semiconductor (as depicted in picture to the right).

Semiconductors and Solar Panels: What’s the correlation?

 Due to the holes that are formed within the  semiconductors, they are ideal to use within  photovoltaic cells. “Why?” you may ask. In  order to induce an electric field inside a photovoltaic cell, two semiconductors are put together,  one P-type semiconductor and one N-type semiconductor. The P and N correspond to positive  and negative respectively. In a P-type semiconductor, there is an abundance of holes while an  N-type semiconductor has an excess of electrons. By putting a P and N-type semiconductor  together, a P-N junction is formed where they meet.

The excess electrons from the N-type semiconductor flow to the P-type semiconductor to fill  the holes. Now, holes are present in the N-type semiconductor and electrons from the P-type  semiconductor move to the N-type material. This process/motion of electrons between the two  types of semiconductors occurs back and forth, and as a result of this back and forth motion of  electrons, an electric field is created at the junction. This electric field causes the electrons to  jump out from the semiconductor to the surface of the material, and from there, they can  move to the electrical current. Other electrons fill the vacated holes and then move out to the circuit, and in this way, the current continues. No electrons are technically “used up” throughout this process. The photons merely give their energy to create electron-hole pairs. The electrons transfer this energy to the electrical circuit in the form of electrical power, and then they return to the cell to recombine with the positive holes. Simply put, this process conveys the idea of the conservation of energy.

Silicon & What Makes It a Great Material for Solar Panels                  

                        

In the earliest successful solar panels, silicon was the semiconductor material that was used, and no surprise, it is still a commonly used material for solar panels today.

Silicon is a perfect example of a semiconductor. On an atomic level, silicon has four valence electrons and has the ability to form four covalent bonds. When it does so with four neighboring silicon atoms, the resulting crystalline silicon solid is implemented into solar panels.

However, pure crystalline silicon is a metalloid that looks shiny, but is brittle and a fairly poor conductor since its electrons are not free to move around. In order to solve this issue, the silicon that is found in photovoltaic cells is not just silicon alone–it has impurities. Usually when one hears of “impurities,” it has a negative connotation, but that is not the case here. In a process calleddoping, impurities (other atoms) are purposely mixed with silicon atoms. Typically, silicon is mixed with phosphorus and boron. For approximately every 1 million silicon atoms, there is 1 phosphorus atom or 1 boron atom. Why phosphorus and boron?

Let’s start with phosphorus. Phosphorus has five valence electrons instead of four. It can still bond with neighboring silicon atoms, but what makes it different is that one of its electrons does not bond with another atom. In comparison to if it was pure silicon, it takes much less energy to cause this extra electron to break free. With several phosphorus atoms mixed with the silicon atoms, several free electrons, or free carriers, become available, and the silicon becomes an N-type material. N-type doped silicon serves as a good conductor.

Mixing silicon with boron forms P-type doped silicon. Boron has three valence electrons, and only forms three bonds with nearby silicon atoms, leaving one “incomplete bond.” This incomplete bond is capable of capturing one of the free electrons from the phosphorus. The capture of a free electron leaves a “hole,” so another electron moves to fill this “hole.” This causes the aforementioned motion of electrons, the creation of the electric field, and the flow of electric current.

In the end, if doping is not done, silicon photovoltaic cells would not work.

A visual representation of how silicon P-N junctions work in solar panels can be found here.

Up Next…

Although silicon is so widely used, it is not the only material that is used for solar panels. Additionally, it does not absorb light as efficiently as other materials do, so in our next post, we will dive deeper into the usage of other materials that enhance the efficiency of the solar panel.

An Introduction into Nuclear Fusion

Have you always wanted to learn more about nuclear fusion, but can’t understand the complex scientific writing? Well then you’re in the right place! In our posts, you’ll find plenty of information about nuclear fusion told in a way anyone can understand. But before we talk about all the cool things nuclear fusion can do, we have to understand how it works and what it does.

Nuclear fusion is a reaction which produces large amounts of energy by combining two hydrogen or helium protons (positively charged atoms), hence the term “fusion.” Fusion is not to be confused with fission, which is the opposite process. While fusion forms larger atoms out of smaller ones, fission splits larger atoms into smaller atoms. The energy released is primarily in the form of heat. Fission creates a large amount of energy, but also releases potentially dangerous radioactive byproducts. This “nuclear waste”  has to be collected and stored in safe facilities to ensure no leaks occur, which costs money and requires resources.  However, fusion releases even more energy, and does not release radiation. Also, hydrogen, which is used in fusion reactions, is the most abundant gas in the universe (Check it out here). Because it is such a sustainable source of energy, many people hope that Fusion will be the energy source for the future.

Fusion reactions combine different isotopes (same element but with a variation in the number of neutrons) of helium and hydrogen, including the Hydrogen isotopes Protium, Deuterium, and Tritium and the Helium isotopes Helium-3 and Helium-4. If the nucleus of the newly created nuclei needs less energy to hold it together than the old ones, energy will be released and mass will be “lost.” Different combinations of the hydrogen isotopes result in an isotope of helium and the release of high-energy particles, as shown in this fusion diagram.  However, since fusion brings two protons together, their positive charges cause them to repel each other. The only way to get protons to overcome this repulsion is to put them at extremely high temperatures and pressures.  In order for the protons to have enough energy to bond with each other, they need to be in an environment that is about 100 million Kelvins (six times hotter than the sun’s core). They also need to be under a lot of pressure in order to push them close enough together to fuse (1×10-15 meters away from each other). At first glance, these temperatures seem unreachable, as no known substance can withstand them.

To compensate for this fact and achieve these conditions, scientists use magnetic or inertial confinement reactors. A magnetic confinement fusion reactor contains plasma (superheated gas) which is spun with a magnetic field around the inside of the reactor so that the plasma does not touch the walls of the reactor. At the same time, the necessary pressure and heat comes from magnetic and electric fields themselves. An inertial confinement reactor (ICR) works differently. To compress and heat the fuel, energy is delivered to the outer layer of the target ( a pellet containing a mixture of Deuterium and Tritium ) using high-energy beams of laser light, electrons, or ions. The most common practice of ICR has used lasers. The heated outer layer explodes outward, producing a reaction force against the remainder of the target, accelerating the energy inwards, compressing the target. This process is designed to create shockwaves that travel inward through the target. A sufficiently powerful set of shock waves can compress and heat the fuel at the center so much that fusion reactions occur. However, due to current technological limitations, fusion reactions have to take place on a very large scale in order to achieve an efficiency high enough that the reaction will be productive.Fusion Diagram

With all these conditions required for nuclear fusion, do you think it could occur naturally? Where in the universe might these extreme conditions be present? Check out our next post to find out! Also, watch this video for a brief recap of the process discussed in this blog post.