Tag: space propulsion

space

SpaceX Starship Propellant Production on Mars

By Espresso Insight on

Making Rocket Fuel on Mars

When humans get to Mars, Elon said the first order of business is propellant production. This means producing liquid methane and oxygen, Starship’s fuel, using only the resources and raw material gathered on Mars.

Why produce propellant on Mars?

The ability to produce rocket fuel without importing it from Earth is critical if humans want to successfully build a self sustaining base on Mars:

  • To ensure humans have consistent fuel supply on the red planet for return missions.
  • To reduce weight by minimizing the amount of fuel carried onboard Starship. Carrying less, we reduce launch mass and enable more efficient flights.

“In-situ propellant-production is critical to the space architecture needed for a long-term human presence on Mars, future interplanetary transport, and eventually, multi-planet colonization.”

– NASA

How does rocket propellant work?

Rocket propellant needed to launch rockets from the surface of a planet into orbit and beyond. Starship uses chemical fuel like methane and oxygen, in contrast to satellites and deep space probes which use electrical propulsion such as ion thrusters.

There are a few different types of chemical based rocket fuel, but they all have a few things in common.

Rocket fuel works similarly to the way gasoline in a car works, via combustion. For any combustion reaction, you need two things: a fuel source, and an oxidizer. The oxidizer accepts electrons. Because oxygen has 6 electrons in its outer shell, it accepts 2 electrons to create 8, a full outer valence shell.

For a combustion reaction, you really just need oxygen and a fuel source. The fuel could be liquid, like gasoline, or solid, like gunpowder. Rocket fuel is complicated – over the years, chemical engineers have tried different combinations of fuels and oxidizers to try to find the optimal rocket propellant:

  • Liquid Hydrogen, for example, is the most efficient. The problem with hydrogen fuel for Mars missions is that it has a boiling point of -423 degrees F, which presents a challenge keeping it in liquid state during long trips and during the friction-intensive high temperature entry burns.
  • RP-1 is another type of rocket fuel similar to kerosene, but is not suitable for SpaceX’s goal of having rapidly reusable rockets because it leaves a large amount of soot residue after use, which requires extensive effort to clean.
Chemical structure of methane. source: science.org.au

There are a few differences between gasoline and rocket fuel: In the case of an automobile, oxygen is readily available in the environment to be used as an oxidizer. In a rocket travelling through outer space, the oxidizer must be carried along with the fuel in a separate tank. Space travel means moving through a vacuum, where you don’t have the luxury of an endless supply oxidizer in the surrounding space.

To get to and from Mars, methane and liquid oxygen will be used. Although Mars doesn’t have an abundance of liquid Methane like Saturn’s moon Titan, the good news is that methane can be readily made on Mars from material that is available in the ground and atmosphere.

Key Requirements for Mars Fuel

It will be in our best interest to implement the following items into the propellant production process:

  • Minimize electrical power needs because all electric power will need to come from batteries, solar power, or nuclear power. There is no way of knowing whether or not Mars will have fossil fuels beneath the surface, and we cannot rely on this.
  • The propellant production process will be heavily dependent on chemical engineering and the ability to complete multiple chemical reactions and separations sequentially. In addition to the desired products of Hydrogen and Methane, the process will produce by-products, many of which are useable for other endeavors on Mars.
    • Nitrogen is one byproduct that is specifically useful because it is inert and non-reactive. The gas can be used used for flushing of tanks and lines through which other gases pass since it is reactive neutral.
    • Oxygen and water byproducts are both potentially valuable feedstock for making propellant oxidizer or for life support/drinking. For this reason, Mars engineers will need to consider options, means, and costs in any facility design with business analysts to determine the costs to market value of any manufacturing byproducts.

Efficiency: one metric ton of propellant per 17 megawatt-hours energy input. Starship needs 240 tons of fuel – which will require 4.1 gigawatt hours of energy input.

How long does propellant production take?

How much time does it take to make enough fuel for launching Starship.

This brings us to outlining the process of creating fuel. There are 4 key steps.

Chemical Reactions to make Methane Rocket Fuel:

Pre-Requisites to fuel creation:

Gathering CO2

Carbon dioxide is highly available in Mars’ atmosphere, 20 times as much as on Earth. We would likely use a type of air pump to gather the CO2.

Separation and removing contaminants

To obtain pure CO2, dust filtration will be important in this step of the process as well as removing the small amounts of ambient gasses including nitrogen, argon, neon, and krypton. Carbon molecular sieves (CMS) will be used to separate the carbon dioxide from the nitrogen, and a Vortex Swirl particle separator will use used as well.

As the reaction proceeds and produces methane and water, a separator will be used to remove the water vapor, leaving pure methane. This will be done by simply allowing the products to cool, so that water goes through the process of condensation then stored in tanks.

An important consideration is: How do we make sure no contaminant gasses are present with additional harmful byproducts?

The diagram below shows the reactions that will be required for producing propellant on Mars.

spaceX mars propellant production
source: SpaceX

The reactions get complicated, and while I started covering them below, I found this super helpful video on YouTube that covers the chemical reactions as well as a lot of other information about making rocket fuel on Mars.

1. Electrolysis of Water

Hydrogen is the critical component that is hard to get, which we must separate from water. We need to pump water from underground wells, use robotic vehicles to mine raw water ore.

– Robotic vehicles, such as the NASA KSC Regolith Advanced Surface Systems Operations Robot (RASSOR) prototype or the OffWorld Inc.5 smart robots, are likely candidates for mining the raw “water ore”.

Then, the water will go through electrolysis to break water into components Oxygen and Hydrogen.

Once this is done, we can use the hydrogen and CO2 from the atmosphere to run the Sabatier methanation reaction.

Note, we will have to do the electrolysis of water twice throughout.

2. Sabatier Reaction: Carbon Dioxide and Hydrogen to create Methane gas and Water.

The Sabatier methanation reaction has been recommended by many of the top researchers as the most likely basis for an ISRU propellant production processing plant on Mars.

This reaction leverages the abundance of CO2 in the Martian atmosphere to create both the methane fuel and water. CO2 is taken from the atmosphere by either freezing the gas into a solid, mechanical compression, and absorption pumping. Freezing will purify the gas, and may be advantageous.

Nickel or aluminum oxide transition metal catalyst is required (why?)

The reaction has to be carried out at high temperatures (why doesn’t it happen at low temp?) There is a principle in chemistry that describes the effect of temperature on reaction speed.
“Higher temperatures mean faster reaction rates; as molecules move about more quickly, reactant molecules are more likely to interact, forming products…” – sciencing.com
Since mars is cold, the reaction has to take place in an insulated container. The good news is that the reaction is exothermic, so once it starts, there isn’t much energy required to keep it going at temperature.

This is used today on the international space station to form water for astronauts to use.

3. Carbon Dioxide solid oxide electrolysis to create Oxygen and CO byproduct.

4. RWGS (reverse water gas shift)

A way to supplement producing methane and oxygen from hydrogen and carbon dioxide.

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space

SpaceX Starship Overview 2021

By Espresso Insight on

Starship Rocket Overview

Important Breakthroughs

  • Propellant production in Boca Chica will be important to optimize the supply chain.
  • Rapidly reusable rockets – like air travel or car travel, you don’t get a new car every time you take a trip.
    • Re-usability will allow flying the booster 20 times per day, and the ship 3-4 times per day. Reason ships can only be used a few times a day: since ship goes to orbit, the track of a satellite is sinusoidal (unless it is equatorial or san-synchronous). you have to wait for the ground path to sync up with the launch site. It takes like 6 hours to sync up.
  • Satellite Delivery: Currently, the company uses Falcon to deliver satellites for Starlink. Starship will be able to deliver satellites further and at a lower marginal cost per launch, as Startship has a much greater payload..
  • SpaceX created the Raptor engine, which has a very high specific impulse. Because Earth’s gravity is quite high, we are just on the cusp of reusable rockets being physically possible. Raptor engine (it will have 6 engines) uses mostly oxygen per unit of fuel (3.5 tons of oxygen for every 1 ton of fuel).
  • Making it to orbit was tough… landing the rocket was tougher, and SpaceX was the first to do so.

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Reducing Launch Mass

  • Steel: the rocket it made of steel. It has the perfect combination of strength and heat resistance. Because of this, the rocket will be able to have a smaller heat shield, and only need a heat shield on 1 side of the ship. This will reduce launch mass.
  • Orbital re-fueling: Starship attaches to another rocket containing fuel while in orbit, making it pace.

Starship Demographics

Image
Raptor Engine. Source: @brandondeyoung_ twitter

SpaceX has published a quite succinct user guide with detailed information.

  • Engine: Raptor
  • Fuel: Methane and Liquid Oxygen (CH4 and LOX)
  • Length: 72 meters
  • Diameter: 9 meters
  • Material: Stainless Steel
  • Payload: 100 tons
  • Nomenclature: SN9 stands for “Serial Number 9”

Starship flights:

Starship performed its first test flight on July 26, 2019 and has so far performed 6 orbital test flights.

Starship SN8 flight recap

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space

Deep Space Travel with Ion Thrusters – an Overview

By Espresso Insight on

Electrically powered ion thrusters are one of the most common propulsion systems for the vacuum of space.

Ion Propulsion Key Takeaways:

ion propulsion system as Xenon exits the engine
source: NASA
  • Ion thrusters allow spacecraft to travel further, faster, and cheaper than other systems.
  • 11.5 times as efficient as chemical propellant.
  • Though efficient, the system provides very little thrust, a fraction of 1 newton. The tiny force, over time, eventually results in big changes in velocity.
    • Cannot be used to launch a rocket from Earth.
    • Are excellent for maintaining satellite orbit, sending smaller probes on long distance voyages to asteroids or outer planets.
  • Can operate continuously for years. Ion thrusters can be used over very long periods of time. For example, Dawn, the spacecraft that was the first to reach a dwarf planet Ceres, used ion thrusters to reach a speed of 10 km/s.
  • Could be used in the near future to power additional missions to Saturn’s moon, Titan, or other places proximate to our solar system, for example.
  • Could one day be used to send people on a thousand year voyage to other stars.

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Even though the amount of force that an Ion Thruster Engine provides is barely the weight of a piece of paper, these systems allow spacecraft to reach enormous speeds.
Although going from 0-60 takes about 4 days, the compounding acceleration of running these engines for years allows them to cover distances of billions of miles through space.

Ultimately, ion engines are the perfect system for long-distance, deep space travel.

What is an Ion Thruster?

An artist’s concept of Dawn, propelled by ion propulsion, approaching Ceres.
source: NASA

We now know that an ion thruster is a method of powering a spacecraft during flight through outer space.

As ions are ejected from the back of the engine, each ion generates a tiny force which slowly begins accelerating the spacecraft.

In the absence of air, there is no friction or wind resistance to slow down a spacecraft’s trajectory. Thus, the tiny, consistent force generated is successful at increasing the speed of a vehicle in space when maintained for long periods of time – often years.

Ion thruster engines are the most useful known method of movement in space, particularly over long durations.

Why are ion thrusters used?

Ion propulsion is specifically valuable for long distance space travel because less propellant is needed to increase speed.

With traditional rockets, chemical-based fuel like methane or hydrogen is used. In these systems, fuel tends to be such a large fraction of a spaceship’s total mass.

Ion thrusters, on the other hand, are effective in generating the most thrust for a given mass of fuel. A lower fuel mass is analogous to a car getting more miles per gallon.

Traditional chemical rockets release energy stored in molecular bonds of propellant (Methane, CH4 for example) and are limited by their low specific impulse, no matter what type of fuel or design is used.

For missions that require a large acceleration (like deep space travel over millions of miles), chemical propulsion is no good because the low specific impulse requires fuel to take up a larger percentage of the payload.

According to the Tsiolkovsky Rocket Equation, as acceleration increases, the amount of fuel needed increases exponentially.

Specific impulse describes how effectively propellant is converted into thrust.

Key differences between ion and chemical propulsion:

ion thruster
source: NASA
  • Compared to chemical engines, ion powered spaceships are able to reach speeds that are more than 11 times as fast.
  • Greater efficiency; high specific impulse, which means its able to generate larger force for a given mass of propellant.
  • Less powerful
  • Ionizes atoms rather than reacting molecules in an exothermic combustion reaction.
  • Acceleration can be sustained for months or years at a time, in contrast to the very short burns of chemical rockets.
  • Less propellant is required, which means we can send smaller, cheaper vehicles on missions.

What are the drawbacks of ion thrusters?

There are a few tradeoffs to using an ion thruster engine. Although sustainable for months or even years, ion thrusters produce only a small amount of force, so it takes a long time to reach high speeds.

Additionally, ion thrusters can only be used in the vacuum of space. They don’t work in the presence of air particles, and the tiny force cannot overcome air resistance.

An ion thruster won’t work for launching a rocket from a planet’s surface, but it can be used for steering, orientation and acceleration once you’re in space.

How do Ion Thrusters work?

What is an ion and how is it different from an atom? | Socratic
source: socratic.org

Instead of igniting propellant, ion propulsion works by taking inert, unreactive gas atoms (such as xenon or krypton), and inducing a positive charge by removing an electron.

An inert gas is used as the propellant because it is unreactive and non-corrosive. Xenon and Krypton are quite unreactive because they contain a full 8 electrons in the valence shell. Elements with a full outer shell are called noble gases.

Xenon is a slightly better propellant than Krypton because it has a larger atomic mass, producing more force.

The process of removing electrons requires electricity. Often, solar panels are a good option to power the ionization. However, for deep space missions where less solar power is available, the energy is too small and acceleration is slowed. Because of this, alternative energy sources are required. Nuclear is one possible option. NASA and Livermore labs working on a nuclear system for electricity production.

Once electrons are ionized (electrically charged), the ion can be accelerated. This is done by applying a voltage to create an electrical field, causing them repel one another (similar to the way magnets behave when you hold them next to each other). The large amount of ions repelling each other produce momentum and force to accelerate the spacecraft.

The voltage causes the ions to leave the engine at up to 90,000 miles per hour. Each individual ion provides a small amount of thrust for the spacecraft.

ion thruster
source: NASA JPL

With a large number of these ions being expelled, a constant force is generated that can move the spacecraft forward, to the left in the image above.

Given the amount of force is small, for reference it takes four days to accelerate from 0 to 60 miles per hour. Though small, when sustained over many years, continual acceleration can cause the spacecraft to reach up to 200,000 miles per hour, fast enough for deep space travel.

Who works with ion thrusters today?

SpaceX Starlink satellites use ion thrusters. Each SpaceX Starlink satellite is able to propel and orient itself to ensure it doesn’t run into other satellites or orbital debris. Starlink uses krypton for the inert gas because it is cheaper (though less efficient) and better suited for their large amount of satellites.

NASA Glenn Research center has done tests on a Hall Effect thruster, known as HERMeS, which is three times as powerful as other systems.

ion thruster
source: NASA

The University of Michigan is developing the X3 Ion thruster, which is also a type of Hall Effect thruster capable of generating 5.4 Newtons of thrust, and has set numerous records.

NASA Dawn Mission has used ion thrusters to travel 4.3 billion miles towards Ceres, a planet in the asteroid belt.

Thanks for reading!

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