Starship: The best rocket ever built?
Space Exploration Technologies Corp. (SpaceX) was established by Elon Musk in 2002 to revolutionize space technology, with the ultimate goal of enabling people to live on other planets.
But first, they needed to develop rockets that are capable of lifting significant amounts of payload into LEO (Low Earth Orbit) and eventually to The Moon and Mars.
Before establishing the corporation, Elon Musk and his partners Adeo Ressi and Jim Cantrell went to Russia to buy a rocket. They didn’t succeed. A little later, the russians came to the States, no deal. Then Elon and Jim went for another meeting to Russia. They couldn’t negotiate the price low enough to be able to afford it, so they flew back home empty handed. On the flight home, Elon opened his computer, did some calculations, and realized that they could actually build rockets instead of buying them. Soon, SpaceX was born.
The first successful launch was the 4th launch of Falcon-1 on 28 September 2008. If this launch failed, the company would have gone bankrupt. But it didn’t. Falcon-1 delivered the dummy payload - a mass simulator - of 165 kg to the desired orbit, making SpaceX the first privately funded organization to reach LEO.
Fast forwarding a few years, as of 18 March 2019, SpaceX has made 75 launches (70 succeeded), 35 booster landings, 20 reflights, and finished developing the Falcon 9 Block V rocket, that is designed to be reused 20-30 times, and carry U.S. astronauts from U.S. soil to the ISS (International Space Station). As that design is finished, most of the company’s resources are put into developing a bigger, cheaper and more powerful rocket.
Introducing: Starship and Super Heavy
This new rocket was called the Interplanetary Transport System (ITS) in 2016, redesigned and renamed to BFR which stands for Big Falcon Rocket (please don’t confuse this with another F word that would also fit the acronym) in 2017. That name created some controversy, so the team decided to rename the second stage “Starship” and the booster stage “Super Heavy” in 2018.
The renaming part is not as important as the redesign. The key here, and why this rocket will never be like any rocket ever before, is because both stages will be fully reusable by design. Booster landings are kind of mainstream nowadays, the mechanism is mastered and will remain mostly the same for the Super Heavy as well. But the Starship is a completely brand new 2nd stage, it will be capable of lifting approximately 100 metric tons into LEO, refuel in orbit and go to the Moon or even Mars. When landing on the Red Planet, Starship could use aerobraking to slow down upon re-entry, and then fire its engines retropropulsively (against the direction of travel) to land vertically. After refueling on Mars using ISRU (in situ resource utilization), it will be capable of escaping Mars’ gravity to head back home. This is an exciting future ahead. As Elon once said:
“YOU WANT TO WAKE UP IN THE MORNING AND THINK THE FUTURE IS GOING TO BE GREAT - AND THAT'S WHAT BEING A SPACEFARING CIVILIZATION IS ALL ABOUT.
IT'S ABOUT BELIEVING IN THE FUTURE AND THINKING THAT THE FUTURE WILL BE BETTER THAN THE PAST. AND I CAN'T THINK OF ANYTHING MORE EXCITING THAN GOING OUT THERE AND BEING AMONG THE STARS.”
But Starship is exciting for many other reasons. Due to it’s reusable nature, it will be capable of doing all the tasks a spaceship needs to do. Currently, SpaceX is using the Dragon and the Crew Dragon to supply the ISS (and launch astronauts this summer), and has a different setup to launch satellites into orbit. Starship can do both, and many more. Here is a list of the tasks Starship is expected to fulfill:
- Launch payloads into Earth orbit (Cargo version)
- Dock with the ISS, carry cargo and astronauts up and down (Crew version)
- Refuel in orbit with a Tanker version of Starship (a modified design, where the payload is fuel itself)
- Land on The Moon vertically, and provide opportunity to build a Moon Base
- Reach Mars, and land on the surface vertically to build a Mars Base
- Reach other Moons of the solar system and land on them (Enceladus, Europa, Titan, etc.)
- Earth-to-Earth transit to carry passengers from one side of the globe to the other side in 30-45 minutes
If you are able to develop the same thing to do many tasks, you are working very efficiently. This in turn will lower the costs, make spaceflight accessible to a wider population and make a human civilization on Mars possible. This is an inspiring future worth working for!
The Anatomy of the Starship
The Raptor Engines
In order to make the Starship more efficient, SpaceX has developed a new rocket engine called Raptor. This will allow much higher thrust levels than the previous Merlin engines (380,000 lbf vs 190,000 lbf), better reusability by requiring less to no maintenance between flights, and burn the liquid methane and LOX more efficiently.
The current Merlin engines on Falcon 9 rockets use rocket-graded RP-1 kerosene, but SpaceX wanted to be able to refuel the Starship on Mars, using local resources, so they decided to go for methane. Methane (CH4), can be synthesized from CO2 in the Sabatier reaction.
“The Sabatier reaction or Sabatier process was discovered by the French chemist Paul Sabatier and Senderens in 1897. It involves the reaction of hydrogen with carbon dioxide at elevated temperatures (optimally 300–400 °C) and pressures in the presence of a nickel catalyst to produce methane and water. Optionally, ruthenium on alumina (aluminium oxide) makes a more efficient catalyst.” It is described by the following exothermic (∆H = −165.0 kJ/mol) reaction:
This will allow SpaceX to deliver as much cargo to the surface of Mars as possible, so they don’t have to carry the fuel that is required for the trip back to Earth all the way to Mars. They will land with close to zero fuel left in the tanks, and use the Martian CO2 in the atmosphere to create methane and oxygen to fuel the rockets.
Producing a rocket engine capable of this high thrust comes with a few problems during landing. During retropropulsive landing, the spaceship slows itself down with the engines to be able to reach velocity 0 by the time it reaches altitude 0. But engines work differently based on different ambient pressures, and this limits the engines capability to throttle down.
If an engine cannot throttle down enough, it will make the landing hard and potentially impossible. Falcon 9 second stages don’t land for this reason, because they can’t throttle down enough with the Vacuum nozzles, to be able to hover the smaller mass second stage.
Starship engines will have dual-bell nozzles that will allow them to work in vacuum with high throttle, but will also enable the engines to throttle down to as low as around 20% at sea level ambient pressure, which makes them capable of landing the spacecraft vertically with a landing profile that can be survived by the human body. You can learn more about rocket engine gas outflow, and the dual-bell nozzles in this article by Marstronauts.
A design feature that is truly remarkable for Starship is the wings. There are three of them, one is still, but two of them are actuated, which means they can be controlled by hydraulics to adjust Starship’s proper angle during atmospheric entry and aerobraking. This is necessary to reduce ballistic coefficient (an object's tendency to fly uncontrollably, like a brick). Cylindrical objects are not stable during re-entry (remember the shuttle was not cylindrical with its delta wings), so having wings is crucial. But this also poses a few problems.
First of all, it increases the risk for potential failure, because moving parts have a worse record than stationary parts, and thus causing mission failure. Moving things can go wrong, and you can’t just go out and use some WD-40 on them. You need redundant systems on spaceships, and although hydraulics can be used in tandems, these wings will definitely get high attention from SpaceX engineers.
Close up of the wings illustrated by @starship_renders Instagram user.
Another risk is Martian dust. It contains perchlorate salts and is electostatically charged so sticks to everything, including these moving parts, so colonists will have to take extra caution to either clean them up before liftoff, or cover them with something and seal them to protect from this dust.
Why Stainless Steel (SS) Instead of Carbon-fibres (CF)?
The current Falcon 9 fleet is built up of carbon-fibre structure, so it would be logical to use a manufacturing process that is already perfected for the next rockets. But actually, CFs are very expensive, $135 per kilogram, with a 35% scrap rate (meaning you have to throw away 35% of the material because you can’t use it), which puts you at around $200 per kilogram. SS is $3 per kilogram.
But the benefits don’t stop there. For a reusable rocket, you need a material that can withstand high temperatures during re-entry, but work well with super cooled cryogenic fluids. CF is limited to around 300 Fahrenheit. You may push it to 350, and there are some CFs that can go close to 400, but that has consequences of structural integrity loss. With stainless steel, you can easily do 1500, 1600 Fahrenheit. That is significantly better, and makes it possible for SpaceX to go without a replaceable heat shield they need to always change.
Talking about the heat shield, Elon said that the windward side will be double layered. Two layers of SS joined by stringers. The outer layer on the windward side will have hexagon tiles as shielding, and there will be a fluid layer between the two SS layers, filled with water or methane. This makes the first ever regenerative heat shield on a rocket, and thus making conventional heat shielding unnecessary. The leeward side will not need shielding, but it will probably be part of the cooling system. The dual-layer SS will serve double purposes. First, heat shielding, second structural strength.
Stainless steel, which has high chrome-nickel content also remains strong if super cooled by the fuel inside. Keeps it’s ductility at around 12-18 %, and becomes stronger, without increasing the risk for fractures. SpaceX will thus use 301 stainless steel in the rocket manufacturing process. This material has all the necessary benefits required for spaceships, is cheap and is easy to work with, unlike CF.
Development and Launch Cost
As SpaceX is a privately funded company, they need to keep an eye on profit margins, and keeping the company alive. This makes them unable to afford unnecessary steps in engineering and design, and in my opinion this is a good thing, a key factor that made them successful. When time and money impose constraint, you are forced to focus. For SpaceX, this resulted in an estimated cost to develop the Starship and Super Heavy at around $5-10 billion, which is ridiculously low, compared to other Spacecraft. In comparison, the most powerful rocket ever flown - the Saturn V - cost around $42 billion to develop calculated to 2018 dollars.
The cost per flight will be ridiculously cheaper than any rocket in the competition, due to Starship’s potential for full reusability. It is estimated, that the first launch will cost $150 million, but it will decrease rapidly, surpassing even the Falcon 1, expected at around $5 million per launch. If Elon will make his plans become a reality for a $100k Mars ticket, calculating with 100 crew members, this puts the full price at around $10 million. But there is a long way to go.