Interview Part 3
On July 30, 2021 Tim Dodd received a tour of Starbase with SpaceX CEO, CTO, and founder Elon Musk. This article includes key takeaways from the over two-hour-long interview.
Musk once again emphasized that manufacturing is underrated and design is overrated; developing a production system is 10-100 times harder than designing the product, which according to Musk, proved especially true with Raptor. Furthermore, as volume production increases the amount of effort that goes into design rounds down to 0.
For example, Musk stated that designing a closed-cycle engine is easy. However, the extremely hard part is getting the cost-per-tonne of thrust under $1,000 – with each Raptor producing 230 tonnes of thrust, this means that each engine must cost less than $230,000 to produce. Musk continued stating that the cost-per-tonne to orbit, and the cost-per-tonne to the surface of Mars, is several orders of magnitude too high on current launch vehicles. For this reason, it is so important to move as much mass and complexity as possible to the ground systems, which Musk is calling “stage 0.” For example, SpaceX has opted to not fuel Starship through the booster, and will rather move the complexity and mass to the ground and fuel Starship from its side.
Super Heavy Design:
Musk noted that Super Heavy’s dry mass should be under 200 tonnes, although the dry mass is a moving target. The engines, including mounting mass, are 2 tonnes; the fuel tank and the liquid oxygen (LOx) tanks are roughly 80 tonnes; and the interstage is around 20 tonnes, including four grid fins that each weigh roughly 3 tonnes. Musk noted that he expects to be able to cut the mass of each grid fin in half, and that the current design is very mass inefficient. He further noted that currently everything is too heavy, including avionics, grid fins, and batteries.
The grid fins are currently electrically powered, using a modified Tesla Model 3 motor to drive them. In continuation of using Tesla parts, the batteries are currently energy optimized instead of power optimized, as a car needs several hours of energy, whereas the grid fins only need two or three minutes of power. Due to these reasons, Musk noted, like much of Starship’s design, the batteries are temporary, and battery mass can drop by a factor of ~10.
The booster is designed to carry 3,600 tonnes of propellant, of which ~78% of that is liquid oxygen. The Raptor burns at a mixture ratio of 3.5 to 3.7, which is fuel rich. A fuel rich mixture burns cooler than the stoichiometric ratio, which would melt the engine. Super Heavy’s propellant residuals (the amount of fuel that cannot be used without risking damage to the vehicle) are on the order of 20 tonnes, Musk notes, which is significantly higher than the 1 tonne propellant residual of the Falcon 9. With optimizations to design Musk noted that the final dry mass of Super Heavy should be between 160 and 200 tonnes.
An interesting note is the grid fins on Super Heavy do not fold in like on the Falcon 9, as they are another mechanism, which adds unnecessary complexity, mass, and failure modes. Additionally, the increase in drag from the grid fins being deployed during ascent is small, assuming they are not at a high angle of attack.
The grid fins on Super Heavy are not evenly spaced 90° apart, like on the Falcon 9. Musk said the reason for this change is Super Heavy requires more pitch control authority, so they positioned the gridfins closer together to increase pitch control.
Musk’s Engineering Philosophy:
Musk overviewed his five step engineering process, which must be completed in order:
- Make the requirements less dumb. The requirements are definitely dumb; it does not matter who gave them to you. He notes that it’s particularly dangerous if an intelligent person gives you the requirements, as you may not question the requirements enough. “Everyone’s wrong. No matter who you are, everyone is wrong some of the time.” He further notes that “all designs are wrong, it’s just a matter of how wrong.”
- Try very hard to delete the part or process. If parts are not being added back into the design at least 10% of the time, not enough parts are being deleted. Musk noted that the bias tends to be very strongly toward “let’s add this part or process step in case we need it.” Additionally, each required part and process must come from a name, not a department, as a department cannot be asked why a requirement exists, but a person can.
- Simplify and optimize the design. This is step three as the most common error of a smart engineer is to optimize something that should not exist.
- Accelerate cycle time. Musk states “you’re moving too slowly, go faster! But don’t go faster until you’ve worked on the other three things first.”
- Automate. An important part of this is to remove in-process testing after the problems have been diagnosed; if a product is reaching the end of a production line with a high acceptance rate, there is no need for in-process testing.
Additionally, Musk restated that he believes everyone should be a chief engineer. Engineers need to understand the system at a high level to understand when they are making a bad optimization. As an example, Musk noted that an order of magnitude more time has been spent reducing engine mass than reducing residual propellant, despite both being equally as important.
Musk noted that SpaceX has produced parts of version 2 of Raptor, called Raptor 2, including the thrust chamber assembly. Teams have finished the design of the turbo pumps, and are expecting to be ready to fire the first Raptor 2 by the end of August. Raptor 2 will reach 230 tonnes of thrust at 298 bar main combustion chamber pressure, with Musk commenting “come on… we have to get 2 more bar out of that thing!” Raptor 2 features a larger throat, which decreases the area ratio; this causes a decrease in specific impulse of around 3 seconds, but increases thrust significantly. Despite having a lower ISP, this allows for booster engines to be more efficient as it decreases gravity losses. Musk noted that Raptor 2 will be significantly cleaner looking than Raptor 1, as they will remove a large amount of plumbing.
Raptor Vacuum has a brazed steel tube wall nozzle extension that has an expansion ratio of around 80, giving the engine a specific impulse (ISP) of 378 seconds. Musk noted that teams are hoping to get the expansion ratio up to 90, which would increase the ISP to 380 seconds. Long term, SpaceX will have three Raptor variants: sea level engine with gimbal, sea level engine without gimbal, and vacuum level engine without gimbal.
It was also stated that SpaceX will move volume production of Raptor to McGregor, but keep the Hawthorne factory for development engines and Raptor Vacuum versions.
Starship Separation System and Attitude Control:
Following Musk’s five step plan, SpaceX has decided to remove the pushing separation system from Starship and will instead rely on conservation of angular momentum to separate the stages. Right before main engine cutoff (MECO), Super Heavy will gimbal its engines, causing the vehicle to start rotating. The latches between Starship and Super Heavy will then release, causing the vehicles to float apart; the whole process is similar to how SpaceX deploys the Starlink satellites. This serves two purposes, as it separates the stages while starting the booster’s flip, which it needs to conduct for the boost back burn.
In addition to this, SpaceX has decided to remove the dedicated hot gas thrusters from Super Heavy. To replace it, SpaceX will use the ullage gas from the tanks for attitude control by having four vents spaced 90° apart. By venting the tanks through these vents, they will be able to control the attitude of the booster during the flip. This has the advantage of using the ullage gas, which would need to be vented either way, to do useful work to the vehicle.
Once again following Musk’s five step process, Musk outlined that he is optimistic that HLS will not need the landing thrusters at the top of the lunar Starship. If SpaceX is able to demonstrate that landing on the moon with Raptor will not create too large of a hole in the lunar regolith, they may remove the thrusters from the lunar Starship. While the exact Raptor configuration for lunar Starship has not been decided, it is likely that it’ll be the same as the Earth variant with three sea-level Raptors and three vacuum Raptors.
Starship Design Philosophy:
Musk noted that SpaceX has polar opposite design methods for Starship and Dragon. He continued, saying that Dragon can never fail, must be tested extreme amounts, and has tons of margin. However, to develop the world’s first fully and rapidly reusable rocket SpaceX must iterate rapidly, which leads to lots of failures. Falcon is in-between, where SpaceX can afford to have a landing failure, but cannot experience a failure during ascent. In response to Dodd asking what SpaceX learned from the Space Shuttle, Musk continued stating that the biggest problem with the shuttle was that its design froze. Due to all Space Shuttle missions being crewed, design changes were high risk and low reward. Musk contrasts, stating the biggest advantage of Starship is “Starship does not have anyone on board so we can blow things up. It’s really helpful.”
In response to this, Dodd asked if Starship will ever have a launch escape system (LES). Musk explained that there are no plans to add LES and that instead Starship will fly a lot and have enough redundancy. Super Heavy will be able to lose several engines during ascent, while still having a fully successful mission, and an engine on Starship. Additionally, Musk stated that due to the inability to have an escape system on the moon and Mars, there is no reason to have one at all.
Furthermore, Musk noted that SpaceX’s goal is to push the envelope with each vehicle, such that it blows up, as this ensures SpaceX is getting lots of data while not having to store many vehicles. He further noted that every Starship has had major upgrades over the previous vehicle; such is the pace at Starbase. Because of this, the first 10 Starship’s that SpaceX gets back will likely not be reflown, as newer vehicles will be so vastly different.
Musk added that this is a freedom that SpaceX did not have with Falcon 9 as it was flying cargo from day one. However, SpaceX had the grasshopper and F9R programs, where they could test reusability technology.
Dodd asked Musk how he was feeling about Starship’s Thermal Protection System (TPS), to which Musk responded “we’ll find out!” He added that the TPS tiles have been holding up well during the sub-orbital flight tests. Each tile is mechanically mounted in a way so that the tiles can move a small amount, ensuring they do not get damaged during the expanding and contracting of the tank and tiles during the temperature changes of fueling and reentry.
Starship’s TPS tiles are currently made in Florida at “the Bakery.” While not all of the tiles are uniform, SpaceX is able to mass produce the tiles due to them largely being the same shape and size, a significant advantage over the Space Shuttle’s TPS system. Additionally, Musk described the tiles as having “no meaningful limit” to their lifespan.
Musk stated that the hardest parts of the vehicle to protect against reentry heating are the flap hinges. Ensuring hot plasma does not get into the hinges and destroy the vehicle is not trivial, as the seal must simultaneously not damage the tiles and survive the heat of reentry; this means a metal seal is required.
With a full heatshield, S20’s dry mass will “hopefully not be much more than 100 tonnes.” Musk added that adding one tonne to the ship removes about two tonnes from the payload capacity, after taking into account the added mass and increase in propellant needed.
Orbital Demonstration Flight:
Musk stated that on the orbital demonstration flight, S20 will reach orbital velocity, but have a positive perigee that is less than 80km. This will ensure the vehicle reenters safely in a controlled manner, even in the event of a failure. On the orbital flight, both the first stage (B4) and the second stage (S20) will be expended, as SpaceX’s goal for the first flight is to “make it to orbit without blowing up”. Additionally, SpaceX would like to demonstrate the ability to position the booster precisely, such that if the booster came down next to the tower, Mechazilla (the catching mechanism) would be able to catch it.
For the first orbital flight, SpaceX will use a crane to stack Starship on top of Super Heavy. While this is a harder way to stack the vehicles, it allows SpaceX to start testing the vehicle before the orbital launch tower is complete.
Musk noted that they will continue to launch from Starbase, Texas as long as the operational difficulties remain low. While Starship is launching from Texas, SpaceX will continue to work on the two oil rigs (named Phobos and Deimos); however this is not a priority for SpaceX right now as SpaceX is just “thinking about the things [they] have to think about.”
Musk noted that the new nosecone design consists of two rows of stretch formed steel, whereas the old nosecone was made from three sections of stamped sections. The new nosecone is made by stretching steel over a big tool, which creates a far smoother and cleaner final product. Musk also noted that SpaceX has stopped all work on the fairing door for now, as it is not needed for an orbital demonstration flight. In a similar manner, SpaceX has not started work on Starship in-orbit refueling, as it’s not needed in the short term.
Musk noted that they are moving extremely fast as “if we operate with extreme urgency we have a chance of making life multi-planetary. It’s still just a chance, not for sure. If we don’t act with extreme urgency, that chance is probably 0.” Shyamal Patel, the director of Starship operations, added to this stating that “I tell the crane operators ‘what would you do if there was an asteroid heading to this planet in 8 days?'”
Elon Musk closed the interview stating he is glad that people are becoming interested in rockets and how they work. He once again emphasized that becoming multiplanetary is very inspiring, and may be the most inspiring thing. Musk hopes the Starship program gives people confidence about the future and that humanity will have an exciting future in space; he hopes that science fiction will become a reality some day.