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.
Love the nitty gritty details 👍
I’d be interested to learn how the plasma flows around the ship during re-entry and if it would make sense to build a shield into the ship’s body to protect the hinges.
“the fuel tank and the liquid oxygen (LOx) tanks are roughly 8 tonnes” Musk said 80
Thank you for pointing it out!
That was fantastic!
Are the not-so-small lift points the actual catch points too? I think Elon tweeted a while ago that such points would be used instead of the grid fins as originally stated. But are we back to the fins now that they don’t fold (simpler=more robust).
Won’t SH need RCS thrusters at the catch for fine maneuvering? The grid fins won’t have control authority at near zero velocity, and it’s hard to see gimbaling being that precise. Elon said 6 bar from tank ullage is enough for vacuum but will it work for this?
And… can you give Elon follow-up questions? Like IF the HLS landing engines can’t be eliminated will they be pressure fed? And 99.99999% surely methalox. And will they need to lift HLS off the surface?
Mr Musk picture is a rare example of a happy Billionaire.
its so difficult to be happy when you are that rich, Mr Musk hit the sweet spot.
Fantastic interview! Musk hasn’t given us this kind of detail anywhere else.
Can you share when will you upload the next part?
Musk’s engineering philosophy is sheer brilliance, loved hearing him articulate their process down to 5 steps
“The engines, including mounting mass, are 2 tonnes”
Listening to the interview the number seems to relate to each engine (58 tonnes for 29 engines)
Since the lunar Starship, and other variants that don’t have aerodynamic surfaces and a TPS, don’t have the ability to re-enter earth atmosphere, there is no reason for them to have sea-level Raptors. That pretty much is a given.
The post said “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.”
It does imply they may have to gimbal the vacuum Raptors to land on the moon. Net result is higher payload capacity, or less fuel to take off from earth.
It’s a minimal implication – Elon said they expect to use 3 regular center and 3 Rvacs for HLS. A regular center one togimbal for control – and having more than one gives redundancy. Anyway, somewhere in these interviews he threw cold water on the idea regular SS would use only Rvacs to get to orbit after separation from SH. All 6 engines will be used.
Vacuum Raptors have much larger exhaust bells so the gimballed positions might not have enough clearance for them. Other than that you are correct that RVac would be preferable for the non-returning variants.
I agree they will require a wider mounting pattern, but just assumed they would do that
While they will need a pretty large thrust to get out of earths gravity well, they need very little thrust to land/taskeoff on the moon. Might still require vacuum Raptors in outer position, with a much smaller engine on gimbal for landing since the minimum throttle on the Raptor may be near 10X what is needed for landing.
Sea-level raptors can still be fired in vacuum, they are just not as efficient since they are subject to under-expansion, which reduces their ISP. But they should still reliably and safely work. Thus, they can be used to control a lunar descent/takeoff, and for in-space thrust vectoring.
Vacuum raptors, however, cannot (or, at least, should not) be fired in any significant atmosphere as they would be subject to over-expansion, which can cause flow separation, which causes combustion instability, that could (will?) lead to the engine blowing up.
I mean “Thrust instability” not “Combustion instability”. Thrust instability can still lead to the engine blowing up.
Elon said that Starship needs the 3 sea level raptors to reach orbital velocity at lift off after booster séparation.
Since the booster powers the flight for nearly the first 3 minutes, it should be around 60km at MECO. This is where a vacuum optimized engine will perform much better than a sea level optimized engine. Since the lunar and Mars Starships will never go back to earth, there really isn’t a reason to carry heavy sea level engines that are not optimal.
Elon says they might need the base engines to gimbal during lunar landing (if not using high engines on moon), and the vacuum ones have too big a nozzle to gimbal enough.
Standard Raptors only throttle between 90-225T of thrust which is matched to earth gravity of 9.81ms^2. Our Moon’s gravity is 1.62ms^2, or about 16% which suggests a standard Raptor engine is over powered for Moon landing operations by a factor of 6x to 10x. Sea level raptors have a target weight of 1,500 kg (3,300 lb) each, which for 3 of them is a lot of extra mass to pack to the moon and back, and a lot of extra fuel. Elon talks extensively about optimizations which are wrong, and the process for proper optimization …. trying to optimize sea level Raptors for moon operations is just plain wrong, when the right approach would be to optimize with a much smaller engine, or to optimize the vacuum Raptor throttle range with as placement that can be gimballed.
Excellent interview thanks Tim and Elon.
The writeup doesn’t (yet?) include the bit about the relative complexity of the launch site infrastructure, or I missed that bit 🙂
I really want a transcript of the story about the fiberglass around the Tesla battery, that illustrates point “the most common error of a smart engineer is to optimize something that should not exist” That is some a story that everyone in every industry should read.
Ugh – the Engineering philosophy is awesome. It can be applied to every kind of project no matter what it is. Thanks for this!
After Elons tweet about not being on is best because of lack of sleep and serious back pain I could help noticing that during the second part. Soooo looking forward to the third part. You guys keep up the good work.
I believe the statement “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” actually was said about the first principle dealing with requirements. Those five principles were the key to the entire discussion for me. Thanks for getting & doing this interview!
Nice summary, thx.
Did the question of improvements to weld quality come up during the interview? Seems like weld/production quality is incredibly important to speed and confidence for each new launch. I don’t recall hearing EM commenting on that in the first two interviews.
If flying lunar regolith takes out 5 engines on landing, can it take off and achieve lunar orbit on 1 engine. In an emergency would a sea level engine work on the moon?
Thank you for the write-up! I was able to digest the lessons much better reading this after watching the video.
One suggestion: for posteriority’s’ sake, embed the video on top of the writeup so it’s more accessable for future search queries.
Using ullage gas venting for maneuvering SH in space or near-space sounds great – Elon said 6 bar is enough force in vacuum. But what about the last moments of maneuvering into the catcher arms? As SH slows the grid fins will have less and less control authority. Trevor, please have Tim ask this as a follow up when/if he can! And will there be a moment or two of hover or near hover?
Why no links to the interview videos on YouTube?
I’d be interested to learn how the plasma flows around the ship during re-entry and if it would make sense to build a shield into the ship’s body to protect the hinges.
“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”
No, Elon was speaking primarily about operating and replacement costs, not just capital costs. If each earth-launched starship (SS+Booter) was expected to last, say, 10 launches, then you’d need the cost per run + 1/10 capital replacement cost to be <$230k. Obviously, for this to work, the launch lifetime must be longer than 10 and the capital replacement costs must be driven down.
Thanks for sharing the tour video footage with everyone Tim!
Great summary of the 5 steps, my sythesis is here: https://www.linkedin.com/pulse/elon-musk-five-steps-moving-fast-john-held/?published=t