Everyday Astronaut - Starbase Tour with Elon Musk

Tim Dodd, the Everyday Astronaut, was invited to interview Elon Musk on a private tour around Starbase on July 30, 2021. The more than two hour long interview was uploaded on youtube in three consecutive videos that shared not only behind the scene views but also insightful information about processes at SpaceX and Elon Musk's considerations. An article summarizing the main points of the interview can be found on the Everyday Astronaut website.

## 2021 - Part 1

### Conversation Starts

* A lot of work is being done for the final push for the launch complex (orbital launch site), Musk calls it basically "Stage Zero". This work is very hard.
* Elon Musk highlights again that he considers manufacturing to be much more difficult than design. More accurately, the design of the manufacturing process is much harder than the design of the object/thing itself.
	+ Example: SpaceX is trying very hard to make the cost per ton of thrust for the Raptor engines under $ 1000.

### High Bay [Booster, 5-step process]

* SpaceX is commited to optimize (fix) cost per ton to orbit and ultimately cost per ton to Mars.
* All designs are weak.
* Booster 4 and Engines:
	+ In the high bay, the B4 fuel (CH4) tank is visible with the inter stage (top ring section with: load points for picking up the booster, holes for grid fins)
	+ Dry mass of booster should be under 200 tonnes, which however is a moving target. Constantly under discussion is the margin for propellant residuals.
	+ Engines weight, as a rule of thumb, around 2 tons; with 29 engines this makes 58 tonnes for the engines.
	+ Fuel and LOX tank itself weight approximately 80 tonnes.
	+ The inter stage (grid fins, batteries, ...) weight around 20 tonnes.
	+ Propellant residuals: 20 tonnes.
	+ All together: 160-200 tonnes; But now, everything is too heavy, even the avionics
	+ For the grid fins actuation, they currently use energy optimized batteries; battery mass could drop by a factor of 10.
	+ The booster is designed for 300 tonnes of propellant, around 78% of it is LOX (by mass), which they burn at 3.7-3.8.
* The grid fins won't fold.
* Elon Musk's 5 step engineering process (must be completed in this order):
	+ **Make the requirements less dumb.** Requirements are always dumb, especially if smart people give them. The threat is that you might not question the requirements enough.
	+ **Try 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 or optimize.** This is the third step and should not be done ahead of the first two steps. It is a common error to optimize something that shouldn't exist in the first place.
	+ **Accelerate the 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.** Hereby it is important to remove in-process testing. Reducing in-process testing in favour of end-in-line-testing reduces the threat of false positives.

### Grid Fins [and SN8-11 RUDs]

* The teeth of the grid fins help both for transsonic and subsonic regimes. The more pointy they are, the more effectively they work.
* There is still a lot of mass that SpaceX can get out of the grid fins.
* There is some sort of L-channel on top of the fuel dome for the motor that is going to rotate the fins with a gear box. They're using Tesla model 3 motors.
* Talking about the RUDs of SN8-SN11, none of the reasons why they exploded was on a risk list!!

### V2 Raptor

* Parts of the new V2 Raptor engines have been produced already, mainly the thrust chamber assembly and the design of the pump is complete. First tests could be seen in about one month.
* Volume production of V2 Raptor will be at McGregor. However, they will keep their Raptor facility in Hawthorne, California, for development Raptors and Rvac production.
* The goal for V2 Raptor is 300 or 298 bar, they achieved 230 bar already. The sea level Raptors will reach 230 tonnes of thrust.
* "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."

### HLS; Stage Separation / Hot Gas Thruster

* In compliance with the 5-step process, SpaceX decided to remove the pushing separation system for stage separation. They plan to use the main booster engines to initiate a rotation by gimbaling them before shut down. This will cause the vehicles to move apart. Only some additional cold gas ACS thrusters will be needed.
* Furthermore, they decided to remove the hot gas thrusters from the booster and use the main tank vents (ullage gas) instead by controlling their direction. The ullage gas would need to be vented either way.
* They even might use this for Starship (second stage) as well.

### HLS

* It is currently planned to use 24 thruster for landing the lunar landing variant of Starship on the moon. If they would use the main engines instead, there would be the threat that they would dig a huge hole/ditch so they couldn't land.
* However, Elon Musk thinks it would likely be possible to use the main engines, but they would need to prove this in an experimental setting.
* They will likely use the same Raptor configuration (3 RS and 3 Rvac) for the HLS variant.

## 2022 - Part 1

-First orbital flight by B7 and S24, likely with Starlink satellites in an electrically actuated dispenser

-Ullage gas thrusters on both ship and booster

-Thrust to weight ratio of full stack will be around 1.5 at lift off

-Ship fwd flaps will need to be improved massively, possibly even removing them or moving them more to the leeward side

-No more suborbital flights

-B7 120 degrees chines layout make sense due to pressure [?]; B7 will have lighter grid fins

-Future boosters will have less grid fins, not more than 3

-Megabay [high bay 2] supports up to 12 work stations, is 30-40 ft taller than high bay 1 and has 4x usable area

## 2022 - Part 2

-Orbital flight will follow same trajectory as originally released by FCC exhibit

-Booster has a 250t catch weight, goal is 200t; mach 0.5 at start of landing burn; chines will be facing perpendicular to catch arms

-HLS mostly digital work currently; focus is orbital test flight

-Starlink 2.0: 7m size, some allready on site, 1.25t weight, only fit in Starship, has higher throughput and useful bits

## 2022 - Part 4

This part centered on the Raptor engines and its development:

* Diagnostic center for Raptor 2s
* Ship engine numbers spotted onsite: RC: SN69, SN73, SN78; RV: SN4, SN5, SN6
* Starlink V2s were onsite

Thrust:

* Raptor 1: 180-185 tonnes at around 250 bar
* Raptor 2: 230-250 tonnes at around 300 bar

Recent developments:

* Eliminated torch igniters in Raptor 2, which were a main point of complexity, cost and failure modes
* Blown up 20-30 engines during testing; melted around 50 combustion chambers
* Raptor 2 currently being updated for **robustness** instead of performance
* Higher percentages of head end and throat film cooling; losing a couple point of efficiency because of cooling
* Hydraulic gimballing on Raptor is mostly contrlled by the vehicle and not the engine itself; electric servo gimbal could be added to engines within a few months
* Moving from bolted interfaces to welded interfaces to reduce leak rates of fluids

Problems and technical info:

* The start sequence is one of the largest points of failure with the Raptor engine, while keeping it running is relatively easy
* Raptor engines have such a small combustion chamber because the propellants are already gaseous and 90% pre-mixed using swirl injectors; the acutal combustion chamber does only around 10% of the mixing
* Main goal of Raptor is to reduce the amount of components susceptible to heat by so much that engine shrouds can be eliminated
* Raptor's oxygen pump inducer has a very flat propeller that cuts the propellant at a shallow angle to reduce bubbles and cavitation
* Inducer and two impellers get the oxygen side of the Raptor mechanism to an average operating pressure of 700-800 bar
* Raptor is one of the only engines that has a completely inline powerhead

General statements:

* SpaceX is most likely never going to make a main engine with lower thurst or lower chamber pressure than their current iterative model
* While ISP is important, what really matters is the area under the force-vs-time curve
* Confirmation that during initial B7 testing, part of the transfer tube collapsed, this has been fixed now

1. ↑ https://everydayastronaut.com/starbase-tour-and-interview-with-elon-musk/
2. ↑ https://everydayastronaut.com/starbase-tour-and-interview-with-elon-musk/
3. ↑ https://everydayastronaut.com/starbase-tour-and-interview-with-elon-musk/
4. ↑ https://everydayastronaut.com/starbase-tour-and-interview-with-elon-musk/