This is a deep-dive that may appear to be out of date. It may also have been blogged here or on the main blog. If it was blogged before, I cannot find it yet. So, at the risk of repeating myself, and since the Starship/Superheavy is still experiencing some troubles with engine outs, I theorize something here that may have been seemingly fixed, but hasn't been really fixed for good.
The possible issue here is the development of ice in the ship and booster, and this ice could be the reason for the engine failures.
Indeed, the AI was quizzed and seemed to deny that this is a problem, but I have an issue with that conclusion. To be sure, I haven't watched all of this deep dive as of this writing, but I think I've seen enough to make a post. So here it is: NOTE: AI dialogue not included, but may be added later...
Updated:
I watched the rest of the video, and I was still dissatisfied. Rather than ask the GAB AI, I asked the Brave AI. I was in luck. It turns out that I am most likely on the right track. It is not clear that the Starship system has done away with the problem altogether, or not. It appears from the Brave AI that they've probably not implemented the most likely permanent fix.
If so, I thought as much because I saw or heard somewhere about icing again. That's what made me come back to this CSI Starbase deep-dive to take another look.
Anyway, here's the Brave AI part that I saved for your perusal below:
## Autologous vs. Autogenous: The Distinction
The term **"autologous"** (as used by SpaceX engineers in CSI Starbase Q&As) refers specifically to the **clean-gas** variant of autogenous pressurization. The distinction:
| | **Autogenous** (current Starship) | **Autologous** (clean / proposed) |
|---|---|---|
| Gas source | Preburner exhaust | Dedicated heat exchanger on a propellant bleed |
| Gas composition | "Dirty" — O₂ + H₂O + CO₂ + trace CH₄ | **Pure** single-component (O₂ or CH₄) |
| Contamination | Icing, dissolution loss | None |
| Hardware | Tap-off valve from preburner | Dedicated heat exchanger + bleed line |
## How Autologous Pressurization Works
The mechanism is straightforward:
1. **Bleed**: A small flow of liquid propellant (O₂ or CH₄) is tapped from the turbopump discharge — the same high-pressure liquid that's already being pumped to the main injector.
2. **Heat exchanger**: That liquid passes through a **dedicated heat exchanger** that is thermally coupled to the engine's waste heat (typically the gas generator or turbine exhaust, or the regenerative cooling loop). The liquid absorbs enough heat to fully vaporize.
3. **Feed back**: The resulting **pure gaseous propellant** is routed back up into the ullage of the tank it came from.
The critical difference from the current "dirty" system: the gas **never contacts combustion products**. It's the same molecule that was in the tank, just in a different phase. No water, no CO₂, no unburned fuel.
## Historical Precedent: The Space Shuttle
The Space Shuttle External Tank used exactly this approach. After SSME ignition:
- A small bleed of LH₂ was taken from the turbopump, heated in a dedicated heat exchanger (warmed by the gas generator exhaust), and fed back as gaseous hydrogen into the LH₂ tank.
- A parallel bleed of LOX was heated and fed back as gaseous oxygen into the LOX tank.
This is the textbook autologous system — clean, single-component pressurant, no contamination.
## Why Starship Ditched It (For Now)
The Raptor 1/2 originally **did** have a heat exchanger for this purpose. SpaceX removed it in favor of the simpler preburner-exhaust tap because:
- One less heat exchanger = less mass, fewer parts, less thermal management
- The preburner exhaust is already at very high temperature (the O₂-rich preburner runs at ~1,500 °C), so it's an almost free heat source
- The "dirty gas" problem (icing) was initially underestimated
The tradeoff is the contamination: water and CO₂ in the LOX tank cause ice formation on cold surfaces, and the pressurant gas dissolves into the liquid (a parasitic propellant loss).
## The Path Back
The CSI Starbase Q&A (referenced in community discussions) explicitly identified autologous pressurization as the fix:
> *"One solution would be autologous pressurization where they could heat the oxygen and methane separately instead of using combustion product to pressurize the tanks."*
For Raptor 3, the question is whether the **auxiliary cooling system** (ACS) — the additional heat exchanger surface added to handle the higher chamber temperatures — has enough thermal capacity to double as the autologous heat exchanger. If so, the engine can produce clean pressurant gas without adding a separate component. This would:
- Eliminate the icing problem entirely
- Eliminate pressurant dissolution losses
- Make the ullage gas safe to use as a **hot-gas thruster** propellant (pure O₂ or CH₄, rather than a reactive mixture)
- Enable **non-pumping mode** operation (pressure-fed burns) for Mars landing and in-space operations, where you can't spin up the turbopumps
It's not yet confirmed as a flight implementation, but it's the logical end-state for the system.
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