I was wondering why the Falcon 9 vents so much gas at liftoff. It’s quite apparent in this video of the crew 2 demo, gas is venting from the top of the rocket and looks like it even ignites at the bottom of the rocket. Also apparent in a cargo launch here, including ignition of the emitted gasses. It’s less apparent during the recent crew launch but again a big trail of gas down the rocket that seems to ignite in the exhaust flume.
It looks like the vehicle is a teakettle over-boiling with flammable gas that gets ignited as it drifts into the rocket plume. I recall Stranger on a Train mentioning something about the fuel in Falcon 9’s being overpressure – is this an effect of that? What’s up and how does this not put the rocket at risk?
Your teakettle analogy is apt. The Falcon 9 is fueled by kerosene and liquid oxygen which are both kept chilled to maximize the fuel density. The oxygen has to be continually topped off right up to the last second because it boils off. The plumes you see are actually condensation forming around the cold oxygen gas escaping from the rocket.
Yeah, it occurred to me that it must be oxygen trailing, otherwise the flight would kaboom when the plume ignited. I’m still surprised that NASA is cool with the oxy venting turning into an afterburner in flight. This seems unwise.
This isn’t true of the Falcon 9. Most rockets *do *keep their liquid oxygen at the boiling point; the temperature here is stable, and as you say the LOX can be topped off as some of it boils away. A rocket can be kept on the pad indefinitely this way.
The Falcon 9 however uses subcooled liquid oxygen: oxygen that is well below the boiling point, and nearly at the freezing point. It does this to increase performance: the fuel is several percent denser (maybe 6-7%), which allows squeezing that much more in the tanks, and furthermore means that much more flow into the engines (which can only pump so quickly due to cavitation limits).
The disadvantage is that every moment on the pad, the oxygen is warming up and becoming less dense. If it stays there for too long, it has to drain or vent the excess; worse, it means the rocket can’t hit its required performance.
Therefore, the propellant loading happens as late as possible: it starts about a half-hour before launch, and finishes at about T-5 minutes. This was an issue for NASA crew launches, because they would have preferred the crew to board only after the propellant load, but 5 minutes isn’t enough time. Eventually NASA got over it and the loading happens while the astronauts are in the capsule.
If there’s a scrub, the tanks are drained and the propellant chilled again. For payloads where timing is not critical, the rocket can spend maybe a couple tens of minutes on the pad before they have to drain, but not much more.
The flare when the plume reaches the engines is likely just the cloud being illuminated, and not any kind of flame.
I feel like I didn’t get an answer to part of my question though. Why does the Falcon 9 seem to vent so prolifically when flying? I don’t see the same thing with (say) a Delta IV Heavy launch, or an Atlas. There’s some boil off before ignition in others, but nothing like the Falcon in flight.
See that orange stuff covering the first stages of both the Delta IV and Atlas V? That’s insulation. Should significantly reduce the boiloff.
To be honest, I’m not sure why SpaceX decided against insulation for the Falcon 9. I guess they assumed that with the subcooled LOX, they wouldn’t be sitting on the pad for long anyway.
BTW, I made a small error above: the Falcon 9 does top off the LOX up until launch. Although it’s true that subcooled LOX won’t boil, there must be enough boiling-point LOX at the surface to make a difference.
Yes. The saturation vapor pressure of a substance increases pretty rapidly with temperature; if you have a pressure vessel containing a substantial amount of liquid, and you heat it to a temperature well above the substance’s atmospheric-pressure boiling point, you can generate very large pressures. It’s a design issue for propane tanks, which are filled with liquid propane: at “room temperature” (~20C) the tank is at about 10 atmospheres (150 psi), but if you let it get up to 50C, it will be at about twice that.
Oxygen would be even worse: by the time it warms up to about -120C, your tank would already be at about 50 atmospheres (750 psi). I can’t see what the pressure would be if the tank actually got up to room temperature, but it certainly be far higher. Even assuming only 750 psi, that’s an awful lot for a pressure vessel 12 feet in diameter. A pressure vessel can be made to withstand that - IIRC, the space shuttle’s SRBs were about 10 feet in diameter and had an internal pressure of around 1000 psi - but you end up with a thick, heavy pressure vessel (the shuttle SRB skins were 2 cm thick steel, whereas the Falcon’s tanks are 4.7mm aluminum). Building the Falcon with a super-strong tank would let it keep every last gram of the precious, precious oxygen - but it’d be so heavy it would never get off of the pad.
I couldn’t find out for sure, but I’d bet that the tanks are maintained at some modest pressure to help stabilize them against buckling failure.
probably decided they didn’t want the weight, and I can imagine coating the outside with foam would make refurbishing for reuse more difficult. so they skipped the insulation and just went with late propellant loading. Delta IV starts prop load about 5 hours before launch.
Delta IV has another good excuse: it uses liquid hydrogen as fuel for its first stage. And that’s so cold, with such a high tank surface area (low density), that it really does need the insulation (see the Shuttle external tank, too). Not a big deal to apply it to the LOX tankage, too, while they were at it.
The Atlas V in comparison only uses hydrogen in the second stage, and kerosene in the first. Though admittedly, I’m seeing mixed information here: Wikipedia says the first stage has insulation, while the user’s guide does not. Also, the orange is not from the spray-on insulation (like on the Centaur upper stage)–it’s from some other coating process. In short, I’m not entirely sure what’s going on with the Atlas.
They are. I don’t think it’s known exactly what the pressures are, but numbers between around 50 to 80 psi have been thrown around. Starship prototypes have withstood pressures of 70-110 psi. Different vehicle obviously, but the basic design principles are the same.
There’s going to be a safety factor on top of whatever the nominal pressure is, but it’s not going to be a lot. Starship is going for a 1.4x safety factor. I wouldn’t expect the Falcon 9 to be more, and it may even be less.
The pressure also helps out the engine turbopumps.
up until the V, the Atlas rocket family used “balloon” tanks and would collapse if there wasn’t at least a certain amount of pressure in them. IIRC they kept them filled with pressurized nitrogen until fueling.
