7 ms·

Isn't the point of the booster to use up the entirety of its fuel to propel the main rocket? Is it normal for there to be enough fuel left in it to cause such a

Isn't the point of the booster to use up the entirety of its fuel to propel the main rocket? Is it normal for there to be enough fuel left in it to cause such an explosion?

jackmott 3y ago
There is often some margin, and it doesn’t take much. Many early failed falcon 9 landings were very explodey
orbital-decay 3y ago
Yes, there's always residual fuel and oxidizer in the piping, it's always accounted for. For the first stage of a rocket of this size, it should be around a couple dozen kg, give or take. And hypergolic components ignite on contact.
perihelions 3y ago
You can't pump a fuel tank entirely empty. (You could try to displace it with an inert filler gas—compressed helium, for example—but then you're replacing fuel mass with filler-gas mass, and that brings its own set of tradeoffs). (I understand it's a particularly deep and difficult engineering problem to solve this in space: in zero-gravity, liquid fuels turn into bubbles floating around in nearly-empty fuel tanks—all sorts of phenomena going on).
- CobrastanJorji 3y ago
  By no means am I an engineer, but the first thing that comes to mind to me is those little toothpaste keys, and I wonder if you could perhaps just reduce the size of the canister as it empties. I sort of imagine a board with a spring on the top end of the canister that is held compressed under pressure but pushes towards the other end as the fuel is used up. Perhaps the pressure is so great when full that this is very difficult?
  - positr0n 3y ago
    If that were possible with the fuels rockets use (liquid hydrogen, liquid oxygen, liquid methane, etc.) it is still probably way cheaper on every axis to just make the tank 3% bigger and ignore the waste. Common axes they care about in rocket design are weight, cost in money, reliability/complexity, and lately reusability.
  - mohaine 3y ago
    These are not cryogenic so something is probably possible but this would add extra weight/mass. You do not want extra weight on your rocket. Every bit of non-fuel weight you add means you have to add extra fuel, and the extra fuel to carry the extra fuel along with any extra tank needed for the extra fuel. It gets bad fast. It just isn't worth it when you can just add that weight as fuel that normally doesn't get used. Ideally would dump in the ocean (which you cleared) so it didn't matter (much).
  - krisoft 3y ago
    They do that for smaller engines. Absolutely. They call it a bladder tank. Here is an article about it: https://www.space-propulsion.com/spacecraft-propulsion/hydrazine-tanks/hydrazine-tank-overview.html https://www.space-propulsion.com/spacecraft-propulsion/hydra... > I sort of imagine a board with a spring on the top end of the canister You are better off doing it with gas pressure instead of a spring, but yeah absolutely. Very good thinking.
- krisoft 3y ago
  > in zero-gravity, liquid fuels turn into bubbles floating around in nearly-empty fuel tanks Perihelions says it absolutely right. (Fitting nick name :)) There is a beautifull footage of the phenomenon from spacex: https://youtu.be/mVAGoWJuDKk?si=AkjH5hAm-dHC0V87 https://youtu.be/mVAGoWJuDKk?si=AkjH5hAm-dHC0V87 From 34s onwards it shows an internal view of the liquid oxygen tank. Just to better understand the picture the camera is inside the cylindrical tank. The viewpoint is facing backwards, so the camera is at the bulkhead closer to the tip of the rocket and it is looking “down” towards the engines. The blue “magical portal” is the surface of the liquid oxygen draining away rapidly by the rocket engines. And then suddenly there is main engine cutoff and the rocket is no longer pushed forward by the engines. That is when you see that the “magical portal” sloshes to the side and then breaks up into many floating bubbles. They float because the whole rocket, including the liquid oxygen is in free fall. Why is this a hard problem? It is absolutely not a problem… unless you want to re-ignite the engine. (Which you might want to do to conduct further maneuvers) If you do that and don’t do anything special to shepherd the fuel to the pumps of the rocket engine they might not suck in a propellant bubble. If they don’t your engine fails to reignite. If the engine ignited, but then the pumps suck in a gas bubble they will break apart. So you absolutely have to coax the fuel to the intake before you try to reignite. How do you do that? There are multiple strategies. At smaller scales you can try to have a flexible bladder. But at large rockets your best bet is to accelerate the rocket forward so all the propellant sloshes to the back and then you can ignite. Once the engine(s) are ignited you don’t have to worry about it anymore (that much) because the acceleration will keep the propellants bunched up at the aft bulkhead. But if you need the engines to be ignited to accelerate, and you need to accelerate to ignite them then you have a circular dependency. How do you in theory solve this? You have some other mean of temporary propulsion. Either a cold gas truster, or a monopropellant rocket with a flexible blader, or a solid propellant engine. It doesn’t have to be powerfull, and it doesn’t have to push you for long. Just enough to collect the propellant while the main engines are reignited. These auxilary motors are frequently called ullage rockets.[1] Now of course how long is long enough, and how much push you need is the black magic with the whole thing. (And of course this has nothing to do with the pictured rockets. I don’t think those are reignitable.) 1: https://en.m.wikipedia.org/wiki/Ullage_motor https://en.m.wikipedia.org/wiki/Ullage_motor
  - 3y ago
numpad0 3y ago
You also don't want the booster to run out of gas while still attached, as anything in a rocket could easily end in a fireball
LorenPechtel 3y ago
Trying to burn your tank dry is asking for an explosion when the pumps suck gas instead of liquid. And if the fuel feed starts to fail while there still is oxygen the engine itself will burn.

Isn't the point of the booster to use up the entirety of its fuel to propel the main rocket? Is it normal for there to be enough fuel left in it to cause such an explosion?

jackmott 3y ago

There is often some margin, and it doesn’t take much. Many early failed falcon 9 landings were very explodey

orbital-decay 3y ago

Yes, there's always residual fuel and oxidizer in the piping, it's always accounted for. For the first stage of a rocket of this size, it should be around a couple dozen kg, give or take. And hypergolic components ignite on contact.

perihelions 3y ago

You can't pump a fuel tank entirely empty. (You could try to displace it with an inert filler gas—compressed helium, for example—but then you're replacing fuel mass with filler-gas mass, and that brings its own set of tradeoffs). (I understand it's a particularly deep and difficult engineering problem to solve this in space: in zero-gravity, liquid fuels turn into bubbles floating around in nearly-empty fuel tanks—all sorts of phenomena going on).

CobrastanJorji 3y ago
By no means am I an engineer, but the first thing that comes to mind to me is those little toothpaste keys, and I wonder if you could perhaps just reduce the size of the canister as it empties. I sort of imagine a board with a spring on the top end of the canister that is held compressed under pressure but pushes towards the other end as the fuel is used up. Perhaps the pressure is so great when full that this is very difficult?
- positr0n 3y ago
  If that were possible with the fuels rockets use (liquid hydrogen, liquid oxygen, liquid methane, etc.) it is still probably way cheaper on every axis to just make the tank 3% bigger and ignore the waste. Common axes they care about in rocket design are weight, cost in money, reliability/complexity, and lately reusability.
- mohaine 3y ago
  These are not cryogenic so something is probably possible but this would add extra weight/mass. You do not want extra weight on your rocket. Every bit of non-fuel weight you add means you have to add extra fuel, and the extra fuel to carry the extra fuel along with any extra tank needed for the extra fuel. It gets bad fast. It just isn't worth it when you can just add that weight as fuel that normally doesn't get used. Ideally would dump in the ocean (which you cleared) so it didn't matter (much).
- krisoft 3y ago
  They do that for smaller engines. Absolutely. They call it a bladder tank. Here is an article about it: https://www.space-propulsion.com/spacecraft-propulsion/hydrazine-tanks/hydrazine-tank-overview.html https://www.space-propulsion.com/spacecraft-propulsion/hydra... > I sort of imagine a board with a spring on the top end of the canister You are better off doing it with gas pressure instead of a spring, but yeah absolutely. Very good thinking.
krisoft 3y ago
> in zero-gravity, liquid fuels turn into bubbles floating around in nearly-empty fuel tanks Perihelions says it absolutely right. (Fitting nick name :)) There is a beautifull footage of the phenomenon from spacex: https://youtu.be/mVAGoWJuDKk?si=AkjH5hAm-dHC0V87 https://youtu.be/mVAGoWJuDKk?si=AkjH5hAm-dHC0V87 From 34s onwards it shows an internal view of the liquid oxygen tank. Just to better understand the picture the camera is inside the cylindrical tank. The viewpoint is facing backwards, so the camera is at the bulkhead closer to the tip of the rocket and it is looking “down” towards the engines. The blue “magical portal” is the surface of the liquid oxygen draining away rapidly by the rocket engines. And then suddenly there is main engine cutoff and the rocket is no longer pushed forward by the engines. That is when you see that the “magical portal” sloshes to the side and then breaks up into many floating bubbles. They float because the whole rocket, including the liquid oxygen is in free fall. Why is this a hard problem? It is absolutely not a problem… unless you want to re-ignite the engine. (Which you might want to do to conduct further maneuvers) If you do that and don’t do anything special to shepherd the fuel to the pumps of the rocket engine they might not suck in a propellant bubble. If they don’t your engine fails to reignite. If the engine ignited, but then the pumps suck in a gas bubble they will break apart. So you absolutely have to coax the fuel to the intake before you try to reignite. How do you do that? There are multiple strategies. At smaller scales you can try to have a flexible bladder. But at large rockets your best bet is to accelerate the rocket forward so all the propellant sloshes to the back and then you can ignite. Once the engine(s) are ignited you don’t have to worry about it anymore (that much) because the acceleration will keep the propellants bunched up at the aft bulkhead. But if you need the engines to be ignited to accelerate, and you need to accelerate to ignite them then you have a circular dependency. How do you in theory solve this? You have some other mean of temporary propulsion. Either a cold gas truster, or a monopropellant rocket with a flexible blader, or a solid propellant engine. It doesn’t have to be powerfull, and it doesn’t have to push you for long. Just enough to collect the propellant while the main engines are reignited. These auxilary motors are frequently called ullage rockets.[1] Now of course how long is long enough, and how much push you need is the black magic with the whole thing. (And of course this has nothing to do with the pictured rockets. I don’t think those are reignitable.) 1: https://en.m.wikipedia.org/wiki/Ullage_motor https://en.m.wikipedia.org/wiki/Ullage_motor
- 3y ago

numpad0 3y ago

You also don't want the booster to run out of gas while still attached, as anything in a rocket could easily end in a fireball

LorenPechtel 3y ago

Trying to burn your tank dry is asking for an explosion when the pumps suck gas instead of liquid. And if the fuel feed starts to fail while there still is oxygen the engine itself will burn.