Starship / Superheavy autogenous pressurization questions

[BarryKirk]

I'll start things off by saying that I'm a long time SpaceX amazing people and I've watched them do one impossible thing after another.

So, if SpaceX says they can do something, I'm inclined to believe that they can.  On the other hand, they are smart enough to realize that some things while doable, may not be worth it.

So, I'm looking at the statement that SpaceX claims that Starship and Superheavy will use autogenous pressurization and I was thinking of all the pro and cons and what makes this such a hard nut to crack.

I haven't seen any discussion about it other then... "oh yes, they will use autogenous pressurization, it will save weight, nothing much to see here, move on...."

But....

I know the big advantages.

1. No COPV and all the problems associated with it.
2. One less gas to deal with at the launchpad.

But, and I haven't researched this one, so, it's very possible others have done autogenous pressurization.

But this one may be a little more difficult for SpaceX then others because... they are using subcooled fuel and oxidizer near the freezing point of the liquids and not the boiling point.

So, how do you create the gas to pressurize the tanks?

It may be similar problems for both the LOX and LNG.

Do you heat up the liquids till they boil?

Where do you put the heaters?

You certainly want to keep the gas formation as far away from the suction pipes for the engines as possible, because getting gas in your intake would be a bad day for your engine(s).

If you put the heaters at the bottom of the tank, then the gas will have to bubble up to the top of the tank.

If you put the heaters at the top of the tank then they will be heating up a gas and will probably have to be larger and heavier.

Also, their is the controls problem.  If you add heat to a liquid near the boiling point, it's a much easier controls problem then adding heat to a liquid near the freezing point.

I suppose you could pump in liquids from the bottom, boil it off and pipe it to the top of the tank.

Then there is the question of transitions.  When you start or stop the engines, the flow of gas into the tank needs to start and stop quickly so you don't over pressure or worse pull a vacuum on the tank.

Then there is the question of having introduced relatively hot gases to the tank, how does that affect sub chilled liquids during loiter times?

One last point, the gas filling the tank will be either Oxygen for the LOX tank or Methane for the LNG tank.  Both of those gasses are substantially heavier then the Helium they are currently using.

Just thinking that I haven't seen much discussion on this topic. 

[jpo234]

Where do you put the heaters?

You have these hot, fiery engines that need cooling...

[meekGee]

You've solved most of the problems yourself as you wrote your post...

Liquid will be piped out, heated also by the engines, and introduced as a gas up top.

The gas will continuously condense into the top of the liquid, and slowly warm it up.  No harm in that - it happens anyway.  The important thing is to have it chilled in the first minute.

The extra mass doesn't matter since it's propellant...  Keep using it along with reserves as you refill the tank.

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Plus, there's no He on Mars, so He is a non-starter...  They have to work this one out, and it's been done before (Shuttle worked that way I believe)

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ABCD: Always Be Counting Down

[Bananas_on_Mars]

STS used autogenous pressurisation of the external tank after engine start. Subcooled fuel and liquid oxygen let's you put more mass in the same volume, because of the higher density. After part of the fuel and oxidiser are already used for orbit insertion, the tanks are only partly fuelled, so there's no problem with the volume increase due to warming up the fuel and oxidiser. I think Starship will use LOX and liquid methane at boiling point temperatures for anything that involves fuel transfer on orbit.

Re "lighter helium", i guess that's only correct if you take the gas mass, but with the necessary sub systems it might not have a mass advantage.
And the helium used for Falcon 9 already makes up for a big portion of the consumables cost of the rocket.
I think i heard that Falcon 9 already uses up a sizeable amount of the yearly helium production of the united states, but can't remember any numbers.

[BarryKirk]

There are two reasons that SpaceX uses subcooled fuel and oxidizer.

1. Higher density in the fuel tank means the same size fuel tank can carry more.

2. Higher density liquids feeding the engines means the same size engine can pump more fuel and oxidizer and thereby has a higher thrust to weight ratio.

Your correct that as the tank empties out, the same size tank can handle the lowered density of the remaining fuel.

However, the reduced thrust to weight ratio of the engine could be a problem.  At least during the initial boost stage.

Once in orbit, the thrust to weight ratio has very little effect.

Another thought is how does this effect the loiter time since you've added a lot of heat into the cryotanks?

[ThomasGadd]

Where do you put the heaters?

You have these hot, fiery engines that need cooling...

What do you use when your engines are cold.

[Kaputnik]

I've tried to find a list of vehicles that use autogenous pressurisation, without any luck.
From what I understand, STS used it on both propellants, and Centaur uses it for the H2 only. ACES (or is it ICES?) may use it for both propellants as part of the Integrated Vehicle Fluids upgrade.
Can anybody add to this list please?

[Kaputnik]

Where do you put the heaters?

You have these hot, fiery engines that need cooling...

What do you use when your engines are cold.

IANARS but I don't think there is a problem here. A cryogenic propellant will want to boil off to some degree, just not fast enough to keep up with an engine draining the tank. So your starting pressure is already there, and by the time you have to start worrying about lack of tank pressure, the engine will be up to temperature.

[Bananas_on_Mars]

I've tried to find a list of vehicles that use autogenous pressurisation, without any luck.
From what I understand, STS used it on both propellants, and Centaur uses it for the H2 only. ACES (or is it ICES?) may use it for both propellants as part of the Integrated Vehicle Fluids upgrade.
Can anybody add to this list please?

Vector is planning to use autogenous pressurisation for their pressure-fed engines using propylene and LOX.

[Tuna-Fish]

However, the reduced thrust to weight ratio of the engine could be a problem.  At least during the initial boost stage.

They need the highest possible thrust out of the engines for about a minute after liftoff. Beyond that, for most of the trajectory they throttle down anyway to reduce first aerodynamic forces on the vehicle and then g-forces on the crew.

Supercooled propellant is just a way to get more bang for your buck for that immediate just-off-the-launchpad thrust, which is what limits just how much fuel you can carry.

[groundbound]

Where do you put the heaters?

You have these hot, fiery engines that need cooling...

What do you use when your engines are cold.

IANARS but I don't think there is a problem here. A cryogenic propellant will want to boil off to some degree, just not fast enough to keep up with an engine draining the tank. So your starting pressure is already there, and by the time you have to start worrying about lack of tank pressure, the engine will be up to temperature.

This is an ignorant observation but it seems like that misses one situation. I don't know enough to understand whether this situation is likely.

If you need to perform a set of maneuvers with a sustained or heavy use of thrusters then it seems like you pretty much have to light up one of the mains. That could possibly end up being either very wasteful or potentially really bad.

Maybe that situation is just outside of the scope of what will happen, but if it is not, some little internal burner or electric heater system might be worth carrying along.

[BarryKirk]

Just a question.. In all other cases of auto genous pressurization, is the fuel LH2?

If so, that’s a very light gas to replace Helium with.

[Slarty1080]

Can anyone give us an educated guestimate of the rough process likely to be used to fire up a raptor engine?

I assume they don’t just open all of the valves fire an igniter and hope for the best? This would help understand at what point the autogenous pressurisation kicks in or how long it takes to build

[livingjw]

Can anyone give us an educated guestimate of the rough process likely to be used to fire up a raptor engine?

I assume they don’t just open all of the valves fire an igniter and hope for the best? This would help understand at what point the autogenous pressurisation kicks in or how long it takes to build

The start sequence is something like the following:
- crack valves and dribble in propellants to pre-chill the engine.
- open valves and propellants flow into their respective pre-burners.
- spark ignites stoichiometric mixture in torches.
- torches ignite pre-burners
- pre-burner exhaust spins turbines attached to propellant pumps. (one for methane, one for LOX)
- main chamber torch ignites gaseous propellants entering chamber.
- pumps start increasing pressure above head pressure and quickly climb to design pressure.

This requires detailed understanding of the combustion processes and the dynamics of the pumps, turbines and valves. It is a tightly choreographed dance.

- Hot gases for autogenous pressurization are available within a few seconds.

- Other heat options are needed for recharging gaseous RCS tanks and for re-pressurizing the main tanks before reaching Mars. They could use waste heat, electricity or combustion.

John

[BarryKirk]

It’s also going to be a very interesting control loop.

Certainly the outer loop will have a process value of tank pressure.

But there has to be a loop controlling the heat flow.

Reaction time has to be fairly quick and temperature loops tend to be fairly slow acting at least with temperature loops I’ve set up in the past.

Having said that most of my experience has been with processes that are very slow acting.

[livingjw]

It’s also going to be a very interesting control loop.

Certainly the outer loop will have a process value of tank pressure.

But there has to be a loop controlling the heat flow.

Reaction time has to be fairly quick and temperature loops tend to be fairly slow acting at least with temperature loops I’ve set up in the past.

Having said that most of my experience has been with processes that are very slow acting.

The gases are tapped off the engine in such a way as to obtain the correct temperature.  I don't know if temperature regulation is active or passive. A active pressure regulation and distribution system controls the flow to maintain the proper tank pressure.

John

[BarryKirk]

It’s also going to be a very interesting control loop.

Certainly the outer loop will have a process value of tank pressure.

But there has to be a loop controlling the heat flow.

Reaction time has to be fairly quick and temperature loops tend to be fairly slow acting at least with temperature loops I’ve set up in the past.

Having said that most of my experience has been with processes that are very slow acting.

The gases are tapped off the engine in such a way as to obtain the correct temperature.  I don't know if temperature regulation is active or passive. A active pressure regulation and distribution system controls the flow to maintain the proper tank pressure.

John

I'm struggling to understand how the gases are tapped off the engine to obtain the correct temperature.  More specifically, what that correct temperature should be.

I'm just going to focus on the fuel tank, I'm assuming the oxidizer tank will have similar issues.

I would guess that liquids are pumped around the engine which heats them up, vaporizes them and then the hot gases are piped to the top of the fuel tank.  This effectively makes the pipes surrounding the engine the heat exchanger.

So, several things, say process values that can be adjusted.

1. The flow (volume/sec ) of liquid introduced to the heat exchanger.

2. Maybe the percentage of the heat exchanger in use at any given time.

Remember that for a fixed size heat exchanger, if one limits the fluid flow, then the output gases are probably going to be hotter.

One has to remember that as the hot gases enter the fuel tank, they will cool off and the pressure will drop.

Since, certainly at the beginning of the stage first burn, the fuel is near the freezing point, it's possible that some of the gases in contact with the super cold liquid could condense back to a liquid themselves.

Here is an issue I see.  Lets say after SECO there is a coast phase.  During that coast phase, the engines, and hence the heat source is turned off.

However, during that time, the gases in the tank will cool off, thus dropping the pressure.  Since SECO occurs when the stage is outside the atmo, there is no danger of the pressure dropping below the external pressure.  And since, the rockets are off, there won't be forces causing the walls to buckle.

However, when restarting for a second or later burn, one has to hope that there is sufficient pressure to prevent buckling before additional gas can be created.

[renclod]

I would guess that liquids are pumped around the engine which heats them up, vaporizes them and then the hot gases are piped to the top of the fuel tank.  This effectively makes the pipes surrounding the engine the heat exchanger.

The SSME has distinct heat exchanger for producing autogenous pressurization gas.

https://commons.wikimedia.org/wiki/File:Ssme_schematic.svg

http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf

[magnemoe]

It’s also going to be a very interesting control loop.

Certainly the outer loop will have a process value of tank pressure.

But there has to be a loop controlling the heat flow.

Reaction time has to be fairly quick and temperature loops tend to be fairly slow acting at least with temperature loops I’ve set up in the past.

Having said that most of my experience has been with processes that are very slow acting.

The gases are tapped off the engine in such a way as to obtain the correct temperature.  I don't know if temperature regulation is active or passive. A active pressure regulation and distribution system controls the flow to maintain the proper tank pressure.

John

I'm struggling to understand how the gases are tapped off the engine to obtain the correct temperature.  More specifically, what that correct temperature should be.

I'm just going to focus on the fuel tank, I'm assuming the oxidizer tank will have similar issues.

I would guess that liquids are pumped around the engine which heats them up, vaporizes them and then the hot gases are piped to the top of the fuel tank.  This effectively makes the pipes surrounding the engine the heat exchanger.

So, several things, say process values that can be adjusted.

1. The flow (volume/sec ) of liquid introduced to the heat exchanger.

2. Maybe the percentage of the heat exchanger in use at any given time.

Remember that for a fixed size heat exchanger, if one limits the fluid flow, then the output gases are probably going to be hotter.

One has to remember that as the hot gases enter the fuel tank, they will cool off and the pressure will drop.

Since, certainly at the beginning of the stage first burn, the fuel is near the freezing point, it's possible that some of the gases in contact with the super cold liquid could condense back to a liquid themselves.

Here is an issue I see.  Lets say after SECO there is a coast phase.  During that coast phase, the engines, and hence the heat source is turned off.

However, during that time, the gases in the tank will cool off, thus dropping the pressure.  Since SECO occurs when the stage is outside the atmo, there is no danger of the pressure dropping below the external pressure.  And since, the rockets are off, there won't be forces causing the walls to buckle.

However, when restarting for a second or later burn, one has to hope that there is sufficient pressure to prevent buckling before additional gas can be created.

Assuming this work like the ACES upper stage you will have both high pressure tanks as an buffer and you will have an heat source to heat the gas, ACES uses an modified car engine it both provides heat and power.
You will also use the high pressure gas to power the reaction control rockets.
In space you can use the huge tanks to hold pressurized gas as they are mostly empty giving you lots of margin.

No its not an simple system, looking at ACES piping was pretty mindbogglingly.
Did not the Shuttle used it but in deep space things get more complex than during an launch.

[Slarty1080]

Snip...

I'm struggling to understand how the gases are tapped off the engine to obtain the correct temperature.  More specifically, what that correct temperature should be.

Snip...

I would have thought the tank pressure would be the overriding factor (within broad bands) rather than temperature, and would be linked to a valve on the fuel coolant loop. If the tank pressure is too low open the valve if it is too high close the valve. Obviously this is simplistic, but conveys the general idea. The difficulty would be in time lag; however this could be mitigated to a large extent by simulation of the exact set up and figuring out what works in what circumstances. So at tank pressure x and manifold pressure y we need to leaving the valve open fully for 10 seconds then shut it and check again in 10 seconds.. or whatever.