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.

--

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)

-----
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.

[livingjw]

- During main engine burns, they can tap off gases of different temperature and mix them to control temperature. They then use a pressure regulator to manage tank pressure.

- Max tank temperature will depend on tank material or possibly other items placed in the tanks. I would guess somewhere between 200 - 500 degrees F.

- After engine cut off in space, the tanks will be nearly empty and tank pressures will stabilize at some level which depends on heat loads, heat radiation, and insulation.

- An engine relight may need some re-pressurization before hand. If so, heating may be needed.

John

[josespeck]

- During main engine burns, they can tap off gases of different temperature and mix them to control temperature. They then use a pressure regulator to manage tank pressure.

- Max tank temperature will depend on tank material or possibly other items placed in the tanks. I would guess somewhere between 200 - 500 degrees F.

- After engine cut off in space, the tanks will be nearly empty and tank pressures will stabilize at some level which depends on heat loads, heat radiation, and insulation.

- An engine relight may need some re-pressurization before hand. If so, heating may be needed.

John

Is this what you want to say?

[envy887]

- During main engine burns, they can tap off gases of different temperature and mix them to control temperature. They then use a pressure regulator to manage tank pressure.

- Max tank temperature will depend on tank material or possibly other items placed in the tanks. I would guess somewhere between 200 - 500 degrees F.

- After engine cut off in space, the tanks will be nearly empty and tank pressures will stabilize at some level which depends on heat loads, heat radiation, and insulation.

- An engine relight may need some re-pressurization before hand. If so, heating may be needed.

John

Is this what you want to say?

No. The combustion chamber tap-off products are fuel-rich and you do not want to dump them in a LOX tank.

The "tap off" that John referred to is from earlier in the cycle. Since Raptor has hot fuel-rich gases coming out of the fuel preburner, and hot ox-rich gases coming out of the ox preburner, they could tap those separately and feed them to the fuel and LOX tanks.

[acsawdey]

- During main engine burns, they can tap off gases of different temperature and mix them to control temperature. They then use a pressure regulator to manage tank pressure.

- Max tank temperature will depend on tank material or possibly other items placed in the tanks. I would guess somewhere between 200 - 500 degrees F.

- After engine cut off in space, the tanks will be nearly empty and tank pressures will stabilize at some level which depends on heat loads, heat radiation, and insulation.

- An engine relight may need some re-pressurization before hand. If so, heating may be needed.

John

Is this what you want to say?

No. The combustion chamber tap-off products are fuel-rich and you do not want to dump them in a LOX tank.

The "tap off" that John referred to is from earlier in the cycle. Since Raptor has hot fuel-rich gases coming out of the fuel preburner, and hot ox-rich gases coming out of the ox preburner, they could tap those separately and feed them to the fuel and LOX tanks.

Questionable whether you'd want to do that as either of those will contain water as a combustion product which will freeze out in the fuel or oxidizer tanks. And the planetary geologists joke about how "ice is a mineral" at those temperatures, certainly not something you want clogging up fuel lines and injectors. Heat exchangers to boil a little methane or lox are simple by comparison.

[Jcc]

- During main engine burns, they can tap off gases of different temperature and mix them to control temperature. They then use a pressure regulator to manage tank pressure.

- Max tank temperature will depend on tank material or possibly other items placed in the tanks. I would guess somewhere between 200 - 500 degrees F.

- After engine cut off in space, the tanks will be nearly empty and tank pressures will stabilize at some level which depends on heat loads, heat radiation, and insulation.

- An engine relight may need some re-pressurization before hand. If so, heating may be needed.

John

Is this what you want to say?

No. The combustion chamber tap-off products are fuel-rich and you do not want to dump them in a LOX tank.

The "tap off" that John referred to is from earlier in the cycle. Since Raptor has hot fuel-rich gases coming out of the fuel preburner, and hot ox-rich gases coming out of the ox preburner, they could tap those separately and feed them to the fuel and LOX tanks.

Questionable whether you'd want to do that as either of those will contain water as a combustion product which will freeze out in the fuel or oxidizer tanks. And the planetary geologists joke about how "ice is a mineral" at those temperatures, certainly not something you want clogging up fuel lines and injectors. Heat exchangers to boil a little methane or lox are simple by comparison.

The heat exchanger coils can use the tap off products as working fluid, then dump overboard.

[livingjw]

- During main engine burns, they can tap off gases of different temperature and mix them to control temperature. They then use a pressure regulator to manage tank pressure.

- Max tank temperature will depend on tank material or possibly other items placed in the tanks. I would guess somewhere between 200 - 500 degrees F.

- After engine cut off in space, the tanks will be nearly empty and tank pressures will stabilize at some level which depends on heat loads, heat radiation, and insulation.

- An engine relight may need some re-pressurization before hand. If so, heating may be needed.

John

Is this what you want to say?

No. The combustion chamber tap-off products are fuel-rich and you do not want to dump them in a LOX tank.

The "tap off" that John referred to is from earlier in the cycle. Since Raptor has hot fuel-rich gases coming out of the fuel preburner, and hot ox-rich gases coming out of the ox preburner, they could tap those separately and feed them to the fuel and LOX tanks.

Questionable whether you'd want to do that as either of those will contain water as a combustion product which will freeze out in the fuel or oxidizer tanks. And the planetary geologists joke about how "ice is a mineral" at those temperatures, certainly not something you want clogging up fuel lines and injectors. Heat exchangers to boil a little methane or lox are simple by comparison.

I meant tapping off pure methane gas and pure oxygen gas from downstream of the pumps. They may put a small heat exchanger in the lox pre-burner to heat up some Lox and gasify it. I believe the SSME used this approach. Methane could be tapped after exiting the coolant channels.

John

[Lar]

The heat exchanger coils can use the tap off products as working fluid, then dump overboard.

Dumping any tapoff has a negative effect on ISP I think.

[magnemoe]

The heat exchanger coils can use the tap off products as working fluid, then dump overboard.

Dumping any tapoff has a negative effect on ISP I think.

Yes but this is only used if pressure in tanks are to low for an secondary burn and you need to raise pressure.

Another benefit of the header tanks as its not much volume to pressurize so you might only use them for engine startup even if having fuel left in main tank.

[livingjw]

The heat exchanger coils can use the tap off products as working fluid, then dump overboard.

Dumping any tapoff has a negative effect on ISP I think.

Yes but this is only used if pressure in tanks are to low for an secondary burn and you need to raise pressure.

Another benefit of the header tanks as its not much volume to pressurize so you might only use them for engine startup even if having fuel left in main tank.

- There is no dumping of tapped methane or Lox during engine burn. Liquid propellants are tapped off as needed to keep the tank pressure at the desired pressure during the engine burn.

- After engine shut down, tank pressures will rise with temperature and may have to be vented to maintain desired pressure. This can be minimized by orienting the vehicle so that exposure to sunlight is minimized.

- After Mars transfer burn, the main tanks are empty and all remaining vapor vented to better insulate the header tanks for the long voyage. Tanks would have to be re-pressurized with vapor before Mars entry sufficient to maintain vehicle structural integrity.

John

[josespeck]

- During main engine burns, they can tap off gases of different temperature and mix them to control temperature. They then use a pressure regulator to manage tank pressure.

- Max tank temperature will depend on tank material or possibly other items placed in the tanks. I would guess somewhere between 200 - 500 degrees F.

- After engine cut off in space, the tanks will be nearly empty and tank pressures will stabilize at some level which depends on heat loads, heat radiation, and insulation.

- An engine relight may need some re-pressurization before hand. If so, heating may be needed.

John

Is this what you want to say?

No. The combustion chamber tap-off products are fuel-rich and you do not want to dump them in a LOX tank.

The "tap off" that John referred to is from earlier in the cycle. Since Raptor has hot fuel-rich gases coming out of the fuel preburner, and hot ox-rich gases coming out of the ox preburner, they could tap those separately and feed them to the fuel and LOX tanks.

[Semmel]

I cant draw that nicely, so here is a written discription:

Methane:
Tank -> Pump -> Tap-Off -> Heat Exchanger with Combustion Chamber -> Tank

LoX:
Tank -> Pump -> Tap-Off -> Heat Exchanger with Combustion Chamber -> Tank

Both cycles have nothing to do with each other. Both cycles never touch the combustion products, just the heat generated by the combustion. The heat exchanger might as well be mounted on the pre-burners, I am not sure about that part.

[josespeck]

I cant draw that nicely, so here is a written discription:

Methane:
Tank -> Pump -> Tap-Off -> Heat Exchanger with Combustion Chamber -> Tank

LoX:
Tank -> Pump -> Tap-Off -> Heat Exchanger with Combustion Chamber -> Tank

Both cycles have nothing to do with each other. Both cycles never touch the combustion products, just the heat generated by the combustion. The heat exchanger might as well be mounted on the pre-burners, I am not sure about that part.

Ok. That's the typical Tap-Off cycle. And combustion products can get water, unwanted, into the tanks.

[livingjw]

I cant draw that nicely, so here is a written discription:

Methane:
Tank -> Pump -> Tap-Off -> Heat Exchanger with Combustion Chamber -> Tank

LoX:
Tank -> Pump -> Tap-Off -> Heat Exchanger with Combustion Chamber -> Tank

Both cycles have nothing to do with each other. Both cycles never touch the combustion products, just the heat generated by the combustion. The heat exchanger might as well be mounted on the pre-burners, I am not sure about that part.

Ok. That's the typical Tap-Off cycle. And combustion products can get water, unwanted, into the tanks.

or:

Methane:
Tank -> Pump -> MCC regen output -> Tap-Off -> Tank

Lox:
Tank -> Pump -> Tap-Off -> Heat Exchanger in Lox pre-burner exhaust -> Tank

John

[matthewkantar]

I cant draw that nicely, so here is a written discription:

Methane:
Tank -> Pump -> Tap-Off -> Heat Exchanger with Combustion Chamber -> Tank

LoX:
Tank -> Pump -> Tap-Off -> Heat Exchanger with Combustion Chamber -> Tank

Both cycles have nothing to do with each other. Both cycles never touch the combustion products, just the heat generated by the combustion. The heat exchanger might as well be mounted on the pre-burners, I am not sure about that part.

Ok. That's the typical Tap-Off cycle. And combustion products can get water, unwanted, into the tanks.

No, the plumbing for the loops never have water in them. The propellants for autogenous pressurization go through a heat exchanger, they don't get burnt or mixed, just heated.

[Semmel]

or:

Methane:
Tank -> Pump -> MCC regen output -> Tap-Off -> Tank

Lox:
Tank -> Pump -> Tap-Off -> Heat Exchanger in Lox pre-burner exhaust -> Tank

John

Certainly a good option for the methane.
But I don't know how hot the LOX preburner exhaust will be. Do we have any estimation? 

[livingjw]

or:

Methane:
Tank -> Pump -> MCC regen output -> Tap-Off -> Tank

Lox:
Tank -> Pump -> Tap-Off -> Heat Exchanger in Lox pre-burner exhaust -> Tank

John

Certainly a good option for the methane.
But I don't know how hot the LOX preburner exhaust will be. Do we have any estimation?

- Probably no higher than 1500 F.
- It is how the SSME autogenous tap is done.

John

[josespeck]

I cant draw that nicely, so here is a written discription:

Methane:
Tank -> Pump -> Tap-Off -> Heat Exchanger with Combustion Chamber -> Tank

LoX:
Tank -> Pump -> Tap-Off -> Heat Exchanger with Combustion Chamber -> Tank

Both cycles have nothing to do with each other. Both cycles never touch the combustion products, just the heat generated by the combustion. The heat exchanger might as well be mounted on the pre-burners, I am not sure about that part.

Ok. That's the typical Tap-Off cycle. And combustion products can get water, unwanted, into the tanks.

No, the plumbing for the loops never have water in them. The propellants for autogenous pressurization go through a heat exchanger, they don't get burnt or mixed, just heated.

I said it for this comment:

https://forum.nasaspaceflight.com/index.php?topic=47068.msg1906265#msg1906265

[Jim]

This is how the SSME does it, no combustion products going into tanks.  H2 doesn't need heating but methane might and so a heat exchanger like the LOX side could be employed.

[cuddihy]

Can a single heat exchanger produce enough hot gaseous methane to 1. Pressurize the CH4 tank. 2. Produce bleed gas for the TPS. 3. Produce pressurized gas for the RCS cold gas system?

If it’s only on one engine you’ve got a critical SPoF. If it’s on multiple engines you’ll have tons of piping and complexity with when it’s used from one engine or another. I guess shuttle did it with 3 engines so it’s solvable.

[envy887]

This is how the SSME does it, no combustion products going into tanks.  H2 doesn't need heating but methane might and so a heat exchanger like the LOX side could be employed.

The methane already has a nice heat exchanger in the regen cooling system.

[envy887]

Can a single heat exchanger produce enough hot gaseous methane to 1. Pressurize the CH4 tank. 2. Produce bleed gas for the TPS. 3. Produce pressurized gas for the RCS cold gas system?

If it’s only on one engine you’ve got a critical SPoF. If it’s on multiple engines you’ll have tons of piping and complexity with when it’s used from one engine or another. I guess shuttle did it with 3 engines so it’s solvable.

They probably aren't going to use methane for the cold gas RCS. Flaring gaseous fuel near the vehicle is generally a bad idea in a highly oxygen rich atmosphere.

[livingjw]

Can a single heat exchanger produce enough hot gaseous methane to 1. Pressurize the CH4 tank. 2. Produce bleed gas for the TPS. 3. Produce pressurized gas for the RCS cold gas system?

If it’s only on one engine you’ve got a critical SPoF. If it’s on multiple engines you’ll have tons of piping and complexity with when it’s used from one engine or another. I guess shuttle did it with 3 engines so it’s solvable.

- 1) Very small flow rate is needed to pressurize the tanks. A small heat exchanger in the Lox pre-burner is all that is needed. As stated before, Methane can be tapped after it exits MCC regenerative cooling loop, so it probably does not need additional heating. The mass fraction needed to fill the tanks is the ratio of density of the vapor to the density of the liquid.

- 2) Engines are not on when reentry is taking place. Methane gas is coming from the full methane header tank which is probably pressurized to about 3 bar (45 psi). 

- 3) RCS cold gas system will probably be from separate higher pressure methane tanks and will already be in a warm gaseous state prior to launch and reentry. I would guess a minimum of 66 bar (1000 psi) and a temperature of around 350-400 C (662-752 F)

- During launch, the autogenous pressurization system only needs regulators and valves to work. It relies on the engines for pure gases at the right temperature and pressure. Reentry with 3 engines gives triple redundancy, which should be more than sufficient.

- During launch and reentry, cold gas RCS only requires regulators and valves to work. Warm high pressure gas is made available prior to launch or reentry.

John

[Restless]

This thread seems to assume that sub-cooling will be used for LOX and LNG for hopper and SS and the booster. F9 needed the sub-cooling to stretch fuel for fly back of the booster and extra heavy missions. Has Spacex indicated that sub-cooling will be used at Boca? It adds extra equipment and may not be necessary for the hopper and SS prototype.

[alienmike]

This thread seems to assume that sub-cooling will be used for LOX and LNG for hopper and SS and the booster. F9 needed the sub-cooling to stretch fuel for fly back of the booster and extra heavy missions. Has Spacex indicated that sub-cooling will be used at Boca? It adds extra equipment and may not be necessary for the hopper and SS prototype.

I think the assumption is that they will test like they fly. It helps retire a lot of risk.

[Lar]

This thread seems to assume that sub-cooling will be used for LOX and LNG for hopper and SS and the booster. F9 needed the sub-cooling to stretch fuel for fly back of the booster and extra heavy missions. Has Spacex indicated that sub-cooling will be used at Boca? It adds extra equipment and may not be necessary for the hopper and SS prototype.

I think the assumption is that they will test like they fly. It helps retire a lot of risk.

Agreed. Also, subcooling can be added on later, especially if provisions are made (any through berm piping, for example, is placed even if just stubbed out at both ends (and filled with nitrogen maybe?) after the first few hopper tests. But I think by the time the first prototype starship flies they will be doing propellants as close to final as they can even if they start out not doing so initially with hopper.

[livingjw]

This thread seems to assume that sub-cooling will be used for LOX and LNG for hopper and SS and the booster. F9 needed the sub-cooling to stretch fuel for fly back of the booster and extra heavy missions. Has Spacex indicated that sub-cooling will be used at Boca? It adds extra equipment and may not be necessary for the hopper and SS prototype.

Did I miss something. I haven't assumed sub-cooling in anything I have assumed. Doubtful that it is needed or desirable in this case. I think this post was inadvertently placed here.

John

[guckyfan]

Without looking it up, I am pretty sure Elon mentioned subcooled propellant in 2016. I recall a question, how they will have subcooled propellant for landing on Mars and Elon replied, initially just venting some propellant to vacuum, active coolers maybe later.

[Llian Rhydderch]

This is how the SSME does it, no combustion products going into tanks.  H2 doesn't need heating but methane might and so a heat exchanger like the LOX side could be employed.

That is an awesomely well-done image, Jim, of how another historical launch vehicle design has handled the provision of autogenous pressurization gases for propellant tanks.

Would anyone on this forum be willing to have a go at trying to schematicize a design in that fashion for what might work for Starship and/or Super Heavy?

That would be an awesome way to debate the merits of various approaches, with reference to a particular draft version of a schematic, here in the forum.

[alienmike]

This thread seems to assume that sub-cooling will be used for LOX and LNG for hopper and SS and the booster. F9 needed the sub-cooling to stretch fuel for fly back of the booster and extra heavy missions. Has Spacex indicated that sub-cooling will be used at Boca? It adds extra equipment and may not be necessary for the hopper and SS prototype.

Did I miss something. I haven't assumed sub-cooling in anything I have assumed. Doubtful that it is needed or desirable in this case. I think this post was inadvertently placed here.

John

If the Raptor is designed to take in sub-cooled LOX and LNG, which is denser, then wouldn't it require a slightly different design for propellants that are not sub-cooled, or as dense? Maybe this was a false assumption on my part. My memory says that Merlin was modified slightly to take in sub-cooled propellants, but my memory could be wrong too.

[envy887]

This thread seems to assume that sub-cooling will be used for LOX and LNG for hopper and SS and the booster. F9 needed the sub-cooling to stretch fuel for fly back of the booster and extra heavy missions. Has Spacex indicated that sub-cooling will be used at Boca? It adds extra equipment and may not be necessary for the hopper and SS prototype.

Did I miss something. I haven't assumed sub-cooling in anything I have assumed. Doubtful that it is needed or desirable in this case. I think this post was inadvertently placed here.

John

If the Raptor is designed to take in sub-cooled LOX and LNG, which is denser, then wouldn't it require a slightly different design for propellants that are not sub-cooled, or as dense? Maybe this was a false assumption on my part. My memory says that Merlin was modified slightly to take in sub-cooled propellants, but my memory could be wrong too.

Merlins are reportedly tuned for subcooled props, but can almost certainly burn boiling propellants just fine without hardware changes. Likely with less thrust, but the only time that thrust is really important is on ascent when the props are always subcooled anyway. After coasting they are likely boiling, especially on the 8 hour coast to direct GEO insertion but probably after the booster's rather toasty entry also.

[alienmike]

If the Raptor is designed to take in sub-cooled LOX and LNG, which is denser, then wouldn't it require a slightly different design for propellants that are not sub-cooled, or as dense? Maybe this was a false assumption on my part. My memory says that Merlin was modified slightly to take in sub-cooled propellants, but my memory could be wrong too.

Merlins are reportedly tuned for subcooled props, but can almost certainly burn boiling propellants just fine without hardware changes. Likely with less thrust, but the only time that thrust is really important is on ascent when the props are always subcooled anyway. After coasting they are likely boiling, especially on the 8 hour coast to direct GEO insertion but probably after the booster's rather toasty entry also.

So testing like you fly does not mean that sub-cooled propellants will be used, at least not initially. Thank you for clarifying.

[Semmel]

This is how the SSME does it, no combustion products going into tanks.  H2 doesn't need heating but methane might and so a heat exchanger like the LOX side could be employed.

That is an awesomely well-done image, Jim, of how another historical launch vehicle design has handled the provision of autogenous pressurization gases for propellant tanks.

Would anyone on this forum be willing to have a go at trying to schematicize a design in that fashion for what might work for Starship and/or Super Heavy?

That would be an awesome way to debate the merits of various approaches, with reference to a particular draft version of a schematic, here in the forum.

I was standing in front of the SSME for about 2 hours, with that chart in hand. I couldnt figure out all of it. Granted, its hard if you cant see the colors in the diagram also on the pipes and there were a hell of a lot more pipes on the engine than in the diagram. Not sure you could derive such a diagram from the images of Raptor or even when you would stand in front of it. Maybe livingjw could do it, I cannot.

[livingjw]

Raptor schematic. Started with the SSME schematic and changed it to reflect Raptor as I currently understand it.
Modified to add a heat exchanger in the methane pre-burner turbine exit flow. After adiabatic expansion pressurant gas was still too cold.

Modified (4 Feb 2019) changed source of methane to pump outlet for higher pressure to Lox pre-burner. Probably could use either source, but pressure after cooling MCC is only 10 -15 bar higher than lox pump pressure and and I would like to have more like more like 30 ish.

John

[vaporcobra]

Without looking it up, I am pretty sure Elon mentioned subcooled propellant in 2016. I recall a question, how they will have subcooled propellant for landing on Mars and Elon replied, initially just venting some propellant to vacuum, active coolers maybe later.

Correct.

[livingjw]

There is one possible, but improbable, problem that can happen with sub-cooled propellants.

During NASP, slush hydrogen was tested in a flight weight tank which was to undergo vibration testing. When the tank was shaken, the sub-cooled fluid interacted with the vapor cooling it and causing the tank to partially collapse due to negative pressure.

Since, severe vibrations occur only when the engines are running, there would always be plenty of pressurant gas to counter this.

Just a funny story.

John

[thxbmp3]

This is how the SSME does it, no combustion products going into tanks.  H2 doesn't need heating but methane might and so a heat exchanger like the LOX side could be employed.

That is an awesomely well-done image, Jim, of how another historical launch vehicle design has handled the provision of autogenous pressurization gases for propellant tanks.

Would anyone on this forum be willing to have a go at trying to schematicize a design in that fashion for what might work for Starship and/or Super Heavy?

That would be an awesome way to debate the merits of various approaches, with reference to a particular draft version of a schematic, here in the forum.

Sorry to stray a bit OT, but I had some questions re: the beautiful schematic Jim posted. Isn't SSME an FRSC engine? Why do each of the turbopumps have their own preburners?

Also, do the green areas represent anything (e.g. physically isolated volumes/chambers) or are they just there to highlight the locations of the turbines and pumps?

[meekGee]

This is how the SSME does it, no combustion products going into tanks.  H2 doesn't need heating but methane might and so a heat exchanger like the LOX side could be employed.

That is an awesomely well-done image, Jim, of how another historical launch vehicle design has handled the provision of autogenous pressurization gases for propellant tanks.

Would anyone on this forum be willing to have a go at trying to schematicize a design in that fashion for what might work for Starship and/or Super Heavy?

That would be an awesome way to debate the merits of various approaches, with reference to a particular draft version of a schematic, here in the forum.

Sorry to stray a bit OT, but I had some questions re: the beautiful schematic Jim posted. Isn't SSME an FRSC engine? Why do each of the turbopumps have their own preburners?

Also, do the green areas represent anything (e.g. physically isolated volumes/chambers) or are they just there to highlight the locations of the turbines and pumps?

Wikipedia (where that image is also posted) says Full Flow.

https://en.m.wikipedia.org/wiki/Space_Shuttle_main_engine

[rsdavis9]

This is how the SSME does it, no combustion products going into tanks.  H2 doesn't need heating but methane might and so a heat exchanger like the LOX side could be employed.

That is an awesomely well-done image, Jim, of how another historical launch vehicle design has handled the provision of autogenous pressurization gases for propellant tanks.

Would anyone on this forum be willing to have a go at trying to schematicize a design in that fashion for what might work for Starship and/or Super Heavy?

That would be an awesome way to debate the merits of various approaches, with reference to a particular draft version of a schematic, here in the forum.

Sorry to stray a bit OT, but I had some questions re: the beautiful schematic Jim posted. Isn't SSME an FRSC engine? Why do each of the turbopumps have their own preburners?

Also, do the green areas represent anything (e.g. physically isolated volumes/chambers) or are they just there to highlight the locations of the turbines and pumps?

Wikipedia (where that image is also posted) says Full Flow.

https://en.m.wikipedia.org/wiki/Space_Shuttle_main_engine

It uses 2 preburners but both are fuel rich.
For a FFSC you need one fuel rich and the other oxygen rich.

[Tuna-Fish]

Wikipedia (where that image is also posted) says Full Flow.

https://en.m.wikipedia.org/wiki/Space_Shuttle_main_engine

Don't get your information from wikipedia.

SSME is FRSC, with separate preburners and turbopumps for the oxidizer and fuel sides, but both of those are ran fuel-rich. In total, 76% of the fuel and 11% of the oxidizer go through the preburners, to be united with the rest of the propellants once they get injected into the chamber.

Source: Rocketdyne's presentation on SSME (pdf), see page 18.

[Llian Rhydderch]

This is how the SSME does it, no combustion products going into tanks.  H2 doesn't need heating but methane might and so a heat exchanger like the LOX side could be employed.

That is an awesomely well-done image, Jim, of how another historical launch vehicle design has handled the provision of autogenous pressurization gases for propellant tanks.

Would anyone on this forum be willing to have a go at trying to schematicize a design in that fashion for what might work for Starship and/or Super Heavy?

That would be an awesome way to debate the merits of various approaches, with reference to a particular draft version of a schematic, here in the forum.

Sorry to stray a bit OT, but I had some questions re: the beautiful schematic Jim posted. Isn't SSME an FRSC engine? Why do each of the turbopumps have their own preburners?

Also, do the green areas represent anything (e.g. physically isolated volumes/chambers) or are they just there to highlight the locations of the turbines and pumps?

Wikipedia (where that image is also posted) says Full Flow.

https://en.m.wikipedia.org/wiki/Space_Shuttle_main_engine

I looked at that Wikipedia article meekGee, and am not finding the "Full Flow" claim.  Searching for "full" or "flow" or "staged" don't seem to find any instance of even a statement to that effect, let alone a sourced statement to support the statement. 

Were you possibly looking at some other article than Space_Shuttle_main_engine?

[meekGee]

This is how the SSME does it, no combustion products going into tanks.  H2 doesn't need heating but methane might and so a heat exchanger like the LOX side could be employed.

That is an awesomely well-done image, Jim, of how another historical launch vehicle design has handled the provision of autogenous pressurization gases for propellant tanks.

Would anyone on this forum be willing to have a go at trying to schematicize a design in that fashion for what might work for Starship and/or Super Heavy?

That would be an awesome way to debate the merits of various approaches, with reference to a particular draft version of a schematic, here in the forum.

Sorry to stray a bit OT, but I had some questions re: the beautiful schematic Jim posted. Isn't SSME an FRSC engine? Why do each of the turbopumps have their own preburners?

Also, do the green areas represent anything (e.g. physically isolated volumes/chambers) or are they just there to highlight the locations of the turbines and pumps?

Wikipedia (where that image is also posted) says Full Flow.

https://en.m.wikipedia.org/wiki/Space_Shuttle_main_engine

I looked at that Wikipedia article meekGee, and am not finding the "Full Flow" claim.  Searching for "full" or "flow" or "staged" don't seem to find any instance of even a statement to that effect, let alone a sourced statement to support the statement. 

Were you possibly looking at some other article than Space_Shuttle_main_engine?

I missed the fact that even the O2 side was fuel rich.

Personal fail, not Wikipedia fail.

[Llian Rhydderch]

Yeah, thanks meekGee!

And re the larger point made by Tuna Fish

Don't get your information from wikipedia.

Wikipedia articles should always be looked at as an emergent assemblage of information that should be verifiable, and mostly sourced to reliable secondary sources.

Not all of it is, of course.  But the beauty of Wikipedia is that you, the observer and reader, can go to the source(s) and see for yourself.  If it is not sourced, or the source is a poor source, you can (and in my view, should) quickly add a mere six characters in an edit to politely request it be sourced; the bots take care of the rest.  The large group of spaceflight-related articles on Wikipedia could use a bit more of space-knowledgeable people asking for sources where none is provided.  (DM me if anyone wants to know this simple bit.)

But in all cases, it is not that Wikipedia does not have good information.  It has a large quantity of great information assembled.  But all readers for good science or engineering info should obviously verify a source (that is used in Wikipedia) for yourself before using it. 

So in the SSME FFSC or FRSC case, I just looked carefully, and found no source at all that it is FFSC, nor even a claim that it was FFSC.  Voila. 

[livingjw]

Modified Raptor schematic to add a heat exchanger in the methane pre-burner turbine exit flow. After adiabatic expansion pressurant gas was still too cold.

John

[Lar]

Modified Raptor schematic to add a heat exchanger in the methane pre-burner turbine exit flow. After adiabatic expansion pressurant gas was still too cold.

John

Thanks for that mod, comparing it and the original and the SSME side by side by side is very instructive. The SSME with two stage pumps is a lot more complex...

So I guess I was sort of half right about the possible need for a heat exchanger.

[livingjw]

Modified Raptor schematic to add a heat exchanger in the methane pre-burner turbine exit flow. After adiabatic expansion pressurant gas was still too cold.

John

Thanks for that mod, comparing it and the original and the SSME side by side by side is very instructive. The SSME with two stage pumps is a lot more complex...

So I guess I was sort of half right about the possible need for a heat exchanger.

- You made me check the pressurant temperature after dropping its pressure and sure enough it was too low. It needs to be a few hundred degrees F.

- The SSME really has a three stage hydrogen pump and also a boost pump for both propellants.

- The lack of boost pumps is puzzling. I think that the header tanks might be held at a higher pressure than the main tanks in order to start the engines without boost pumps.

John

[groknull]

[snip]

- The lack of boost pumps is puzzling. I think that the header tanks might be held at a higher pressure than the main tanks in order to start the engines without boost pumps.

John

10's of kg of boost pump mass removed from each engine could be applied to header tank structure and redundant header tank fill and pressurization systems.  All useful for pump down for cruise portion of flight.