eberkain

Where is tank pressure mod?

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With all the different part switch modules, how come there is no mod for changing the tank pressure on a given part?  

I imagine something like where you can double the fuel inside a part and triple its dry mass weight, etc...  whatever the balance should be I'm not the expert.  

Currently I end up just clipping tanks together to get the performance I am after inside the form factor that looks right.  Fewer parts for a small mass penalty would be good with me. 

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On 3/9/2020 at 7:20 PM, eberkain said:

With all the different part switch modules, how come there is no mod for changing the tank pressure on a given part?  

I imagine something like where you can double the fuel inside a part and triple its dry mass weight, etc...  whatever the balance should be I'm not the expert.  

Currently I end up just clipping tanks together to get the performance I am after inside the form factor that looks right.  Fewer parts for a small mass penalty would be good with me. 

If you’re looking for that level of detail you might find Real Fuels to your liking.

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Posted (edited)

But it's for 1.7.3

Though, they say it works in 1.9.1

Edited by kerbiloid

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I’m confused how increased pressure would increase storage density of liquid fuels. Unless we’re talking about neutron star levels of pressure, liquids are effectively incompressible. I’m not aware of any successful rockets that run on gaseous tankage either. 
 

however there’s nothing wrong with creative clipping if you just want your craft to look a particular way. General etiquette is to call out extensive clipping when you share craft, as some players prefer to stick with somewhat realistic non-clipped construction 

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On 3/14/2020 at 4:18 PM, fourfa said:

I’m confused how increased pressure would increase storage density of liquid fuels. Unless we’re talking about neutron star levels of pressure, liquids are effectively incompressible.

If that is the case, then why is such a big deal made about Starship being able to hit some specific tank pressure?  

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Posted (edited)

Couple of reasons @eberkain. One, boosters spend a lot of their lifespan in the low atmosphere surrounded by the ambient 14.7psi. Not just flight, but stacking, fueling, defueling, destacking, lots of potential abort situations etc. Many rocket stages will collapse under their own weight if they’re not inflated with substantial pressure inside. Then in flight, the volume of fuel getting sucked into the turbopumps needs to be replaced with something - pressurized gas - or the internal pressure will drop below the external ambient pressure (while still in atmo at least) and the tank will implode. Bad news. This is the routine reason that applies to all rockets. The gas could be compressed nitrogen; Saturn/Apollo used cryogenic helium, some rockets even use a fraction of their own propellants that run around the engines as coolant, turn from liquid to gas, then get routed back to the tank as pressurant.
 

The headlines about Starship pressure tests are another reason having to do with the particular propellants chosen. Oxygen and methane are gases at ordinary temperatures. Cool them to very low (cryogenic) temperature and they turn liquid. But how to keep them liquid on a hot Florida or Texas afternoon?  Insulation helps (the orange stuff on the outside of the space shuttle external tank is foam insulation). But increased pressure helps a lot more - if you can hold the tanks at high pressure, the tendency of cryogenics to boil is much reduced. Then there’s a safety factor - imagine the pressure relief valve that maintains that desired pressure fails and sticks closed, let’s say on the hottest Florida summer day with hours and hours of launch delays. The cryogenics are going to continue to slowly boil and increase pressure in the tank, and you definitely don’t want the tank to rupture and explode - especially when there will be people on top.  So you design for the pressure you need to minimize boil-off, then add a safety factor (and triple up on safety release valves too).
 

For Starship that safe pressure level looks to be 8.5bar (about 125psi). SpaceX has been experimenting with new methods of tank construction, and testing their experimental tanks to destruction.  Some of those tests, rather than using explosive propellants, have been done with inert nitrogen instead. Now it looks like they’re starting to test with real propellants, which they have to do eventually because they’ll be different temperatures than liquid nitrogen. They need to prove to NASA or the FAA or themselves that the tanks are safe. 
 

But none of it is to cram more propellant in the tank by compressing it to make it more dense. SpaceX does, however, cram more fuel and oxygen in its Falcon 9 tanks by making them more dense - by making them COLDER. This works, and gives you a few percent more fuel for the same volume tank. This is one of the reasons the Falcon 9 payload has gotten upgraded significantly over the years. The downside is if there is a launch delay, the propellants warm up and start to boil off more aggressively.  Delay too long, they have to call an abort, detank and try again with a fresh batch of super-cold LO2.  Not only that - but usually a rocket is filled with propellant with no one on board and minimal staff around in case of an accident.  Then the long and complex process of boarding astronauts and sealing up the cockpit happens.  But that takes too long with super-cooled densified propellant, it'll warm up and boil off too much.  So SpaceX has to board and seal up the crew first, then fill up the rocket with them on board (they call this "load-and-go").  Lots of people consider this more dangerous; some don't.

Its a complex topic and easily mixed up!

Edited by fourfa

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