Starship Methox RCS Thrusters

[Semmel]

I've seen diagrams both ways... the RCS tanks can store the mixed gas right?

You mean mixed gas and liquid? As a mixture of oxygen and methane at high pressure is not going to stay at that pressure for long.

[rakaydos]

How severe is the mass penalty for multiple small tanks over monolithic large tanks?

I'm wondering about reduncancy, and whether it would be worth giving individual RCS blocks their own pressure vessels. Replenished from a central source, but with fuel on hand if the central source fails.

[Pete]

How severe is the mass penalty for multiple small tanks over monolithic large tanks?

I'm wondering about reduncancy, and whether it would be worth giving individual RCS blocks their own pressure vessels. Replenished from a central source, but with fuel on hand if the central source fails.

The added failure modes will likely make this sort of system *less* reliable than a simpler system.

[livingjw]

How severe is the mass penalty for multiple small tanks over monolithic large tanks?

I'm wondering about redundancy, and whether it would be worth giving individual RCS blocks their own pressure vessels. Replenished from a central source, but with fuel on hand if the central source fails.

- Little penalty for the pressure tanks. Tank weight is proportional to volume everything else being equal.

- Some penalty for additional mounting fixtures and plumbing.

- System Designs should be "fail safe" where every possible; that is, one component failure should not bring down the system. A for and aft RCS system with cross feed capability would probably be adequate.

John

[ThomasGadd]

I've seen diagrams both ways... the RCS tanks can store the mixed gas right?

You mean mixed gas and liquid? As a mixture of oxygen and methane at high pressure is not going to stay at that pressure for long.

Sorry I was clear... they will be extracting from the LOX and LCH4 header tanks into a new GOX and GCH4 RCS tanks. 
Can that new (GOX and GCH4) be a single tank? 

[ulm_atms]

I've seen diagrams both ways... the RCS tanks can store the mixed gas right?

You mean mixed gas and liquid? As a mixture of oxygen and methane at high pressure is not going to stay at that pressure for long.

Sorry I was clear... they will be extracting from the LOX and LCH4 header tanks into a new GOX and GCH4 RCS tanks. 
Can that new (GOX and GCH4) be a single tank?

If I saw this config I would never ride on the thing even if paid.  When you mix GOX and GCH4...or any oxidizer and fuel.....you get a bomb, plain and simple.

I state this with certainty...you will never see a single tank with both in it.

[ThomasGadd]

I've seen diagrams both ways... the RCS tanks can store the mixed gas right?

You mean mixed gas and liquid? As a mixture of oxygen and methane at high pressure is not going to stay at that pressure for long.

Sorry I was clear... they will be extracting from the LOX and LCH4 header tanks into a new GOX and GCH4 RCS tanks. 
Can that new (GOX and GCH4) be a single tank?

If I saw this config I would never ride on the thing even if paid.  When you mix GOX and GCH4...or any oxidizer and fuel.....you get a bomb, plain and simple.

I state this with certainty...you will never see a single tank with both in it.

I understand that argument but there is no ignition source. 

Automobile gasoline tanks have air and gaseous gasoline in them. 

[ulm_atms]

I've seen diagrams both ways... the RCS tanks can store the mixed gas right?

You mean mixed gas and liquid? As a mixture of oxygen and methane at high pressure is not going to stay at that pressure for long.

Sorry I was clear... they will be extracting from the LOX and LCH4 header tanks into a new GOX and GCH4 RCS tanks. 
Can that new (GOX and GCH4) be a single tank?

If I saw this config I would never ride on the thing even if paid.  When you mix GOX and GCH4...or any oxidizer and fuel.....you get a bomb, plain and simple.

I state this with certainty...you will never see a single tank with both in it.

I understand that argument but there is no ignition source. 

Automobile gasoline tanks have air and gaseous gasoline in them.

In very minute quantities of ox to gas.  It may flame up if you smoke will filling up...but it won't go boom..and only flame on the outside where there is more air.  When you start the car, it also pulls a vacuum on the tank to keep that exact thing from happening.  That's why leaving the gas cap off sets off the check engine light..it's a vacuum error.  It's also about the correct fuel to air mixture.  The gas tank is really too fuel rich to burn.

But....

If you have GO2 and GCH4 under pressure in the same RCS tank...it's going to go boom if ignited...no question, as since the tank would have to be at the correct mixture ratio to work at all...it will ignite with very very little energy.  As far as ignition sources...static shock can easily happen.  In a compressed state...it takes so little energy for ignition.  Just the act of pressurizing the system may be enough to set it off.

[Pete]

The correct technical term for a high-pressure vessel containing a near-stoichiometric mixture of Oxygen and Methane gas is...

a Bomb.

All it needs is an ignition source.
Which could be almost anything, really.
A bit of heat, the tiniest spark, a shock to the containment vessel, a trace of contaminant in the contents or vessel...
Really about anything.

And 1 kg of Methane/Oxygen mixture contains about 3 times as much energy wanting to be released as 1kg of TNT.

[CorvusCorax]

I was thinking, could a MethOX thruster be "boosted"

We have quite a number of Tesla packs on board, in the car they are rated for 500kW each, but let's assume they can provide 2MW short term boost when using space rated next gen (plaid) packs, and put a few in parallel, let's say with a 10 MW Thermoelectric system

(for example using microwaves or a magnetic field to induce a heating current in the plasma within the thurster's throat.

Thus, turning the purely chemical thruster into a electrochemical hybrid engine.


To double the specific impulse - let's say from 350s  to 700s, one would have to quadruple the temperature.

At an assumed Isp of 350 s, at MethOX thruster.
350s = 3433 m/s / 9.81 m/s²
3433 m/s = 3433 N * s/kg
That means the specific fuel consumption is around 0.3 kg per second and kiloNewton

A 50 Kilonewton thruster would therefore have a fuel flow of 15 kg/s

Assuming a temperature of aprox 3500 K of the methox exhaust, the system would have to superheat the plasma in the throat center (but hopefully not anywhere close to the wall)  to 14000K.

Both steam and Carbon Dioxide are in the ballpark range of a specific heat capacity of 2kJ per kg and Kelvin.

The energy required to heat 15kg of gas from 3500K to 14000k is therefore roughly 10^4 K * 15 kg * 2kJ = 3*10^5 kJ
we have that per second, which makes it 3*10^5 kW of power

that's 30 Megawatt.

So I guess a doubling of ISP at full thrust isn't possible, but at 1/3 of the thrust it might start to be feasible.
Especially if the bats can be recharged using solar panels.

All we need is a 10 MW induction coil around the thruster's throat...  Hmmm... we usually have cryogenic liquids on board anyway, high temperature superconductors anyone?

Edit: Had a slight misconception there. After all, when doubling the ISP at the same mass flow, the thrust would double, too. That means our 50 kN thruster operating at 1/3 throttle, boosted with 10MW  would still put out 2/3 of its regular thrust or around 33 kN.  *slaps head*

[DistantTemple]

Really:

which makes it 3*10^5 kW of power

that's 30 Megawatt.

3*10⁵ = 300,000 in my book!

so thats 300,000KW = 300MW

So has such a coil ever been made? Or any other way of transfering that power.

Items on this scale that come to mind are include
"A new gas-fired power plant with a total capacity of 300MW has entered commercial operation in Lincolnshire.

The £100 million Spalding peaking plant is highly flexible and will provide backup for renewable generation sources like solar and wind power."

[livingjw]

OT  Might want to take this discussion to advanced concepts.

John

[alang]

I've seen diagrams both ways... the RCS tanks can store the mixed gas right?

,
You mean mixed gas and liquid? As a mixture of oxygen and methane at high pressure is not going to stay at that pressure for long.

Sorry I was clear... they will be extracting from the LOX and LCH4 header tanks into a new GOX and GCH4 RCS tanks. 
Can that new (GOX and GCH4) be a single tank?

If I saw this config I would never ride on the thing even if paid.  When you mix GOX and GCH4...or any oxidizer and fuel.....you get a bomb, plain and simple.

I state this with certainty...you will never see a single tank with both in it.

I understand that argument but there is no ignition source. 

Automobile gasoline tanks have air and gaseous gasoline in them.

Were you around for the discussion of obscure ignition sources for the composite overwrapped pressure vessels (COPV's) submerged in liquid oxygen! This was after the SpaceX on pad explosion.
Apologies for not having a link, but a lot of knowledgable  people agreed that there will always be an ignition source.

[ThomasGadd]

I've seen diagrams both ways... the RCS tanks can store the mixed gas right?

,
You mean mixed gas and liquid? As a mixture of oxygen and methane at high pressure is not going to stay at that pressure for long.

Sorry I was clear... they will be extracting from the LOX and LCH4 header tanks into a new GOX and GCH4 RCS tanks. 
Can that new (GOX and GCH4) be a single tank?

If I saw this config I would never ride on the thing even if paid.  When you mix GOX and GCH4...or any oxidizer and fuel.....you get a bomb, plain and simple.

I state this with certainty...you will never see a single tank with both in it.

I understand that argument but there is no ignition source. 

Automobile gasoline tanks have air and gaseous gasoline in them.

Were you around for the discussion of obscure ignition sources for the composite overwrapped pressure vessels (COPV's) submerged in liquid oxygen! This was after the SpaceX on pad explosion.
Apologies for not having a link, but a lot of knowledgable  people agreed that there will always be an ignition source.

I was, but there is a big difference between LOX and LCH4 and there gaseous versions. 
On further thought though they will keep the gases stored separately to pressurize the main tanks. 
If there is no engine heat available they can use these tanks pressurize the main tanks

[edzieba]

All quiet on the hot-gas RCS front? The plan back at the Mk.1 unveil was for Mk.1 & 2 to use cold-gas RCS only, with hot-gas RCS added to Mk.3. Obviously with the switch to the SN series that timeline is no longer valid, but if hot-gas RCS is as imminent as it was previously going to be we should be seeing some evidence of component testing.

[cdebuhr]

All quiet on the hot-gas RCS front? The plan back at the Mk.1 unveil was for Mk.1 & 2 to use cold-gas RCS only, with hot-gas RCS added to Mk.3. Obviously with the switch to the SN series that timeline is no longer valid, but if hot-gas RCS is as imminent as it was previously going to be we should be seeing some evidence of component testing.

The could be heavily into testing hot gas thrusters ... how would we know?  I'd expect most of the test campaign to happen in McGregor, and these things will be so tiny compared to things like Raptor, or even Merlin, that I doubt anyone would even notice.  I expect the first sign of hot gas RCS at BC will be when they install RCS packs that look different from what came before, and barring any tweets from Elon, we won't really know until they light them off for the first time.

[livingjw]

Did this a while back. Notional schematic of gaseous RCS system.

[Jcc]

Did this a while back. Notional schematic of gaseous RCS system.

I assume the pumps, etc, will be able to operate in microgravity. How do you avoid the possibility of feeding them gas instead of liquid, or is that not an issue?

[livingjw]

Did this a while back. Notional schematic of gaseous RCS system.

I assume the pumps, etc, will be able to operate in microgravity. How do you avoid the possibility of feeding them gas instead of liquid, or is that not an issue?

- Pumps are small and are only used to recharge the RCS system when needed. Capillary flow devices in the tanks can be used to insure propellants are maintained at the pumps entrances.

- Also, you are taking a small amount of liquid from almost nearly full header tanks. As an alternative, you could get liquid from residual propellants in the main tanks, but that would be harder. You could also pump gases from the main tank, but the pumps would be large and take much more power.



John

[rakaydos]

Bumping this for it's relevance to the Lunar Starship's landing system.