Rocket engines- why do they have little exhausts on them?

[jimmymcgoochie]

A lot of the rocket engines in KSP have little exhausts on the side of the main nozzle, often with a lot of pipes and tubes tangled around the nozzle. In some mods, these are animated and seem to spit out little clouds of smoke and fire as the engine runs. What are these for?

My best guess is some kind of cooling system for the nozzle itself, or possibly a system that diverts part of the exhaust to an alternator that generates power. 

[mikegarrison]

In KSP it's obviously just for show. In real life, this is usually the exhaust of the gas generator. Essentially it's a jet engine that runs the fuel pumps for the rocket engine.

https://en.wikipedia.org/wiki/Gas-generator_cycle

Edited December 16, 2019 by mikegarrison

[έķ νίĻĻάίή]

3 minutes ago, mikegarrison said:

In KSP it's obviously just for show. In real life, this is usually the exhaust of the gas generator. Essentially it's a jet engine that runs the fuel pumps for the rocket engine.

I’m probably being stupid but maybe the engine ignitors? Like, if they spit out sparks the that would light the engine?

Right? No...

[mikegarrison]

This is a picture of a SpaceX Merlin engine. It's a gas generator cycle engine. You see there to the right another nozzle that is firing out a very smoky exhaust? That's the exhaust from the gas generator. (It's obviously running quite rich to be putting out that much unburned carbon.)

It's pointed down because even though it doesn't make much thrust compared to the main rocket engine, it does make a little bit of thrust. So they might as well point it down.

8 minutes ago, έķ νίĻĻάίή said:

I’m probably being stupid but maybe the engine ignitors? Like, if they spit out sparks the that would light the engine?

Right? No...

No, generally you can't light the engine from outside. The flame is not going to propagate back up the nozzle.

Edited December 16, 2019 by mikegarrison

[Xd the great]

1 hour ago, mikegarrison said:

In KSP it's obviously just for show. In real life, this is usually the exhaust of the gas generator. Essentially it's a jet engine that runs the fuel pumps for the rocket engine.

https://en.wikipedia.org/wiki/Gas-generator_cycle

Some engines also use it for roll control.

[magnemoe]

10 hours ago, Xd the great said:

Some engines also use it for roll control.

Smart and make sense as you can not do that with one engine. 

[kerbiloid]

The 2nd stage of R-29.

[RealKerbal3x]

IIRC (correct me if I’m wrong) it’s an open-cycle gas generator. Basically, some of the fuel is diverted to spin a turbine that runs the fuel pumps, before being vented.

On closed-cycle gas generator engines, it’s the same except the fuel is pumped back to the combustion chamber to contribute to thrust instead of being wasted.

[magnemoe]

1 hour ago, RealKerbal3x said:

IIRC (correct me if I’m wrong) it’s an open-cycle gas generator. Basically, some of the fuel is diverted to spin a turbine that runs the fuel pumps, before being vented.

On closed-cycle gas generator engines, it’s the same except the fuel is pumped back to the combustion chamber to contribute to thrust instead of being wasted.

Most engines are open cycle, exceptions is the space shuttle engines, the Russian ones for the N1 and Raptor for starship is closed cycle. Assuming Blue Origin also uses closed. Electron uses electrical pumps. 

[kerbiloid]

ICBM and SLBM like the close cycle. Proton as well is ICBM, too.

Edited December 17, 2019 by kerbiloid

[farmerben]

What sort of fuel pumps are used with hypergolic fuel?

[mikegarrison]

1 hour ago, kerbiloid said:

ICBM and SLBM like the close cycle. Proton as well is ICBM, too.

Modern US military missiles are all solids.

1 hour ago, farmerben said:

What sort of fuel pumps are used with hypergolic fuel?

Typically pressure-fed, I think.

[Reactordrone]

2 hours ago, mikegarrison said:

Modern US military missiles are all solids.

Typically pressure-fed, I think.

Depends on the size. Titan II engines used an open cycle gas generator, Soviet RD-250 engines for their large ICBMs used oxygen rich staged combustion or closed cycle.

[kerbiloid]

6 hours ago, farmerben said:

What sort of fuel pumps are used with hypergolic fuel?

Sometimes they have a starter mode when a small amount of both propellant components gets brought into the turbopump combustion chamber, self-ignite, produce the exhaust gas which then partially runs the turbopump, partially raises pressure in the fuel tanks till the engine shutdown. When the turbopump reaches working frequency, it starts feeding the engine combustion chamber.

The largest one (RD-270) had two turbopumps per engine, separate for each component.

2 hours ago, Reactordrone said:

Soviet RD-250 engines for their large ICBMs used oxygen rich staged combustion or closed cycle.

RD-250 is hypergolic, the cryogenic ICBM have been stopped since R-16.
"Oxidizer"-rich, not "oxygen".

Edited December 18, 2019 by kerbiloid

[Zeiss Ikon]

13 hours ago, kerbiloid said:

ICBM and SLBM like the close cycle. Proton as well is ICBM, too.

Modern American ICBM and SLBM are solid propellant.  The Titan family was the last liquid fuel ICBM in the US inventory, and it's been out of that service since the 1980s.  There were never liquid fuel SLBM in US service.

[kerbiloid]

I didn't say about American ones.

There are Soviet ones and their clones to the South-East.

Edited December 18, 2019 by kerbiloid

[mikegarrison]

1 hour ago, kerbiloid said:

I didn't say about American ones.

That's why I specified "US".

[farmerben]

 

This is an LR-87 engine from the Titan II and Gemini programs.  It uses open cycle fuel pumps with their own exhaust pipe.  If what I"m reading is correct.  This engine with very small modifications could run on RP1+LOX, H2+LOX, or Aerozine 50 plus Nitrogen tetroxide.

My understanding of open cycle fuel pumps is that they have to run fuel rich to not overheat, and because oxygen is highly corrosive to steel.  The hypergolics do not have those particular problems.  It seems as though it would be easier to close the fuel cycle with hypergolics (particularly since the fuel and oxidizer have a monopropellant capability before they are even combusted). 

Did they not go for closed cycle just because they were in a big hurry?

Were they exhausting hydrazine rich gas through that tube into the open atmosphere?

 

[mikegarrison]

9 hours ago, farmerben said:

My understanding of open cycle fuel pumps is that they have to run fuel rich to not overheat, and because oxygen is highly corrosive to steel.  The hypergolics do not have those particular problems.  It seems as though it would be easier to close the fuel cycle with hypergolics (particularly since the fuel and oxidizer have a monopropellant capability before they are even combusted). 

Did they not go for closed cycle just because they were in a big hurry?

Combustion is a lot harder than many people realize.

Dumping the exhaust from a turbopump into a rocket combustion chamber is not easy. You don't want to destabilize the combustion, for one thing. Also, fluid only flows from high pressure to low pressure, so that means the outlet pressure of your pump turbine would have to be higher than the combustion chamber of your rocket. And the whole thing would become prone to feedback loops that could drive resonances.

The main purpose of your turbopump is to pump, and the main purpose of your rocket combustion chamber is to burn fuel. It's much easier to avoid compromising either of those by using the open-cycle architecture.

[farmerben]

 

In this picture it looks like the pump exhaust is hydrazine rich.

 

[DDE]

On 12/18/2019 at 1:06 AM, farmerben said:

What sort of fuel pumps are used with hypergolic fuel?

Pretty much the standard ones - material compatibility is always a concern. RD-253 uses an oxidizer-rich PB, RD-0234 does too and its been rated to use ClF5 with minimum modifications.

On 12/18/2019 at 4:18 PM, farmerben said:

My understanding of open cycle fuel pumps is that they have to run fuel rich to not overheat, and because oxygen is highly corrosive to steel.

As an aside, the 'closed-cycle'/staged combustion 

On 12/18/2019 at 12:36 AM, magnemoe said:

Assuming Blue Origin also uses closed.

They're into a variety of weird cycles. Say, they're trying to use the tap-off cycle, where they use the main TC as the gas generator, even though they have to cool the exhaust down before routing it into the turbine.

[Nuke]

On 12/17/2019 at 1:06 PM, farmerben said:

What sort of fuel pumps are used with hypergolic fuel?

i want to say that these are pressure fed engines and dont have them. they stick an air bladder in the fuel tanks, and pressurize them with helium. this expands to displace fuel as it is used, effectively pushing the fuels out the tanks.  simply opening the valves starts the engine, the fuel does the rest.

Edited December 22, 2019 by Nuke

[farmerben]

1 hour ago, Nuke said:

i want to say that these are pressure fed engines and dont have them. they stick an air bladder in the fuel tanks, and pressurize them with helium. this expands to displace fuel as it is used, effectively pushing the fuels out the tanks.  simply opening the valves starts the engine, the fuel does the rest.

They definitely do this, and it works as you say,.  But, I'm not sure if it's fast enough for first stage boosters.   I've also heard the helium bladders are required to prevent the fuel tank from imploding in atmosphere, which it could do with a powerful enough turbopump sucking on it.  

Hypergolic-monopropellants offer some interesting oppotunities.    N₂H₄ transitions into N₂ and 2H₂  irreversibly, and with substantially less heat that combustion.  So it could be fed back into the tank as a pressurizer.  N₂O₄ breaks apart in a reversible manner, so it is a different beast entirely.  

I don't have certainty on this, but it seems to me that:  Hydrazine and dinitrogen tetroxide could support unique types of fuel pressure systems that do not work with more stable fuels.  And there is a possibility reducing weight and number of parts by doing so.  

Somebody has probably already thought about this and written a paper on it.  If you know about it tell me.  

 

[magnemoe]

5 hours ago, Nuke said:

i want to say that these are pressure fed engines and dont have them. they stick an air bladder in the fuel tanks, and pressurize them with helium. this expands to displace fuel as it is used, effectively pushing the fuels out the tanks.  simply opening the valves starts the engine, the fuel does the rest.

Large hypergolic engines uses turbo pumps like the Proton and many Russian ICBM. 
Probes and spaceships uses pressure feed engines. 
One interesting future option might be to use electrical pumps like electron for this, 

Soyuz uses an old system there the turbo pump is run on H2O2, benefit of this is that its basically an steam turbine who is far less stressed. 

[DDE]

5 hours ago, farmerben said:

Hypergolic-monopropellants offer some interesting oppotunities.    N₂H₄ transitions into N₂ and 2H₂  irreversibly, and with substantially less heat that combustion.  So it could be fed back into the tank as a pressurizer.  N₂O₄ breaks apart in a reversible manner, so it is a different beast entirely.  

You're overthinking it.

The R-36 just feeds some propellant into the others' tank, where they combust to provide the pressurization.