Poll

When will full-scale hot-fire testing of Raptor begin?

Component tests - 2017
3 (0.6%)
Component tests - 2018
21 (4.2%)
Integrated tests -  2017
19 (3.8%)
Integrated tests -  2018
237 (47%)
Integrated tests -  2019
181 (35.9%)
Raptor is not physically scaled up
33 (6.5%)
Never
10 (2%)

Total Members Voted: 504


Author Topic: SpaceX Raptor engine (Super Heavy/Starship Propulsion) - General Thread 1  (Read 869945 times)

Offline STS-200

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #720 on: 11/24/2017 01:47 pm »
Everything downstream of the pumps has a lower total pressure. That's why the propellants flow. This is true as long as there is pressure in the tanks even if the pumps are not rotating. Preburner combustion greatly increased the volume of the propellants and hence their velocity (dynamic pressure) not their static pressure. Dynamic pressure spins the turbine.

It seems I lack some basic physics here. How can the preburner ignite the propellant, increase its volume and flow rate and not increase the static pressure?
It doesn't increase the flow rate (mass-per-second). It does increase the velocity of the propellants.
This happens because the propellant is heated in the preburner - it may simply be a gas getting hotter, or a liquid vaporising to gas. Either way, the volume increases, requiring the propellant to accelerate so mass flow rate remains the same.
Dynamic pressure - the pressure caused by he motion of the gas - rises, as the gas is moving faster. Static pressure falls to compensate, as total pressure stays constant (in an ideal device, in the real world it will always drop a bit).


Quote
Rakaydos gave a good reason why there is more pressure on the turbine than on the tank. But if it's all that simple, why would the merlin need to prestart the turbopumps with high pressure helium? They could do the same/similar procedure with the gas generator.
Tank head start is possible, but it's a slow process that could easily give rough starts/problems with startup sequences. Venting Helium (or other start gases) through the turbine is something that can be precisely controlled, is highly predictable and spins up the turbine very quickly.
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Offline livingjw

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #721 on: 11/24/2017 02:13 pm »
Everything downstream of the pumps has a lower total pressure. That's why the propellants flow. This is true as long as there is pressure in the tanks even if the pumps are not rotating. Preburner combustion greatly increased the volume of the propellants and hence their velocity (dynamic pressure) not their static pressure. Dynamic pressure spins the turbine.

It seems I lack some basic physics here. How can the preburner ignite the propellant, increase its volume and flow rate and not increase the static pressure? Rakaydos gave a good reason why there is more pressure on the turbine than on the tank. But if it's all that simple, why would the merlin need to prestart the turbopumps with high pressure helium? They could do the same/similar procedure with the gas generator.

- My mistake. I should have said: Everything downstream of the PRE-BURNER has a lower total pressure. The static pressure will rise in the pre-burner as combustion products back pressure the turbine.  But, it cannot increase above the pressure upstream of the pre-burner injectors.

- The start mode I outlined is what NASA SP-125 (pg 68) calls as "main tank head start". If this type of start takes too long (> 3 seconds or so) a "turbine spin start" may be added to the system to decrease the start time. I do not know which method the Merlin uses. I would guess a "main tank head start". Does anyone know?

Also see SP-125 pg 181 for different types of starts.

Offline Semmel

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #722 on: 11/24/2017 03:02 pm »
Everything downstream of the pumps has a lower total pressure. That's why the propellants flow. This is true as long as there is pressure in the tanks even if the pumps are not rotating. Preburner combustion greatly increased the volume of the propellants and hence their velocity (dynamic pressure) not their static pressure. Dynamic pressure spins the turbine.

It seems I lack some basic physics here. How can the preburner ignite the propellant, increase its volume and flow rate and not increase the static pressure? Rakaydos gave a good reason why there is more pressure on the turbine than on the tank. But if it's all that simple, why would the merlin need to prestart the turbopumps with high pressure helium? They could do the same/similar procedure with the gas generator.

- My mistake. I should have said: Everything downstream of the PRE-BURNER has a lower total pressure. The static pressure will rise in the pre-burner as combustion products back pressure the turbine.  But, it cannot increase above the pressure upstream of the pre-burner injectors.

- The start mode I outlined is what NASA SP-125 (pg 68) calls as "main tank head start". If this type of start takes too long (> 3 seconds or so) a "turbine spin start" may be added to the system to decrease the start time. I do not know which method the Merlin uses. I would guess a "main tank head start". Does anyone know?

Also see SP-125 pg 181 for different types of starts.

I need to digest all this but I think the Merlin has a face start sequence (I have no idea what that means). I remember Mueller in an interview reporting that they blew up 100 engines before they go it right.

Online meekGee

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #723 on: 11/24/2017 03:53 pm »
I need to digest all this ...

Semmel - I think the key question here is, ignoring the actual combustion chamber, does the powerpack run a thermodynamic cycle.

If yes, then startup would be like a jet engine's, which cannot be done by simply "lighting it up".

But I think the power pack is different. The power extracted from the exhaust is not used to pump fuel into the pack, but to pump it into the combustion chamber. I think that's why it is possible.

However, with all the phase changes that are going on, it is far from trivial, and the explanation upthread is too simplistic - I wouldn't take it literally.
« Last Edit: 11/24/2017 03:54 pm by meekGee »
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Offline cppetrie

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #724 on: 11/24/2017 05:11 pm »
Everything downstream of the pumps has a lower total pressure. That's why the propellants flow. This is true as long as there is pressure in the tanks even if the pumps are not rotating. Preburner combustion greatly increased the volume of the propellants and hence their velocity (dynamic pressure) not their static pressure. Dynamic pressure spins the turbine.

It seems I lack some basic physics here. How can the preburner ignite the propellant, increase its volume and flow rate and not increase the static pressure? Rakaydos gave a good reason why there is more pressure on the turbine than on the tank. But if it's all that simple, why would the merlin need to prestart the turbopumps with high pressure helium? They could do the same/similar procedure with the gas generator.

- My mistake. I should have said: Everything downstream of the PRE-BURNER has a lower total pressure. The static pressure will rise in the pre-burner as combustion products back pressure the turbine.  But, it cannot increase above the pressure upstream of the pre-burner injectors.

- The start mode I outlined is what NASA SP-125 (pg 68) calls as "main tank head start". If this type of start takes too long (> 3 seconds or so) a "turbine spin start" may be added to the system to decrease the start time. I do not know which method the Merlin uses. I would guess a "main tank head start". Does anyone know?

Also see SP-125 pg 181 for different types of starts.

I need to digest all this but I think the Merlin has a face start sequence (I have no idea what that means). I remember Mueller in an interview reporting that they blew up 100 engines before they go it right.

Face shut-off.....not face start. AIUI the engine is shut down by closing the pintle completely on the pintle injector. The center of the injector carries the oxidizer and the outside ring carries RP-1.

Offline livingjw

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #725 on: 11/24/2017 07:22 pm »
I need to digest all this ...

Semmel - I think the key question here is, ignoring the actual combustion chamber, does the powerpack run a thermodynamic cycle.

If yes, then startup would be like a jet engine's, which cannot be done by simply "lighting it up".

But I think the power pack is different. The power extracted from the exhaust is not used to pump fuel into the pack, but to pump it into the combustion chamber. I think that's why it is possible.

However, with all the phase changes that are going on, it is far from trivial, and the explanation upthread is too simplistic - I wouldn't take it literally.

- By powerpack I assume you mean a turbo-pump assembly with its associated gas generator. Yes, it runs a thermodynamic cycle.

- This "powerpack" can be started with only main tank head pressure and igniters, but may be slow to spool up. If this is the case a "spin turbine" may be added. The tank pressure is the initial motive force.

- The power extracted from the exhaust (along with the tank pressure ~3 atms) is used to pump fuel into the powerpack as well as the main chamber.

- The gas generator, or pre-burner, gasifies the propellants either fuel rich or oxidizer rich. This is well understood. I fail to see the problem in my start sequence?

- According to Sutton, the F1, MA-3 and SSME are all started using "tank head" starting.

John
« Last Edit: 11/25/2017 01:22 pm by livingjw »

Online meekGee

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #726 on: 11/24/2017 08:38 pm »
I need to digest all this ...

Semmel - I think the key question here is, ignoring the actual combustion chamber, does the powerpack run a thermodynamic cycle.

If yes, then startup would be like a jet engine's, which cannot be done by simply "lighting it up".

But I think the power pack is different. The power extracted from the exhaust is not used to pump fuel into the pack, but to pump it into the combustion chamber. I think that's why it is possible.

However, with all the phase changes that are going on, it is far from trivial, and the explanation upthread is too simplistic - I wouldn't take it literally.

- By powerpack I assume you mean a turbo-pump assembly with its associated gas generator. Yes, it runs a thermodynamic cycle.

- This "powerpack" can be started with only main tank head pressure and an igniters, but may be slow to spool up. If this is the case a "spin turbine" may be added. The tank pressure is the initial motive force.

- The power extracted from the exhaust (along with the tank pressure ~3 atms) is used to pump fuel into the powerpack as well as the main chamber.

- The gas generator, or pre-burner, gasifies the propellants either fuel rich or oxidizer rich. This is well understood. I fail to see the problem in my start sequence?

- According to Sutton, the F1, MA-3 and SSME are all started using "tank head" starting.

John

I'm not sure about the cycle.

In a jet engine, you have a clear "cycle", since the far field inlet and outlet conditions are sinked into the same atmosphere, and mechanical power extracted from the exhaust goes into compressing the inflow.

Here, the conditions in the far field inlet are simple the tanks (with head pressure), and the outlet goes into the combustion chamber.   Mechanical power extracted from the exhaust goes into pumping the combustion chamber - not into the powerpack.

If you include the combustion chamber, then far field outlet conditions are the cold hard cynical vacuum of space.

If there's no cycle, then in theory you could just "light it up", but as you say, practicalities may dictate that the spin up will be impractically slow.

Whichever way, I don't think it's an intractable problem. For all that I know, they might put an electrical motor on the shaft...
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Offline Semmel

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #727 on: 11/24/2017 09:33 pm »
For all that I know, they might put an electrical motor on the shaft...

That was the idea I had and didn't voice because of the danger of baseless speculation. Once running, the motor would generate electricity to heat up and gasify some of the propellant in the tanks to create the autogenous pressure. Safes the running of hot fuel pipes in favor of electrical cables. No idea what is lighter but it probably would safe a lot of headaches with the hot pure oxygen.

Again, total speculation on my part and probably wrong.

Offline MegabytePhreak

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #728 on: 11/24/2017 09:56 pm »
Mechanical power extracted from the exhaust goes into pumping the combustion chamber - not into the powerpack.
No, the the powerpack fuel is also pumped by the powerpack. This is certainly a requirement for raptor, since in FFSC the pressure in the preburner must be greater than chamber, and since all fuel goes through the preburners, there would be nowhere else to pump to.

For a GG like Merlin, it may not be 100% necessary theoretically , but it would suck for ISP to run the pumps on just 3 atm of pressure drop, which is all you get if you rely on tank head to push fuel into the preburner. The turbopump would also need to be physically  much larger.

Offline livingjw

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #729 on: 11/25/2017 12:26 am »

I'm not sure about the cycle.

In a jet engine, you have a clear "cycle", since the far field inlet and outlet conditions are sinked into the same atmosphere, and mechanical power extracted from the exhaust goes into compressing the inflow.

Here, the conditions in the far field inlet are simple the tanks (with head pressure), and the outlet goes into the combustion chamber.   Mechanical power extracted from the exhaust goes into pumping the combustion chamber - not into the powerpack.

If you include the combustion chamber, then far field outlet conditions are the cold hard cynical vacuum of space.

If there's no cycle, then in theory you could just "light it up", but as you say, practicalities may dictate that the spin up will be impractically slow.

Whichever way, I don't think it's an intractable problem. For all that I know, they might put an electrical motor on the shaft...

- I'm sure of the cycles both gas generator and pre-burners.

- Before starting, the main chamber is at what ever the outside pressure is (which could be vacuum).

- Mechanical power extracted from a gas generator's or pre-burner's exhaust all goes into pumping the propellants!

- The propellants then either go to the main chamber or gas generator / pre-burner for combustion.
 In a gas generator cycle only a small portion of the propellants is burnt and it is exhausted separately from the main chamber. In a pre-burner a larger portion of the propellant is burnt and it is exhausted into the main chamber. The pre-burner obviously needs to be at pressure higher than the main chamber.

- No electric motors. Pumps for large rocket engines require 10s of thousands of horse power.

John
« Last Edit: 11/25/2017 12:31 am by livingjw »

Online meekGee

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #730 on: 11/25/2017 01:21 am »
For all that I know, they might put an electrical motor on the shaft...

That was the idea I had and didn't voice because of the danger of baseless speculation. Once running, the motor would generate electricity to heat up and gasify some of the propellant in the tanks to create the autogenous pressure. Safes the running of hot fuel pipes in favor of electrical cables. No idea what is lighter but it probably would safe a lot of headaches with the hot pure oxygen.

Again, total speculation on my part and probably wrong.
Saying something is possible or even a good idea is not speculation...

Saying something "might be in place", or "may have happened" is.

So wrt electric drive, it'll be heavier than a gas starter, but much more reliable and controllable.

I have no doubt it was on the trade table, but who knows what they ended up with.
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Online meekGee

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #731 on: 11/25/2017 01:26 am »

I'm not sure about the cycle.

In a jet engine, you have a clear "cycle", since the far field inlet and outlet conditions are sinked into the same atmosphere, and mechanical power extracted from the exhaust goes into compressing the inflow.

Here, the conditions in the far field inlet are simple the tanks (with head pressure), and the outlet goes into the combustion chamber.   Mechanical power extracted from the exhaust goes into pumping the combustion chamber - not into the powerpack.

If you include the combustion chamber, then far field outlet conditions are the cold hard cynical vacuum of space.

If there's no cycle, then in theory you could just "light it up", but as you say, practicalities may dictate that the spin up will be impractically slow.

Whichever way, I don't think it's an intractable problem. For all that I know, they might put an electrical motor on the shaft...

- I'm sure of the cycles both gas generator and pre-burners.

- Before starting, the main chamber is at what ever the outside pressure is (which could be vacuum).

- Mechanical power extracted from a gas generator's or pre-burner's exhaust all goes into pumping the propellants!

- The propellants then either go to the main chamber or gas generator / pre-burner for combustion.
 In a gas generator cycle only a small portion of the propellants is burnt and it is exhausted separately from the main chamber. In a pre-burner a larger portion of the propellant is burnt and it is exhausted into the main chamber. The pre-burner obviously needs to be at pressure higher than the main chamber.

- No electric motors. Pumps for large rocket engines require 10s of thousands of horse power.

John
Depends where you draw the boundary of the control space.

If around the powerpack, then no, power doesn't go to pump propellant into it.  It is fed by head pressure.  Power goes into pumping into the main chamber.

If around the whole motor, then yes, but then a rocket engine as a whole - does it run a thermo cycle?  I'm not sure.  It's very different from a jet engine...

Anyway, yes, electric motors would have to be huge or only act as primers of some sort...  and they would have been visible in the CAD models.
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Offline RoboGoofers

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #732 on: 11/25/2017 01:36 am »
An electric starter/generator could be used to power electric gimbaling or electric grid fin actuation (only when the turbine is spinning, of course). I have a hard time believing they'll stick with open hydraulics for bfr/bfs
« Last Edit: 11/25/2017 06:35 am by RoboGoofers »

Offline livingjw

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #733 on: 11/25/2017 12:19 pm »
- I have not seen or heard of anything that indicates the use of an electric motor on either Merlin or Raptor.

- Again, the gas generator or pre-burner is fed from the output of the pumps!

John

Offline livingjw

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #734 on: 11/25/2017 01:00 pm »
Depends where you draw the boundary of the control space.

If around the powerpack, then no, power doesn't go to pump propellant into it.  It is fed by head pressure.  Power goes into pumping into the main chamber.

If around the whole motor, then yes, but then a rocket engine as a whole - does it run a thermo cycle?  I'm not sure.  It's very different from a jet engine...

Not that much different.

John

Offline envy887

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #735 on: 11/25/2017 03:06 pm »
Depends where you draw the boundary of the control space.

If around the powerpack, then no, power doesn't go to pump propellant into it.  It is fed by head pressure.  Power goes into pumping into the main chamber.

If around the whole motor, then yes, but then a rocket engine as a whole - does it run a thermo cycle?  I'm not sure.  It's very different from a jet engine...

Not that much different.

John

They are similar, and turbojets use electric motors to spin up the turbines to get the compressors feeding air pressure. Why couldn't a FFSC engine spin the turboshaft with a motor, just to get greater than tank head pressure in the preburner?

A key difference is that a turbojet has zero pressure differential between the inlet and the burner before spinning the compressor up, while the rocket has several atmospheres (~50 psi) of pressure pushing oxidizer into the burner. So a turbojet can't do a head start, while a SC rocket engine can.

Offline livingjw

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #736 on: 11/25/2017 04:40 pm »

They are similar, and turbojets use electric motors to spin up the turbines to get the compressors feeding air pressure. Why couldn't a FFSC engine spin the turboshaft with a motor, just to get greater than tank head pressure in the preburner?

A key difference is that a turbojet has zero pressure differential between the inlet and the burner before spinning the compressor up, while the rocket has several atmospheres (~50 psi) of pressure pushing oxidizer into the burner. So a turbojet can't do a head start, while a SC rocket engine can.
- Yes, you could use an electric motor to spin up the turbine for faster starting, or you could use another high pressure gas source, or you could just use tank head pressure like the F1, MA-3 and SSME. I'm just saying that I have seen no evidence that the Raptor or Merlin use any type of spin up system. Does anyone know different?

- Yes, without a pressure difference the turbojet needs something to spin it up, but if the turbojet had high pressure at its compressor face as happens in forward flight, it can start without a starter motor.

John
« Last Edit: 11/25/2017 04:41 pm by livingjw »

Offline ChaoticFlounder

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #737 on: 11/26/2017 12:48 am »
To corroborate what JW was saying

https://blogs.nasa.gov/J2X/2013/12/

And as JW mentioned, take a look at SP-125 and SP-8107 and Rocket Propulsion Elements by George Sutton

Jet engines and LPRE's are very similar as well, see Kuznetzov Design Bureau and NK-33 / NK-15

C

« Last Edit: 11/26/2017 12:53 am by ChaoticFlounder »

Offline biosehnsucht

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #738 on: 11/26/2017 12:58 am »
They said it uses autogenous  pressurization, so use some of those gases.

I am pretty sure the tank pressure provided by autogenous pressurization system is not enough to start the spin of the turbines. If that was the case, F9 would be able to do the same with LOX and RP1 but they use high pressure helium instead. Probably a lot of it. But I am not an expert and happy to be proven wrong.

Pretty sure the issue is that RP-1 can't be autogenously pressurized the way that Methane and Oxygen can be. So you could in theory use autogenous pressurization of the LOx tank on Falcon 9 but you'd still need Helium for the RP-1 side of things...

Offline lrk

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Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #739 on: 11/26/2017 03:41 am »
I'm just saying that I have seen no evidence that the Raptor or Merlin use any type of spin up system. Does anyone know different?

Merlin uses a spin up system driven by high-pressure helium.  The actual valves used to control the flow of fuel and LOX are spring-actuated (built into the pintle in the case of the combustion chamber, not sure about the preburner but presumably it has something similar?), so in order for fuel to even be injected the pressure must be high enough, which requires first spinning up the turbopump somehow. 
« Last Edit: 11/26/2017 03:42 am by lrk »

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