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Quote from: livingjw on 11/24/2017 04:10 amEverything 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?
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.
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.
Quote from: livingjw on 11/24/2017 04:10 amEverything 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.
Quote from: Semmel on 11/24/2017 06:38 amQuote from: livingjw on 11/24/2017 04:10 amEverything 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 ...
Quote from: livingjw on 11/24/2017 02:13 pmQuote from: Semmel on 11/24/2017 06:38 amQuote from: livingjw on 11/24/2017 04:10 amEverything 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.
Quote from: Semmel on 11/24/2017 03:02 pmI 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.
Quote from: meekGee on 11/24/2017 03:53 pmQuote from: Semmel on 11/24/2017 03:02 pmI 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
For all that I know, they might put an electrical motor on the shaft...
Mechanical power extracted from the exhaust goes into pumping the combustion chamber - not into the powerpack.
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...
Quote from: meekGee on 11/24/2017 08:38 pmFor 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.
Quote from: meekGee on 11/24/2017 08:38 pmI'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...
Quote from: meekGee on 11/25/2017 01:26 amDepends 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.
Quote from: docmordrid on 11/23/2017 09:16 amThey 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.
They said it uses autogenous pressurization, so use some of those gases.
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?