Quote from: Dudely on 08/01/2014 02:25 pmI think the dracos can pulse because their fuel is hypergolic.I thought it was because they were pressure fed... much easier to smoothly change the throttle than a pump-fed system. Or maybe it is both...
I think the dracos can pulse because their fuel is hypergolic.
Quote from: abaddon on 08/01/2014 06:24 pmQuote from: Dudely on 08/01/2014 02:25 pmI think the dracos can pulse because their fuel is hypergolic.I thought it was because they were pressure fed... much easier to smoothly change the throttle than a pump-fed system. Or maybe it is both... There's a big difference between opening and closing a valve and spinning up turbo pumps.
Quote from: Nomadd on 08/01/2014 06:29 pmQuote from: abaddon on 08/01/2014 06:24 pmQuote from: Dudely on 08/01/2014 02:25 pmI think the dracos can pulse because their fuel is hypergolic.I thought it was because they were pressure fed... much easier to smoothly change the throttle than a pump-fed system. Or maybe it is both... There's a big difference between opening and closing a valve and spinning up turbo pumps.Yes, that's exactly the point he was making. If you want a responsive thruster able to pulse rapidly, it has to be pressure fed. No matter if the propellant is hypergolic or cryogenic.
Quote from: Lars_J on 08/01/2014 09:40 pmQuote from: Nomadd on 08/01/2014 06:29 pmQuote from: abaddon on 08/01/2014 06:24 pmQuote from: Dudely on 08/01/2014 02:25 pmI think the dracos can pulse because their fuel is hypergolic.I thought it was because they were pressure fed... much easier to smoothly change the throttle than a pump-fed system. Or maybe it is both... There's a big difference between opening and closing a valve and spinning up turbo pumps.Yes, that's exactly the point he was making. If you want a responsive thruster able to pulse rapidly, it has to be pressure fed. No matter if the propellant is hypergolic or cryogenic.I believe that's the case with closed cycle TP, but on an open cycle, like that Gas Generator, couldn't you just put a throttling valve between the TP pump and the injectors and simply restrict flow while reducing the gg gas flow?
Quote from: baldusi on 08/02/2014 01:54 amI believe that's the case with closed cycle TP, but on an open cycle, like that Gas Generator, couldn't you just put a throttling valve between the TP pump and the injectors and simply restrict flow while reducing the gg gas flow?That's essentially what you do with face throttling. You have to avoid pump stall, but that just means throttling the GG, which is how the Merlin apparently does it. http://jasc-controls.com/jasc-industry-listing/space/space-fluid-management/bi-propellant-valve/
I believe that's the case with closed cycle TP, but on an open cycle, like that Gas Generator, couldn't you just put a throttling valve between the TP pump and the injectors and simply restrict flow while reducing the gg gas flow?
Quote from: HMXHMX on 08/02/2014 03:42 amQuote from: baldusi on 08/02/2014 01:54 amI believe that's the case with closed cycle TP, but on an open cycle, like that Gas Generator, couldn't you just put a throttling valve between the TP pump and the injectors and simply restrict flow while reducing the gg gas flow?That's essentially what you do with face throttling. You have to avoid pump stall, but that just means throttling the GG, which is how the Merlin apparently does it. http://jasc-controls.com/jasc-industry-listing/space/space-fluid-management/bi-propellant-valve/ Why would you need to throttle the GG at all? Split the flow downstream of the main fuel valve (and/or the main oxidiser valve), prior to the injector, and add a second flow path back to the tank, controlled by a valve. Now, if this back-to-tank valve is fully closed, the engine operates as it did previously. If it's open, then the pressurised fuel/oxidiser that exits the turbo-pump simply makes its way back to the tank instead of the combustion chamber.You wouldn't get rid of the combustion stability, and transition forces issues that would be introduced by pulsing/rapid deep-throttling, but you would - in my head atleast - be able to throttle the engine, without needing to change the speed of the turbo-pumps, or throttling the gas generator.There would be new problems of course. One I can think of is with respect to maintaining the back-to-tank flow path at cryogenic temperatures - in case of the LOX, as well as avoiding getting filler gas bubbles flowing back into the pipe etc. You could do it only on the fuel side, but that would mean that whenever you throttled down, you'd be burning oxidiser-rich, and probably a lot hotter than the design of the nozzle/combustion chamber was designed to cope with.
Quote from: baldusi on 08/02/2014 01:54 amQuote from: Lars_J on 08/01/2014 09:40 pmQuote from: Nomadd on 08/01/2014 06:29 pmQuote from: abaddon on 08/01/2014 06:24 pmQuote from: Dudely on 08/01/2014 02:25 pmI think the dracos can pulse because their fuel is hypergolic.I thought it was because they were pressure fed... much easier to smoothly change the throttle than a pump-fed system. Or maybe it is both... There's a big difference between opening and closing a valve and spinning up turbo pumps.Yes, that's exactly the point he was making. If you want a responsive thruster able to pulse rapidly, it has to be pressure fed. No matter if the propellant is hypergolic or cryogenic.I believe that's the case with closed cycle TP, but on an open cycle, like that Gas Generator, couldn't you just put a throttling valve between the TP pump and the injectors and simply restrict flow while reducing the gg gas flow?That's essentially what you do with face throttling. You have to avoid pump stall, but that just means throttling the GG, which is how the Merlin apparently does it. http://jasc-controls.com/jasc-industry-listing/space/space-fluid-management/bi-propellant-valve/
LOX @ +100F! Wasn't expecting that. ;-)
I presume, then, that it's not especially easy to run a GG on its own, without actually firing the engine attached to it? Just that if you could, it would open up possibilities for very low thrust for manoeuvring and landing burns...
Quote from: MP99 on 08/03/2014 07:31 amLOX @ +100F! Wasn't expecting that. ;-) Question for the real rocket scientists around here: If this valve claims to tolerate LOX at 100 degrees Fahrenheit, then what is the state of the "liquid" oxygen then? Gaseous, or pressure-induced liquid?
Quote from: Kaputnik on 08/03/2014 10:45 amI presume, then, that it's not especially easy to run a GG on its own, without actually firing the engine attached to it? Just that if you could, it would open up possibilities for very low thrust for manoeuvring and landing burns...GG exhaust is low pressure, low velocity, so you aren't going to get useful thrust out of it. If you changed that, you'd just make it less efficient at powering the pumps.
Quote from: rpapo on 08/03/2014 09:05 amQuote from: MP99 on 08/03/2014 07:31 amLOX @ +100F! Wasn't expecting that. ;-) Question for the real rocket scientists around here: If this valve claims to tolerate LOX at 100 degrees Fahrenheit, then what is the state of the "liquid" oxygen then? Gaseous, or pressure-induced liquid?I have no idea why they wrote that into the spec. Perhaps someone else does?
Quote from: SoulWager on 08/03/2014 12:28 pmQuote from: Kaputnik on 08/03/2014 10:45 amI presume, then, that it's not especially easy to run a GG on its own, without actually firing the engine attached to it? Just that if you could, it would open up possibilities for very low thrust for manoeuvring and landing burns...GG exhaust is low pressure, low velocity, so you aren't going to get useful thrust out of it. If you changed that, you'd just make it less efficient at powering the pumps.Depends on your definition of "useful." The 1.5E6-lbf F-1 generates 40K-lbf thrust with its GG exhaust after spinning the turbine and being used as a heat exchanger fluid.