Expander Cycle Rocket Engines - Using Waste Heat To Drive Your Rocket

0:08 scott manley here on saturday morning my twitter was abuzz with discussions about rocket engine cycles involving such luminaries as Tory Brunel as CEO of ula and you know I'll people got particularly interested in the expander cycle rocket and then of course Saturday afternoon the Dragons Scituate situation metaphorically and literally exploited but I do want to come back to the expander engines because a lot of people seem to have a lot of questions and a lot of interest now I did mention expander cycle engines in my more general video on fuel cycles but to give you a quick summary when you have a high-performance rocket engine you need to have a high power pump that takes the fuel from the fuel tank and the oxidizer tank and it forces them into the combustion chamber at higher pressure now you can do this in some cases say with an electric pump driving or an electric motor driving a pump that most rockets do this with the turbine driving a pump hence the term turbo pump know the powder they exhaust the gas to power that turbine in most rockets that comes from something called the gas generator that's where you're burning a small amount of the fuel and oxidizer and there's basically a miniature version of the rocket engine blowing the exhaust gasses over the turbine and driving the main pump and then an output from that main pump goes into the engine but for expander cycle engines what they instead do is they take the cryogenic fuel and the flawed around the outside of the combustion chamber and they take that heat boil the fuel and then use the vaporized fuel to drive the turbines so of course most rockets use this technique to cool the combustion chamber but with the expander cycle engines this is the primary way in which the power the turbo pump this engine cycle was actually used in one of the most successful engines in history the RL 10 this design was developed of sixty years ago and it first flew in 1963 on an Atlas and since then has been used as an upper stage on Atlas Titan Dell it's even been used on this from one and it'll also be used in future rockets in the SLS in the Vulcan on the Omega and it might also be used as landing engines for a lunar landers all the expander cycle engines that have flew and work with liquid hydrogen as a fuel and liquid oxygen as an oxidizer now the boiling point of liquid hydrogen is about 20 Kelvin and you know the engineering challenges of working with hydrogen frequently Revolver I'm stopping it from boiling but of course when you need it to it's great that it boils at such a low temperature I mean it turns out that starting up many of these expander cycle engines is literally a case of letting the gas flow into the engine which will be at some ambient temperature which is well above the boiling point of hydrogen and from there that gives them just enough thermal energy to start the pumps properly and start the fuel engine cycle but once the engine starts properly it gets his energy from the combustion chamber in the rl10 the aunt hydrogen is flowing through the walls of the combustion chamber keeping it of course it very low temperatures but inside of the combustion temperature chamber the temperatures are thousands of degrees Celsius and of course will melt practically anything you can think of in the design of the rl10 the hot liquid hydrogen comes into the engine through its pump and from there it's driven through the walls of the combustion chamber and the nozzle where it absorbs a lot of heat it boils it's temperature rises by a couple of hundred to three few hundred degrees Celsius and then from there the expanded gas flows through a turbine which drives the turbo pump and then as the gas leaves the turbo turbine it then is goes into the main engine and of course meets the oxygen now there's obviously also an oxygen turbo pump in the case of the RL 10 the turbo pump for the oxygen is driven by a gearbox hydrogen is very bulky and you need to have pump a lot more of it so you don't need nearly as big a turtle of pump for the oxygen in the case of the RL 10 also I believe they use a two-stage pump for the hydrogen and a single-stage pump for the oxygen this is actually a very efficient system in fact the rl10 that's used in the Delta for upper stage is currently the most efficient chemical rocket flying has a specific impulse of 462 seconds although a lot of that is due to its very large nozzle extension which I love showing because of the way it fits up inside the stage and then extends out during stage separation the only other expander cycle engine I know is flowing is the Chinese YF 75 D which is used in the upper stage of the long march 5 Europe is developing an engine along these lames it's called DaVinci it's going to use separate turbo pumps for the oxygen and the fuel Russia has the rd 0 146 which in theory may one day fly on their angora rocket whenever that goes up and yeah there's a few other things floating around anyway for comparison let's look at another hydrogen oxygen engine the rs.25 that's the engine that powers the space shuttle and will power the SLS this is a vastly more complicated design using pre burners and separate turbo pumps of the fuel and the oxidizer and the stresses on the rs.25 turbines are way higher with the working fluid having temperatures of you know four to five hundred degrees Celsius and containing energetic combustion products that just want to react with anything meanwhile the turbine on the RL 10 it runs on neutral hydrogen gas in this temperature is well in some cases basically room temperature in exchange for all that extra complexity the rs.25 does generate a lot more thrust with much higher combustion chamber pressures the RL 10 by comparison has only ever operated as an upper stage and it never is used to lift a spacecraft against Earth's gravity expander cycle engines like the RL 10 have problems generating high thrust because the energy to drive the expanding hydrogen comes from the heat absorbed by the walls of the combustion chamber and basic geometry says if you are to scale up the rocket by scaling up the combustion chamber that the the area of the walls where the heat is being absorbed will grow as the size squared where is the volume of the fuel combusting will grow as the size cubed so as you scale it up eventually runs out of enough heat flux to run this and so you you basically run into this kind of limit where you can't get any more thrust from an expander cycle engine and there's a few different tricks here and there you know you can change the geometry of the combustion chamber to make it longer and thinner but you know that causes its own trial and problems but yet if you talk to people it suggested that the peak thrust or the you know top thrust you could get from this is probably about a hundred and fifty kilonewtons or fifteen tons of thrust however there are more powerful expander cycle engines which get higher thrusts is more correct to call the RL 10 a closed expander cycle because all of the propellants all of the liquid oxygen all of the hydrogen end up flowing through the combustion chamber the output from that turbine has to flow into the combustion chamber so that none of it is wasted now since all the output from the turbines has to then go through the combustion chamber that means the turbine output pressure has to be higher than the pressure inside the combustion chamber and that means the pump that's powering everything has to have enough pressure to push all the fuel through the cooling channels drive the turbines and then get into the combustion chamber and the performance of that turbine is driving the pump that actually depends on the pressure drop across that turbine you know lots of rocket engine design boils down to just balancing the pressures and the flaws so everything goes in the correct direction and you're maximizing amount of power you're getting out the other end an alternate design simply dumps the turbine exhaust overboard into the vacuum of space and this means that the pressure difference can be a lot higher which means the turbine can generate more power which means the pump can pump more fuel which means the combustion chamber can be bigger and have higher pressures and you might naturally call this an open expander cycle engine but rocket designers that have actually built these things prefer the much cooler sounding expander bleed cycle Japan has developed the Ali 5 series of engines using this cycle for the upper stage of their h1 and their h2 launch vehicles but there are further forthcoming h3 it will be powered by 2 or 3 Le nain engines running on the same expander bleed cycle so this will be an expander cycle engine powering a booster a first stage by moving to the bleed cycle the engines are able to generate 1 point 5 mega Newton's or about 150 tons of thrust and that's more than ten times what you could theoretically get from a closed expander cycle as you can guess you know you lose some efficiency you drop a bit 10% in terms of specific impulse but you still get to keep those nice cool benign turbine environments so you can you're cheap out on the hardware there in the US Blue Origin have also been testing an expander bleed cycle variant of the B III which has previously powered the new Shepard the new variant is called the B III you and two of them are supposed to power the upper-stage of the new glenn the blue origin website specifies a combined thrust of 1,100 kilonewtons and as about 550 kilonewtons per engine which is a bit of a step down from the stock B III but I am sure it gets better specific impulse incidentally that B III that powers the new Shepherd that's also kind of a rocket science curio because it uses something called the combustion tap off cycle instead of having a dedicated gas generator or a pre burner it actually had it takes gases exhaust gases from the combustion chamber and uses that to drive the turbines it's the only a rocket engine that's flown with this design as far as I can tell so I've just grade two basic types of expander cycle engines but of course rocket scientists are always inventing things and there is a fantastic article and expander cycle engines on the NASA website by a guy called William green and he worked on the j-2x which was supposed to be a better upper stage for the SLA project is currently shut down but the articles are still up there and they make fantastic reading and I'm just gonna give you a quick rundown of some of these variants so now if you remember that all these propellant floors require pressure or differences between input and output of each step and pushing all that fuel through the cooling channels in the combustion chamber that requires extra pressure so one idea is called the closed split expander cycle where you have a single shaft that's driving two separate turbo pumps in the fuel and some part of the fuel goes directly to the combustion chamber and the other part goes through the cooling channels and then drives the turbo pump and in theory this might get you some better performance but of course it increases the engine complexity a second concept is the closed dual expander cycle and this uses your fuel expansion to drive the fuel pump but it also uses the oxidizer to cool the combustion chamber and then uses that oxidizer to drive the oxidizer pump now a significant engineering problem in creating in running rocket engines like this is making sure that the fuel and the oxidizer remain separate and if you have a turbine connected to a pump and the turbine is being driven by fuel and the pump is pumping oxidizer it's possible for the oxidizer to leak through the your bearing between the two things so you need a very complicated sealed bearing what they will one of the things the true or one of the tricks they use is they pump helium into the bearing to make sure the fuel and the oxidizer never touch so by using oxidizer they are liquid oxygen boiling and then driving it to use it allowing it to drive the turbine you keep this separate until the very last minute so you can have much simpler bearings of course you're now having a lot more complicated plumbing maybe this works maybe it's not I have only ever seen designs on paper for this there's also concerned about having spinning metal in a pressurized oxygen environment because of its propensity to you will make metal burn and of course if you're a rocket scientist you can combine both these tricks to a closed dual split expander cycle because why not a final idea where's to augment the heating with a gas generator and the heat exchanger so you're burning some of the fuel and oxidizer and you're using that to heat the hydrogen even more and so you need that gives you a bit more thrust you're obviously dumping the grant gas generator over but it's kind of a halfway step between a fool open expander and a fool closed expander I actually know that saw that William Greene has a patent on this so again it's been designed there's a patent that hasn't ever been flown so yeah that's my rundown off expander cycle engines they basically take the problem off the heat and the exploit that to drive the engine itself and it's a sound concept the rl10 has been around for almost 60 years and it will be around for decades to come because it is a fantastic engine and a fantastic concept I'm Scott Manley fly safe [Music] [Music]

Channel: Scott Manley

Views: 326,498

Keywords: rocket science, hydrogen, oxygen, expander, expander cycle, turbopump, rl-10, nasa, rocket engine, turbine, pump, vinci

Id: HSUhHYdyGJI

Length: 14min 43sec (883 seconds)

Published: Tue Apr 23 2019