Yeah, but the second stage is completely different and also fully reusable. Plus the engine design and fuel choice is also very different. So even the first stage has some differences, even if the exterior design is similar.
If active bleed cooling can retire the refurbishment problems of ablative heat shields and the inspection and retention problems of ceramic tiled radiative heat shields, then it really could be a quantum leap toward operational reusability. Just refuel and go.
I think people get pretty enamored by aerospikes. Theres little advantage over just a high chamber pressure, and basically no real advantage for an upper stage. At the expense of low thrust to weight ratio. The reentry method playing well with the aero spike concept is clever, but Im not sure its really so much better than a conventional approach.
You seem to forget that while this is an upper stage engine, they also want to land propulsively using this engine on the stage’s return to Earth. That means that the engine does have to fire both in a vacuum and at sea level (if only briefly), which means the traditional massive upper stage bell nozzle wasn’t ever even an option. It was either an aerospike, a reversible extending nozzle, or something crazier like a TAN. With that in mind, and then adding in the aerospike’s dual use as a heat shield, I think the aerospike was clearly the best option.
I’ve always been a fan of TAN and TAN-like concepts, but it’s better-suited for something like a sustainer architecture, both because the weight penalty is high and because it increases sea level thrust at takeoff. A landing engine needs deep throttling capabilities, not high thrust.
The other option, I suppose, would be to go with a stepped/dual-bell nozzle, but those are very large and would be difficult to shield. I agree that this concept, while admittedly novel, was always going to be their best bet.
The other thing is, I can’t actually come up with a way to make a reusable TSTO vehicle that is simpler than this. It’s like Starship, but without the need for a carefully controlled reentry, or hundreds of tiles, or aerodynamic surfaces, or aerial maneuvers, etc. In trade for not having to deal with all that, all Stoke has to do is actively cool a heatshield.
The only simpler option I could come up with would be to put the heat shield on the nose, like the earliest Falcon 9 upper stage reuse concepts. But that introduces huge center-of-mass issues and would require auxiliary landing engines.
Can’t find any info on booster but it is using methane.
Where are you seeing that their booster is using methane? I would have assumed they were going with hydrogen — perhaps on a gg cycle — both because of the attractiveness of propellant commonality and the low aspect ratio of the booster.
Using hydrogen for cooling isn’t ever as effective as just using water. Water has like 4 times the mass-specific heat of vaporization of hydrogen, not to mention like 20-40 times the volume-specific heat of vaporization, and this advantage isn’t fully undone by the low boiling temperature of hydrogen. Plus, water doesn’t burn like hydrogen does. And is cheaper and doesn’t have the extreme temperature changes of hydrogen.
Water-cooled active shield would be superior in /nearly/ every way.
The mass-specific heat of hydrogen is 3.4X that of water. Assuming inconel or chromium-zirconium bronze construction of the heat shield and coolant channels, the temperature needs to be kept under ~1100 K. One gram of liquid hydrogen will absorb about 4.5 times as much heat going from its boiling point to 1100 K than one gram of water will absorb going from its melting point to 1100 K.
While it’s true that water has a higher enthalpy of vaporization than liquid hydrogen, that’s only the case at standard pressure. But it’s my understanding that at these temperatures and pressures, we’re dealing with supercritical fluids, where the enthalpy of vaporization drops to zero. If you’re trying to operate your water-cooled heat shield at standard atmospheric pressure, then it’s going to undergo that phase change at 100°C, where its enthalpy of vaporization is 2.26 kJ/g. That’s nice and all, but the total energy absorbed by heating one gram of water from 0°C to 100°C and then boiling it off is going to be 2,676 J, which is wildly less than the 14,107 J you’ll absorb by heating one gram of supercritical hydrogen from 20 K to 1100 K without any phase change.
(It’s possible that there’s some fundamental assumption I’m missing, so if so, please feel free to correct me.)
Water is definitely much denser than hydrogen and easier to handle, so that’s very nice. But those are really the only advantages. While water doesn’t burn like hydrogen does, superheated water steam is much more corrosive than hot hydrogen, particularly with the sort of metals from which one is wont to fashion heat shields.
And then finally, as explained by the patent, the liquid hydrogen can be routed through the existing engine nozzle cooling channels and expanded through the existing engine expander turbine to operate the existing fuel turbopump to maintain constant coolant flow, and then dumped out of the engine nozzle through the existing combustion chamber injectors to provide dump film cooling for the engines. Good luck trying to do that with water.
I may be misunderstanding the design, but is the actively-cooled heat shield only actively-cooled during reentry, a time when the engine isn’t running? Or is that same area hot during engine burns, such that regenerative cooling (followed by that heated hydrogen being sent into the combustion chamber) could be used during that phase as well? If the latter is true, perhaps being dual-use like this means that some cooling systems which would be necessary anyway are also being used for reentry, reducing the mass of including separate systems for each (and offsetting the disadvantages of bleeding hydrogen during reentry instead of bleeding water).
Yeah, that’s the big exciting part of the patent (for me at least): a straightforward way to use the same expander cycle cooling manifold and pump system as an active heat shield.
BTW IIRC some of the GH2 from the turbine drive was exhausted though the baseplate, which was a diffusion bonded Titanium with lots of holes drilled in the bottom plate, creating the “Aero-plug” effect. It was somewhere between 10-20% the full length of an equivalent engine with a nozzle of that expansion ratio.
I’m reminded fondly of the rather-wistful proposal by Bill Greene to add a gas generator to a closed expander cycle engine, not to directly drive the turbine but to simply increase the amount of heat that the coolant can pick up and thus improve thrust over the ~150 kN maximum you can otherwise get out of an ordinary closed expander cycle.
The Stoke engine has much more available heat in its re-entry configuration than in its actively-firing configuration due to the whole surface area issue, which makes me wonder if they could be using something like gas generator augmentation to amp up the heat during ordinary engine operation.
Your discussion reminded me of this because like Greene’s proposal, it’s a different way to dump exhaust somewhere, and it would be particularly fascinating if Stoke was able to dump at least part of the turbine exhaust in such a way as to provide an aeroplug effect during engine firing and dump film cooling during re-entry.
“It’s designed to serve a purpose… not to be an aerospike.”
So, it very well may not be an aerospike, but merely an engine with many combustion chambers in a ring?
I interpret that to mean they are designing an engine to accomplish a particular purpose, and that it happens to have the properties of an aerospike because those properties were necessary, not because they set out to make an aerospike “work”.
Their patent explicitly calls it an aerospike engine.