Stoke Space Technologies

[Action]

I would agree an expander cycle makes the most sense with the plug concept shown in their patent --plenty of area to soak up heat and run the turbine.

We have a patent number for them?

US20210381469A1 is for a fairly standard plug nozzle/truncated aerospike arrangement.
WO2021112934A1 is for an actively cooled heatshield using the expansion of the working fluid to drive the coolant pump.

Prior art: Rocketdyne built a 15k-lbf expander cycle aerospike fifty years ago.  Unfortunately, this is the only picture I could find of it without digging though paper files.

There's a better picture of it on page 16 of this PDF (15 of the original document):

https://ntrs.nasa.gov/api/citations/19920013861/downloads/19920013861.pdf

[john smith 19]

There's a better picture of it on page 16 of this PDF (15 of the original document):

https://ntrs.nasa.gov/api/citations/19920013861/downloads/19920013861.pdf

Yes, and it shows what I had not realized. The plug is hanging on 2 trunions on 2 chains.  My bad.

The report doesn not mention the original AIAA paper describing that engine, which from memory is around 1974.

It was a dual expander, with (IIRC) the inner side being O2 cooled and the outside of the chamber segments cooled by H2, then feeding to seperae turbines to drive each pump, obviously only workable with both propellants being cryongens but avoiding the complex must-never-fail seals you need if say the fuel is driving the oxidizer pump. Given the size of the engine I always suspected they borrowed the turbomachinery from a couple of RL10 rather than do actual new development.

The interesting variant is the plug cluster engine, which just groups a set of identical, normal engines around a suitably designed plug. In principle no new engine design needed, just synchronizing the engine controls (which might be non-trivial if they are sealed black boxes) together.

This is probably not viable for Stoke's stage 2 given it's size and the size of available engines.

[Asteroza]

I would agree an expander cycle makes the most sense with the plug concept shown in their patent --plenty of area to soak up heat and run the turbine.

We have a patent number for them?

US20210381469A1 is for a fairly standard plug nozzle/truncated aerospike arrangement.
WO2021112934A1 is for an actively cooled heatshield using the expansion of the working fluid to drive the coolant pump.

Prior art: Rocketdyne built a 15k-lbf expander cycle aerospike fifty years ago.  Unfortunately, this is the only picture I could find of it without digging though paper files.

There's a better picture of it on page 16 of this PDF (15 of the original document):

https://ntrs.nasa.gov/api/citations/19920013861/downloads/19920013861.pdf

The concave plug bottom I assume is for better recirculation heat transfer, versus convex bottomed aeropikes which primarily want to use the baseplate as the reentry heatshield?

The paper has an interesting illustration of the scarfed nozzle and concave bottom for a lunar lander plug nozzle cluster aerospike (so vacuum optimized?)

[Action]

There's a better picture of it on page 16 of this PDF (15 of the original document):

https://ntrs.nasa.gov/api/citations/19920013861/downloads/19920013861.pdf

Yes, and it shows what I had not realized. The plug is hanging on 2 trunions on 2 chains.  My bad.

The report doesn not mention the original AIAA paper describing that engine, which from memory is around 1974.

It was a dual expander, with (IIRC) the inner side being O2 cooled and the outside of the chamber segments cooled by H2, then feeding to seperae turbines to drive each pump, obviously only workable with both propellants being cryongens but avoiding the complex must-never-fail seals you need if say the fuel is driving the oxidizer pump. Given the size of the engine I always suspected they borrowed the turbomachinery from a couple of RL10 rather than do actual new development.

The interesting variant is the plug cluster engine, which just groups a set of identical, normal engines around a suitably designed plug. In principle no new engine design needed, just synchronizing the engine controls (which might be non-trivial if they are sealed black boxes) together.

This is probably not viable for Stoke's stage 2 given it's size and the size of available engines.

These may be the papers you're looking for.  Unfortunately, I can't find a free copy of either.

https://arc.aiaa.org/doi/abs/10.2514/6.1974-1080

https://arc.aiaa.org/doi/abs/10.2514/6.1973-1245

If I recall correctly, there was once a lot of interest in developing aerospikes as a compact and lightweight way of putting a very high expansion ratio on a second stage or in-space propulsion system.  This application avoids any lingering uncertainty about aerospike performance in the atmosphere at low Mach.

[Edit: spelling]

[john smith 19]

These may be the papers you're looking for.  Unfortunately, I can't find a free copy of either.

https://arc.aiaa.org/doi/abs/10.2514/6.1974-1080

Yes that's the paper I had in mind.  I actually have a poor photocopy of it somewhere but I couldn't recall the report number.

If I recall correctly, there was once a lot of interest in developing aerospikes as a compact and lightweight way of putting a very high expansion ratio on a second stage or in-space propulsion system.  This application avoids any lingering uncertainty about aerospike performance in the atmosphere at low Mach.

[Edit: spelling]

True. It was looked at for the "Space tug" that was meant to be the complement to the Shuttle for taking commsats to GTO.

The other benefit for US is high expansion ratiowithout a long interstage length. Lower weight rocket, stiffer rocket, longer propellant tanks for same overall length, all of which can be quite attractive.

[Robotbeat]

These may be the papers you're looking for.  Unfortunately, I can't find a free copy of either.

https://arc.aiaa.org/doi/abs/10.2514/6.1974-1080

Yes that's the paper I had in mind.  I actually have a poor photocopy of it somewhere but I couldn't recall the report number.

If I recall correctly, there was once a lot of interest in developing aerospikes as a compact and lightweight way of putting a very high expansion ratio on a second stage or in-space propulsion system.  This application avoids any lingering uncertainty about aerospike performance in the atmosphere at low Mach.

[Edit: spelling]

True. It was looked at for the "Space tug" that was meant to be the complement to the Shuttle for taking commsats to GTO.

The other benefit for US is high expansion ratiowithout a long interstage length. Lower weight rocket, stiffer rocket, longer propellant tanks for same overall length, all of which can be quite attractive.

You get the same shortness advantage by clustering engines or chambers.

[sevenperforce]

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. There’s 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 I’m not sure it’s 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.

[TrevorMonty]

Active cooling during reentry seems to be driving design.

[sevenperforce]

Maybe once they get it flying in a TSTO configuration, they can experiment with attaching dumb drop tanks and launching it like ROMBUS.

This design approach also feels a lot less squirrely for potential crew capsule integration than something like Starship. Much more straightforward to attach a New Shepard-like capsule with an independent onboard abort motor to the top, giving you full abort capability at (almost) any point during launch or landing. No real way to do that with Starship.

[JEF_300]

"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.

Their patent is also several years old, and doesn't look very much like their current engine at all, which is why I still have some doubts.

[john smith 19]

You get the same shortness advantage by clustering engines or chambers.

True.

But do you get the same Isp, without increasing the chamber pressure with engines that size?

[john smith 19]

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.

I agree with all of you points except this one.

On the latest Skylon/SABRE thread there I attached a file that described some work on a new LH2 pump for an expander engine (the RL60). The key enabler of this was in fact the developmend of an adquately strong Copper based alloy for the tubes making up the combustion chamber. This radically enhanced the thermal conductivity, enabling much more energy to be transferred to the H2 coolant.

Once this alloy was available (mid 90's IIRC) much bigger expanders were possible, without rsorting to stuff like putting an HX in the combustion chamber for example.

People's intuition about the thermal conductivities of materials can be mistaken.  Stainless steels are metals, but not very good heat conductors (which is usually a good thing). Alumina is a ceramic, but is quite a good one in some grades (of which there are a lot)

Aluminum alloys 10x SS
Alumina ceramic (some grades) 2.7x SS
Zirconia 0.14x SS (that's 1/7)

IOW an Alumina heat shield could actually spread the heat better than a steel one. 

[su27k]

https://twitter.com/stoke_space/status/1574872621875097600

Pretty cool: We won a $1 million grant from the National Science Foundation to test our actively cooled metallic heat shield that protects our second stage during re-entry. Thank you for the continued support NSF! 🌎 @NSFSBIRn @NSF

[JEF_300]

https://twitter.com/stoke_space/status/1577747395253006336?s=20&t=pixjcMlkkToipNiBQtAJLg

[Daniels30]

Stoke Space aims to build rapidly reusable rocket with a completely novel design

https://arstechnica.com/science/2022/10/stoke-space-aims-to-build-rapidly-reusable-rocket-with-a-completely-novel-design/

[TrevorMonty]

Stoke Space aims to build rapidly reusable rocket with a completely novel design

https://arstechnica.com/science/2022/10/stoke-space-aims-to-build-rapidly-reusable-rocket-with-a-completely-novel-design/

Some bits from article.

1.65t to LEO fully reuseable. ISP of their aerospike (not stated but best description)  design is better than some variants of RL10. Regeneratively cooled heatshield which we knew.

Nothing on booster design so they may use same engine design and fuel on this or do something more conventional. Regenerative cooling does reduce wear and tear on booster heatshield.

[JEF_300]

I noticed this in the article:

Stoke's answer was using a ring of 30 smaller thrusters. (The tests last month only employed 15 of the 30 thrusters). In a vacuum, the plumes from these nozzles are designed to merge and act as one.

Eric Berger knows what an aerospike is guys, and he knows his audience. If that's what it was, he would just call it that instead of writing... whatever that is (WTF does "merge and act as one" mean here?).

Neither the word 'aerospike' nor the word 'plug' shows up once in this whole article.

[JEF_300]

Here are a couple more fun quotes.

"What you’re seeing in the photos of the test is a high-performance upper-stage engine that can operate within atmosphere at deep throttle to support vertical landing but then also perform at a higher ISP than some variants of the RL 10 engine in space," Lapsa said.

Let's note that there was a sea level variant of the RL-10 made, so there's plenty of wiggle room in that quote. Still, taking it at face value, it's an impressive claim.

Given Stoke's background in rocket engines, Lapsa said it made the most sense to try a regeneratively cooled heat shield. The vehicle's ductile metallic outer layer will be lined with small cavities to flow propellant through the material to keep it cool during reentry.

Here we should note the word propellant. So no water cooled heat shield.

Engine tests are an important step, but they're only the first step of many. Next up for the company is "hop" tests with a full-scale version of the second stage at the Moses Lake facility in central Washington.

I don't think I'll ever get sick of Hop tests.

[GWH]

Lapsa worked for one of them, first helping Bezos develop the powerful BE-4 engine and then as director of Blue Origin's BE-3 program.

“I love Jeff’s vision for space," Lapsa said in an interview with Ars. "I worked closely with him for a while on different projects, and I’m basically 100 percent on board with the vision. Beyond that, I think I would just say that I will let their history of execution speak for itself, and I thought we could move faster.

Attached is an image of New Goddard, Blue's first vehicle.

Also attached is an image of a mock up of Stoke Space's upper stage.

The resemblance is amusing, if nothing else.

I wish them all the luck in executing their upper stage design with purpose, and not a pointless dead end concept vehicle.

[john smith 19]

Eric Berger knows what an aerospike is guys, and he knows his audience. If that's what it was, he would just call it that instead of writing... whatever that is (WTF does "merge and act as one" mean here?).

Neither the word 'aerospike' nor the word 'plug' shows up once in this whole article.

Which is a very roundabout way of describing what sounds quite a lot like a plug nozzle.

I'm pretty sure all of the old Rocketdyne aerospike/plug nozzle engine patents expired decades ago. OTOH if you describe them as such patenting the resulting design would likely be impossible due to prior art.

What's not clear (because nothing has been shown so far) is what is sitting between these nozzles. One of the cousins of the plug nozzle is a set of individual engines ringed around an "expansion surface" IIRC that allows them to generate an "open wake" at high altitude and a "closed wake" at low altitude (essentially how the nozzle does altitude compensation).

In some ways this the biggest issue the classic single-high-expansion-nozzle-engine as an S2.  While the Isp increase is substantial so are the flow separation side forces when you ignite it during landing.

The other options are having multiple engines, with most optimised for full high altitude operation and the rest for low altitude/landing and the expansion deflection nozzle which appears to have been very poorly tested in the landmark study on AC nozzles (by Wasco in 1968). A re-examination of the data and re-testing indicates that in fact ED can deliver high expansion ratio at ground level without flow separation (EG 100:1)

BTW it's important to remember that a)Combustion chambers are less prone (but not immune) to combustion instability as they get smaller and b) Turbo pumps get harder to design as they get smaller as the losses do not scale down with the dimensions (so as a proportion of the design thrust  they get bigger, which basically stuffed the MIT "Micro Rocket" project    )

Which suggests that if you want an easier life overall you'd want a small(ish) number of relatively large pumps driving several small chambers at once. The small chambers can also be easier to fabricate and you can do a lot of inital testing on a single chamber before freezing the design and making a full set. Which should also lower the required development budget.

With a bit of margin in all pumps a single chamber failure need not kill the whole stage (although logically you'd also want to shut down the chamber on the opposite side of the vehicle and increase thrust around the ring in a graduated way to maintain control authority for TVC.

So you end up with a package that is a combined structure/engine/heat shield that (properly designed) could have a lower mass than the combined mass of three separate subsystem masses.

Of course the devils in the details.......