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Here’s the technical information. It uses a piston engine to spin two smallish propellers. At first look it seems inefficient: the propellers are so small (diameter 1.7 ft so radius is 0.26 m) so the air mass flow is low and hence the velocity of the jet must be high, meaning high power needs for the thrust. (Mind you, it’s efficient compared to a peroxide rocket or a low bypass turbofan engine that have been used for jetpacks in the past.)

But let’s look closer. Engine specs. It’s a two stroke 2.0 liter V4 that produces about 150 HP at 6000 RPM and it weighs 60 kg. The quite high RPM means it produces quite high power.

If the engine spun at a lower speed, you could use bigger props, but the engine would produce less power.

If it spun at a higher speed, you could have higher power for little mass growth in the engine and then use a gearbox to still use the same size or even bigger props (if you geared it down further). But gearboxes are heavy, expensive and often unreliable.

6000 rpm is 100 Hz (rpm is a totally weird unit for spin rate anyway, why is it always used?). Speed of sound is 320 m/s. Hence at 100 Hz the supersonic radius would be 0.5 meters (100 1/s * 2 * 3.14 * 0.5 m  = 307 m/s). At the Martin Jetpack’s 0.25 m blade radius it’s about half the speed of sound. At the 7058 max RPM it’s 180 m/s or about 60% of Mach 1 – the transonic region should be easily avoided. Maybe they could be even slightly bigger.

It’s a compromise design. With the small props you can use relatively high engine speeds so your engine stays light – and you avoid a complex expensive failure-prone gearbox. The machine also stays safe as there is no free flailing propeller. With a larger propeller (or two) you would have to give up the shroud(s) since their weight would be prohibitive. On the other hand, fuel consumption would go down and hence the range could increase. It’s a fascinating design space.

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I do appreciate that the model is so different from Apollo that it takes time and thought to understand what it is about; I did not see it at first myself — but once I got past my preconceptions, I found the logic of this approach overwhelming. This is simply what exploration looks like in a world where the budget doesn’t double for a few years and then halve again. You build a piece at a time and as soon as you can start doing things with the pieces, you do so.

Jeff Greason about the Flexible Path, commenting on Rand Simberg’s superiorly excellent Popular Mechanics piece.

Rand Simberg in the article:

I would claim that in fact, this is the most visionary space policy that the nation has ever had, including Apollo. It finally, forthrightly declares a national goal of large numbers of humanity living off planet, with many of them going on excursions into the solar system, and it harnesses the vital element of private enterprise and competition to make it happen in a way that will drive costs down instead of up.

May I add that yours truly proposed something of a flexible path of his own in 2006, though only for launching.

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“Rockets are special” is an interchangeable meme with “rockets are expensive”. Well, hopefully they get less special. Armadillo’s been making progress, from the latest update:

For the very first time, a complete system was operated without the presence of any of the manufacturer’s representation on site. This may seem like a small thing in light of the fact we all knew it would eventually come, but getting there is always a good thing.

Some day launch vehicles will be Refuel And Go Again.

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Or what you are going to call it, an unrealized proposal from Aerojet around 1984. PDF Found on NTRS.

The idea was to have two turbopumps (like on SSME), but instead operate on the expander cycle. Two heat exchangers, two turbines, two pumps. One for each propellant.

 

aerojet_cycle

Both propellants go through a heat exchanger and an expander driving a pump

 

This is a LOX-hydrogen engine. Also this means that since there is the same propellant on both sides of the axle, in the turbine and in the pump, no elaborate seals are needed. Original intent for these engines was for in-space reusable stuff, that needs to be operated many times and for a long time without maintenance. Size was in the RL10 class, about 70 kN. (RL10 has grown though.)

aerojet_margin
Simplicity and margin were claimed

Think for example if you let a fired turbopump sit in space for a long time. Will some fuel leak to the oxidizer side through the seals? This could avoid that. (You can use helium purges too though but then you’ve got one more fluids you need to tank.)

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Sandman

Armadillo flying to 600+ meters with a "mod". I say, it looks like the East German Sandman!

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It seems to be such a marvelous rocket engine. A high thrust to weight, high ISP high tolerance design that is still top of the line after sitting forty years in storage – it will be used for Orbital’s Taurus II as a first stage engine.

I drew a diagram on how the engine works internally.

NK-33 Simplified Flow Diagram

NK-33 Simplified Flow Diagram

It’s a full flow oxygen rich staged combustion design. Also note that there’s a separate geared booster pump to raise the pressure of the small fraction of kerosene going to the gas generator. This is most likely because the turbine lowers the pressure, hence the gas generator has to be at a much higher pressure than the chamber.

In the modern launcher business, Russia probably still has the best expertise on first stage engines.

With a modest chamber pressure, the full flow oxygen rich gas generator gas wouldn’t need even to be that hot. I don’t know why specifically these kinds of engines have not been tried more often. Maybe SpaceX can do something, they’re one of the few companies designing new engines anymore. This is the logical path after gas generator engines. Remember that perhaps the best known GG engine, F-1, originated from around 1958… It could be seen as an anachronism that the US companies are still sticking to gas generator cycle engines.

With modern day machining capabilities and turbine / pump computer optimization, one should be able to do quite a lot relatively easily.

Source for the data that I made the graphic with: Liquid Propellant Rocket Engines, lpre.de, in Russian.

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Masten

Made huge strides in the last few days. A half-L1 done. They might be able to compete with Armadillo on L2, though I’m somewhat skeptical since they’re only going to assemble the new L2 vehicle soon.

Suddenly looks like there are two viable big VTVL sounding rocket companies!

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