Random video blogging

Modern manufacturing technologies enable strange shapes and could produce unconventional pumps. What is good for small scale rockets if turbines and centrifugal pumps have too much tip losses?

Rotary engines have vastly better power to weight ratios and vibrate less than piston engines. They’re also expensive and hard to build (cue modern manufacturing again). Here is one demonstrated in a paramotor, Parajet Cyclone using a Rotron engine. Love the music.

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The Turn-Boost-Glide-Back approach for first stages

In a patent by the famous Barnaby Wainfan. EDIT: corrected the link. This patent was filed in 2006 and granted in 2008.

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Automotive X-Prize Winners

Huh, it always takes a long time to find anything on web pages that are so cluttered up. Here. No idea what the MPGe or miles per gallon equivalent is.

EDIT:

Here’s ERA’s video (they didn’t win, although they were very close. I think they were penalized for driving too fast):

Holy crap. Their vehicle has 1000 Nm torque and does 0 to 100 km/h in 6 seconds.

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On Pressure Fed Rockets and Scaling

Well, scaling seems to be my pet issue. I recently wrote something not entirely well reasoned in a comment at Paul Breed’s. (For some reason Chrome complains about blogrolling.com malware there so continue if you’re sure you’re safe.)
So let’s make it better. (A word of caution though, I’m quite sleep deprived now.)
For those who like to jump into conclusions, it’s going to be like this all over again.

Assume a pressure fed rocket first stage has a certain propellant chemistry, tank and thus chamber pressure and must operate in the atmosphere, hence has a certain exit pressure (0.1 MPa or 1 bar or 15 psi is the optimal). Then it has certain thrust per nozzle area.

Now, the rocket needs thrust to lift off. If we assume a constantly scalable shape, its mass will be base area times length times density.

Since the maximum fittable nozzle exit plane also depends on the base area, we find that for a certain area, the rocket can only have so much mass – or that the rocket has a maximum density times length parameter. If we assume the propellants have been picked early on, density is set and the rocket only has a height constraint.  Each pressure and propellant chemistry basically has a “characteristic length” that can’t be exceeded. Otherwise it can’t lift off.

The higher the exhaust velocity, the smaller the nozzle, so raising chamber pressure reduces the needed nozzle size per thrust and the rocket can be lengthened.

For small rockets, I’d hunch that they have little length and thus they don’t really have to worry about this. They can be as thin (and thus long) as practical, to try to avoid drag losses.

For upper stages, the thrust to weight needed is less and the weight even less so it’s even less of a problem – except that the expansion ratios can be huge since there’s no back pressure anymore. Still, with small rockets, pretty huge expansions might be possible without having much problems because the second stage is very small (=also short) anyway and thus there’s little mass per nozzle exit plane area.

On really tall rockets like Saturn V, the thrust per base area has to be huge, hence it had to have those base extensions for the corner engines (note how the N-1 had a conical shape with a wider base, the engines had a bit higher pressure but the upper stages were kerosene – these cancel out a bit but the base of the rocket had some empty space) . Similarly with STS, putting so much thurst on the tiny orbiter’s tail required high chamber pressures and some tail shaping

I don’t have any numbers handy, but if we assume a 10 m tall 1000 kg/m^3 density (water) rocket, then it has 10,000 kg per m^2 or the thrust required for a 20 m/s² acceleration is 200,000 N/m^2. This is easily achievable. With an exhaust velocity of 2000 m/s, the mass flow needs to be 100 kg/(s*m²) to produce that thrust. Again with the exhaust velocity that mass flow means a density of 0.05 kg/m^3. Air’s density is 1.2 kg/m^3 at 300 K, so that’s 20 times less dense which means hotter, the density is like hot air at 6000 K. Though the molecules might be mostly lighter OH instead of N2 and O2, making that rocket exhaust at 3000 K for the density. Rocket exhaust isn’t that hot – it’s cooler and denser and thus more thrust per unit area.

For a second stage we can look at the pressure fed AJ-10 from Delta 2: 1.7 meters diameter (certainly constrained), 40 kN of thrust. For a T/W of 1, density of 1000 kg/m^3, we get 4 tonnes and 1.7 meters of depth. Quite a stubby stage with a roughly spherical tank! Isp is 321 s. The real Delta II second stage weighs 7 tons and the payload is some too, but reusable rockets won’t have such high performance first stages (nevermind solids!), so they might need more T/W.

Oh, BTW, I assume three stages to orbit for pressure feds though I haven’t looked it that closely.  Mass ratios and ISP:s I’ve only hunched.

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Humans Fast, Machines Slow

Rand Simberg talks about impedance matching. So I’d like to make a post of my comment there (I’ve always wondered why this obvious alternative gets mentioned so little…)

What to do when you arrive at Mars or Earth with your solar electric propelled vessel?

So, the problem with most low fuel demand velocity change schemes is that they only give slow accelerations. Low fuel high velocity change means solar or nuclear electric propulsion and aerocapture mainly.

High delta vee aerobraking is hard to do in one pass – it gets dangerous because of atmospheric variability and potentially other reasons.

Simple: detach a small capsule with the humans that goes directly to the surface (with only days of life support) and leave the untended craft to do multi-pass aerobraking. Hitting van Allen belts a few more times or taking a long time doesn’t matter that much with no humans onboard.

You could also potentially ultimately leave the long distance craft at some Lagrange point instead of LEO. (Cue some clever and complex maneuvers to save fuel – maneuvers that take long.)

Something similar could also be done when a long distance stack is assembled in LEO: send the humans there only after it’s through the belts. They can go with a smallish capsule again. Potentially at some Lagrange point, or in space without any fixed reference, just along the way. It could be dangerous though if the capsule doesn’t have much life support.

Many of these things have potential delta vee penalties as well as timing inflexibilities, but they could have enough other benefits that they should be considered.

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RD-180 Engine Diagram

RD-180 flow diagram

This is a bit different from the NK-33 done previously, but it’s still a full flow oxidizer rich staged combustion lox-kerosene engine.

It has no gears and no flexible shaft coupling between the pumps like the NK-33, making it a real one axis engine – except that it has separate booster pumps at the engine inlet. The fuel booster pump is powered by the fuel tapoff after one main pump stage and the oxidizer booster pump by the turbine exhaust. The starting is also different, but I omitted the starter hardware from the already complicated diagram, as it’s connected to many places – the main chamber, the gas generator and the first main fuel pump inlet. Also various valves, controllers and the tank heat exchanger are left out. And naturally I left out the other chamber and nozzle as well.

Both the RD-180 and the NK-33 have the same amount of pump stages – 3 for fuel entering the gas generator and 2 for fuel entering the main chamber and 2 for the oxidizer (all enters the gas generator).

Perhaps it can be thought, simplified, that the boost pumps are only hydraulically and not axis coupled to the main shaft system, and hence both can be better optimized to their environment (like lower rpm for the boost pumps) and hence the system can reach higher pressures than the NK-33, where the two oxidizer pump stages are on the same shaft. Or then it’s the later materials or more advanced pump design, after all the engines have some ten to twenty years between them.

Source for the drawing and explanation is this patent no. 6226980. Also, lpre.de has awesome pictures of the hardware, including the shaft with all the pump stages included. I assume it’s machined from one solid piece. Also pictures of the pipe stack injector / mixer and more diagrams of the engine operations. I don’t know much Russian (having finished half a course back somewhen), but if you know most of the cyrillic alphabet (helps if you know math as it’s very close to Greek), it’s practically quite easy to read as there are so many loan words – gasogenerator should not be a mystery to anyone. 🙂

The workhorse Soyuz RD-107 and RD-108 engines are completely different as they use a hydrogen peroxide gas generator design – very old-fashioned – but the RD-0124 used on the more modern Russian upper stages is the third interesting kerosene staged combustion engine that might become even more actual if Orbital are going to use it as a second stage engine on their Taurus II (currently they are moving on with solids). The fourth staged combustion engine is the RD-120 that’s bigger than RD-0124 and is used on the Zenit second stage. And then there’s the often overlooked forefather of staged combustion, Proton’s RD-253 / RD-275, that uses hypergolic propellants. The RD-0120 hydrogen engine of Buran / Energia is interesting as a comparison to the similar SSME. So there’s still plenty of study subjects in the Russian engine families.

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The US Air Force Tries To Do Reusables

But is not a real RLV program. It’s just a narrow test for one technology. Hence I think naming it Reusable Booster System Pathfinder is misleading.

Overspecification

They overspecify the problem by requiring a glide landing. Why is it superior to powered landing? At the moment, there’s no clear reason to believe it is! Both need to be developed further to understand their advantages and drawbacks. To my knowledge, there have been only six liquid rocket VTVL prototype manufacturers so far: McDonnell Douglas, JAXA (who was the contractor?), Armadillo Aerospace, Blue Origin, Masten Space Systems and Unreasonable Rocket. Only a few of those have flown to higher than a few hundred meters. The design and operations space is mostly totally unexplored.

Nevermind the large number of other alternatives to boostback. Jon Goff had a recent “lecture series” about these.

I understand that this is just one program, but this should not gain the status of the reusables approach of the air force – stuff like that easily happens.

Master Design Fallacy

They also discard evolution and competition – instead just requiring a single masterfully designed prototype before something operational. Sure, this is much better than starting a multi-billion dollar program without a first lower cost prototype, but nevertheless, it sucks. Somebody brief them on newspace! Rand Simberg, Monte Davis, Jonathan Goff, Clark Lindsey, or one of the numerous people who get it. Or one of the prominent company leaders: John Carmack, Jeff Greason, David Masten.

An Ideal Program

Just specify some boost delta vee points and let companies demonstrate progress towards that. A popup tailflame lander would perhaps give more vertical velocity while some good glider or even a booster that has engines for cruising back could boost far down range to give lots of horizontal velocity. There ain’t a clear winner – there might not even be and multiple approaches would have their uses.

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The Martin Jetpack and Its Design Tradeoffs



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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NASA’s Rejuvenation

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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A Small Step for Rocketry

“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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