Sankrithi Channels Wallis

Most people with interest in aerospace history know of Barnes Wallis’ geodetic structures, most famously used on the Vickers Wellington. He first invented them for the R100 airship, basically a weave of thin aluminum shapes going in different directions, forming a grid. No bulkheads or even wing spars needed, but it was quite complicated to build. Fabric was used as a cover, first linen and later thin steel wire mesh. It became outdated when airplanes were pressurized and moved to aluminium monocoque structures.

Well, now, airplane manufacturing technologies are changing again, after some 70 years of riveted aluminium sheets, bulkheads and spars. Composites laid by robots enable fancy shapes, and optimizing the strenght carefully in various directions. Boeing has been looking at a 737 replacement. So far, pressure hulls have had to be cylinder or ball sections – only that way the thin skin can be in pure tension, the only force it can really resist. Often a double bubble has been used, with the cabin floor dividing the lower and upper half into two circular sections and also keeping the left and right side together at the same time. A circular frame is not very space efficient for humans though, so Sankrithi et al at Boeing figured out how to put fibers in a grid to enable a roughly elliptical shape that is wider than it is tall. The advantages are not entirely clear to me from their description, since they seem to say the ratio of seats per circumference is similar to a circular frame, but it is nevertheless interesting. (Also, it’s strange that Free patents online shows that the patent was filed in 2009, yet Google shows it was filed in 2005, yet they seem the same at first glance).

This has some relation to the X-33 where they also attempted non-cylindrical composite pressure vessels. The technology has advanced since though.

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Experts on the Internet, X-33, Gigaprograms and Real Progress

Experts on the Internet

A lot of internet discussion is ignorant speculation, rumor spreading, ranting and flaming. But that’s not all. The freedom and self-organizing nature of enables massive diversity. Newsgroups, mailing lists, IRC, forums, Twitter – and sometimes there’s something there.

Michael Tobis comments on his experience of reading about the Iranian riots on Twitter – way before anything was said in the printing press. Being his usual self, it acts as a motivation for a longer article about the resignation of reporting on important issues (global warming changing earth significantly being Michael’s issue because of his expertise in that).

I’ve long been saying related things  in relation to space issues.  Now, the traditional media defends its views and sheepish forwarding of NASA public facade material as the right way. Maybe some examples are necessary. The aerospace developments of national agencies are full of failures. All ventures have failures. It’s just that aerospace has so few successes – especially rocketry.

What if Nasaspaceflight.com had existed during X-33? NASP is moot here since it was a secretive military project – hence no insight possible there.

Would X-33:s failures and their reasons have been predicted much earlier? Ares I and V had their critics from before day one. Technical critics. Budgetary. Industrial ones.

What is important and makes things different from mere ranting, or “armchair generals”, is that the NASA and ULA engineers provide, on their free time, insight into engineering matters. Instead of the public affairs that the rest of the media reports on. They have a passion for what they do and want to succeed and advance. If they see hopeless technical incompetence at the top level, they will voice their objections – it is practically their duty as citizens.

X-33 – Marching Towards Certain Failure

X-33:s first failure was trying to use very unproven technology (composite multi-lobed cryogenic tanks) in a billion dollar magnitude program. The technology could have easily been proven on a much smaller scale, very cheaply and fast, before starting the whole X-33 project. Competent engineers should have seen that one as a real high risk with easy reduction possibilities. You don’t risk billions just for fun, if you can easily avoid it! You risk it for politics though.

The table above is from NASA’s tank report (pdf in references), with tests done on tank samples done after the failure, revealing the gross inadequacy of the material for the intended purpose

If, on the other hand, the composite tank was seen as a high risk but not necessary technology for reaching X-33:s goals, then X-33 should have proceeded with the metal tank. In other words, if the composite tank was an optional “nice to have” component. But NASA:s Ivan Bekey testified otherwise – that X-33 had no use without the carbon fiber tank.

All around the X-33 seemed quite big and hugely ambitious on multiple fronts for an experimental vehicle anyway. What were the other objectives besides composite tanks? Could they have been tested in a faster and less expensive vehicle? The metal TPS comes to mind as one. Did it have even the inadequate bench background of the tank? There were military programs from the fifties to the eighties that had developed such things in labs – maybe there was something there.

What about the lifting body shape? The successing Venturestar kept changing shape constantly in simulations and grew big wings. It could very well be that Lockheed Martin and NASA simply didn’t know what they were doing, on any level really, and should not have started building X-33 in the first place. The knowledge base was not at the level to justify going that far yet. The close to existing J-2 derived aerospike engine was perhaps the biggest justification for the size and shape of X-33. But the potential reward of finally getting an aerospike engine flight tested just made the fall that much heavier – the large vehicle necessitated by this turned out to be unworkable. A failure on a lesser scale would not have been as hard. Close to ten years later, no aerospike has yet flown. There have been spike nozzles in hybrids and solids but no aerospikes, where the physical spike is cut off and replaced by a gas jet.

What should have been done to enable the X-33 building?

• Bench tests of composite tanks (basic, room temp, progressing to multi-lobe, cryogenic). Test cryopumping as well (this has been done somewhat since).
• Possibly aerodynamic tests with a much smaller vehicle (or generations) as a glider, first released from a helicopter, then an airplane and finally with a sounding rocket. Alternatively with conventional engines. Possibly horizontal takeoff to reduce test costs.
• Aerospike engine small scale tests. Perhaps contract a smaller company for that, like Armadillo and XCOR have done tests cheaply for NASA methane engines.

If any of these solutions proved unfeasible, then no reason to build the Lockeed style X-33.

The Competitors

Rockwell had a shuttle shaped cylindrical tank vehicle with wings, which seemed pretty simple on the outside. McD had the DC-X growth model. At least both had some heritage in working hardware. There is very little engineering information available about the competitors so if anyone wants to help, drop me a note. Would they have succeeded?

Probably both would have failed as well, in the role of traditional X vehicles of developing new capabilities, mainly because of being too large. Both of the other potential X-33:s would have had a composite hydrogen tank as well (though possibly axisymmetric, even conical or cylindrical), so they could have had similar failure possibilities, though perhaps they would have had a different (sensible) development approach. As is evident from lab tests in the references, cryogenics and composites are hard to fit together.

The Shuttle thermal protection system  is notoriously work intensive, and as far as I know, the Rockwell proposal had quite similar tiles in its proposal. On the other hand, surface loading could have been less since the vehicle had its own tanks and high mass ratio. Also the SSME:s are very work intensive when reused. It was partly more of a rehash of existing technologies, which would perhaps have had moderate chance of success. If it worked, maybe one could try different technologies in it, if it was cheap to fly and could do incremental envelope expansion, while still having high enough performance to really stress test things like TPS or vacuum test less maintenance intensive engines. Heat loads on the composite structure would have been an interesting problem area as well.

McD’s precursor for their X-33 design, the small flying DC-XA program was cut prematurely (after having survived agency changes and funding problems) after a crash from a trivial easily avoidable failure, an unsecured hose. It could have made sense to do DC-XA again, to try the high speed properties, flying at different angles of attack and test the turnaround maneuver that it should perform after re-entry for landing. It would also have made sense to keep in the DC-XA scale and try lots of other solutions in the same vehicle (or fleet). It’s cheaper to test when at small scale. Only when the low capabilities of the vehicle would have been exhausted and good enough solutions found, would it have made sense to move to a bigger vehicle.

Conclusions

All  in all, space is no different from other fields, that rationality is the most effective way to reach sustained progress. It is obvious to any engineer worth their salt that one should retire as much risk as possible, as cheaply and as fast as possible before moving to the big bucks and long development time game.

Sadly, aerospace seems like a hopelessly irrational field in this regard. There are historical reasons for that attitude. Crash programs like Apollo or military ones have left their mark too deep – the field is unable to grow to a rational mature one. It is evident when looking at NASA’s troubled history with manned spaceflight. Since Mercury, Gemini and Apollo, it has not been able to build much incremental progress. STS was a partial success in capability – but it has stifled progress. Everything must always be started over, and at giant scale – making the unavoidable multiple tries very costly, both in time and money, and even utterly shameful in case of failures. A gigaprogram with failure as no option is a recipe, not for sustained progress, but for either a great disaster, or stagnation. A gigaprogram with failure inevitable is waste incredible.

So, the media of today should examine the world in such a perspective. Simplistic “against NASA / for NASA” analysis serves no one. There have been such incredibly farces lately that I’ve had to double check I wasn’t reading the Onion.

I speak for many, when I say, we don’t want delusional Programs, we want rational Progress!

Some sources:

1 Final Report of the X-33 Liquid Hydrogen Tank Test Investigation Team, NASA Marshall

2 Cryopumping in Cryogenic insulations for a Reusable Launch Vehicle, Johnson et al., NASA Langley

3 Proceedings of the RAND Project AIR FORCE Workshop on Transatmospheric Vehicles, Chapter 3: Design Option and Issues, containing X-33 general overview and info about the competitors, Gonzales et al, RAND Corporation

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Aircraft Structure Woes

I’ve been reading quite a lot about aircraft design and structures. Interesting contrast with rocketry. I’ve also been toying around with a few possible home designs and builds. I might write a more proper history / introduction of that someday.

The following is a very much simplified history of aircraft structures. By no means 100% correct, but should give you the general picture.

Development History

Basically, aircraft started with a wood frame, wood ribs and fabric covering. Later during the first world war, there was at times some plywood covering at places, later there were often steel tube frames (in the fuselage), aluminium ribs and skin and sometimes corrugated stiff skin (Junkers). Then, between the wars the planes got ever faster, and the fabric couldn’t hold up anymore. Duraluminium panels were used instead (still with wood interiors), then aluminium structures and stringers and finally just before the second world war, a semi-monocoque structure where the aluminium skin held a significant portion of the loads (besides providing the aerodynamic shape), resulting in a good weight reduction. (Boats had used the semimonocoque structure.) DC-3 and Spitfire are good examples. There were some oddities in the late war, like De Havilland who had the Mosquito and Vampire which used plywood skin with balsa core as a composite. Also, Vickers made bombers had geodesic grid construction all around.

Then, a long time passed until again in the late sixties and early seventies people started experimenting with something new, fiberglass. Again from the boat building industry. There are a few alternatives on exact fiberglass construction: to use a mold, or then use a foam core on which to laminate the fibers that then stayed inside the final aircraft part. Ken Rand built one of the first composite planes in foam core and Burt Rutan later got famous with it.

Current Status

Sailplanes went totally composite decades ago as the performance is vastly superior to aluminium. From there the technology has been slowly seeping to the rest of the aviation.

Nowadays all big commercial aircraft (over 6 passengers or so) are aluminium semi-monocoque construction. The skins and stringers are joined with rivets. The technology is about 70 years old. There is an increase in composite parts, where light weight and stiffness are required. For example a Finnish factory manufactures some spoilers for Airbus. Glass fiber or carbon fiber in an epoxy matrix, all made in a mold in a high pressure and temperature autoclave.

There is more variation at the very light end. The cheapest hang gliders are usually aluminium tubes for structure and dacron fabric for the airfoil. More expensive ones have carbon fiber parts and sometimes some foam, still fabric covered. Ultralight trikes have an aluminium or steel tube chassis attached to a big hang glider. Sometimes with some fiberglass fairings to shield the pilot from wind.

Ultralight aircraft beyond trikes (the requirements for an ultralight are weight under some 450 kg and a low stall speed below about 70 km/h over here) are a various bunch. Ranging from tube-fabric to wood-fabric, fiber-fabric to totally fiber made (with foam core or molded). A few completely Al craft also exist.

Light aviation is mostly either totally aluminium (Cessna, Piper etc), or totally molded composite (Diamond, Grob etc). There are homebuilt experimentals that don’t fit the ultralight category since they are too big or fast. Some of them use foam core, like Rutan’s design Long-Ez, and its derivatives like Cozy.

Ecological Niches

There are reasons why different structures in homebuilt aircraft exist.

Glass fiber is good for making arbitrary smooth and stiff shapes, lending itself very well to highly efficient aerodynamic shapes – that’s why it’s used in sailplanes and efficient small travel planes, where small drag is important. Some kits have lots of premolded skin parts. Some only have drawings, and you cut your own foam, and laminate by hand on top of it. Laminating is messy by hand unless you use the modern vacuum bag technique. Complex composite structures might also develop cracks that are not visible on the surface, making them dangerous. Examples: KR-2, Long-Ez, Cozy.

Aluminium on the other hand is lasting, rugged, easily inspected and can take the weather. It’s used for some bush planes which have STOL capability and where absolute aerodynamic cruise efficiency is not so important. Unfortunately, there are a huge amount of cut, bent and riveted parts in even a small ultralight aircraft, and build times are many thousands of hours taking many years from start to first flight. Examples: Zenair CH701 STOL and its numerous copies.

Tube and fabric is especially good when you want to dismantle your craft for transportation. That is true for hang gliders and many ultralights. It is also possibly quite easy to construct. There is a French bolted aluminium tube design (wrapped with dacron fabric) currently manufactured in Ukraine which requires no welding at all and should be very quick to build, the Skyranger. Rans Coyote is one too.

Welded steel tube is something that requires a builder who is a good welder, and the complicated structure needs lots of cuts and welds, which makes it time consuming.

Wood – it is a mixed bag. It has good strength to weight ratio, comparable to aluminium or steel, but there are moisture problems and the issue that the raw material is by its nature always somewhat uneven. Solutions like plywood try to get around this: if one ply has a weaker spot, it doesn’t weaken the whole structure much. The material could be potentially cheap, though it depends on where you are. Wooden rib construction is quite complicated and time consuming, requires lots of sawing, cutting and gluing and the number of ribs and reinforcements is huge. Examples: Mag-01, Junqua Ibis.

My Own Ideas

I’ve dropped a few hints on the way as to what could be my preferred approach, if I were to build my own airplane. But I’ll handle more of that in a later post, as I’m now in such a hurry and gotta go. You can learn a surprising amount when researching the history – airplanes are such a common subject of dreams and experimentation that a great many things have been tried.

p.s. Changing the blog title picture now.

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