Stratolaunch to fly in 2017?


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The competition heats up: In an interview Paul Allen has revealed that he hopes to begin flight tests of his gigantic Stratolaunch airplane, the largest ever built, later this year.

No word on the rocket that this air-launched system would launch, however. In fact, it appears that no one seems interested in providing one. This could change once the plane is flying.

4 comments

  • Gealon

    My area of engineering does not extend to aerospace, but I do understand structures under dynamic loads and I feel compelled to point out, my opinion of course, that the design of this plane is unsafe. To have three such heavy loads, (the left and right fuselages and the rocket) all supported by a single wing structure at the center of gravity of the entire airframe, introduces not only significant stress on the wing, but without bracing to prevent twisting of the fuselages relative to each other, this design is a recipe for loss of vehicle and crew. To expect the wing to be able to stand up to the torsion load of this multi-ton aircraft oscillating in flight (with one fuselage pitching up and the other down) is unrealistic. The aircraft would twist apart at either the inner wing roots or somewhere along the inner wing.

    There is an easy fix to stabilize any of these gyrations of the aircraft and that would be to connect the tail booms with a single airfoil, like for example on the P-38. Connecting the tails in this manner would afford the connecting structure leverage over the bulk of the airframes and allow it to prevent any of the twisting motions I foresee being the doom of this aircraft. The airfoil though would have to be raised above the attached rocket however, not because of the danger of rocket exhaust, as the engines do not fire until after release, but rather to give the foil a clean airstream in which to operate, which the rocket would disrupt.

    Observing the vehicle I can only imagine that this feature was not implemented so as to save on development time, as it would require a new vertical stabilizer/rudder designed to support the foil. This to me is surprising as it seems a fairly rudimentary thing to pick up on, this aircraft is big, heavy, will be under tremendous loads and is going to twist apart the moment one fuselage pitches up or down relative to the other. Why not build in some air frame stability? Instead what I’m seeing here is a push to keep as much of the existing aircraft as possible, ie, keeping all of the original aero-surfaces on the tails. Now I can’t say for certain if the tails were redesigned from the information I have at hand, I do see a square structure at the base of the vertical stabilizer that I’m pretty sure is not standard on a 747, but if they were redesigned, then why overlook such a glaring defect?

  • Dick Eagleson

    The Stratolaunch aircraft is a Burt Rutan design. It’s pretty much a very large evolution of his prior White Knight and White Knight 2 designs, both of which fly just fine. The mass of all three aircraft seems to be concentrated fairly close to the wing roots. The fuel tankage, I believe, is in the wings and both fuselages, but, in the latter case, doesn’t extent too far back or forward from the wing roots. None of Rutan’s rocket carrier aircraft designs would appear to be good candidates in which to go hurricane hunting, but in normal conditions, they show no tendency to twist apart in flight either. Carbon fiber composites are very light and can stand a lot of torsional stress. So much so, in fact, that many modern race cars use carbon fiber composite tubes as driveshafts.

    If you haven’t followed the history of this project, you can be forgiven for misunderstanding some things about it, especially given that the accompanying illustration is badly out of date and represents an early version of the design and not what the plane looks like now, though the two fuselages remain untied at both front and back. The plane does, indeed, incorporate componentry from two 747’s, but no elements of either donor plane’s airframe are incorporated except for engine nacelles and – possibly – engine pylons. I’m not certain about the latter. The original design of the twin fuselages made them appear, in profile, to be narrowed, slab-sided versions of the 747 fuselage’s shape. Even then, though, none of the donor plane’s airframe components were planned for repurposing. Subsequent refinement to the design, based on CFD and wind tunnel work, now have the twin fuselage profiles looking more like the CGI flying reptiles ridden by the nine ghost kings in the Lord of the Rings film trilogy than of anything else that comes readily to mind.

    Besides powerplants, the borrowings from the donor planes are mainly landing gear and one flight deck. The Stratolaunch airframe is entirely bespoke and constructed, as previously noted here, of carbon fiber composite, not metal.

  • pzatchok

    I do believe White night developed wing stress fractures to the point that it could not fly for a year until it was fixed.

    And even now they are not actually overworking it.

    Ever see a fully loaded B-52 take off?
    It looks like a gooney bird. Wings flapping and the fuselage going up and down. Its a fully metal wing and they still watch for stress fractures even after 50 years of proven flight time.

    Carbon fiber is nice but has a huge problem. When it fails it goes spectacularly.
    It also needs to be thinker to be just as stiff as metal of the same weight.

  • Edward

    Gealon,
    In undergrad classes, we learned to analyze stresses for homogenous materials. Homogeneity allowed us to ignore some terms in the general stress equation, greatly simplifying stress analyses.

    When I later took an introductory composites class, the first thing that the professor did was to reintroduce those extra terms, complicating the calculations and concepts. Cross-coupling stresses not only complicate the equations, but can add beneficial reactions to a design.

    Although I eventually did not do a lot of work with composite materials, I worked with an engineer who had used the cross coupling factor in helicopter blades. As the rotor increased in speed, the radial forces of the blades were used to induce a twist to part or all of each blade in order to reduce the angle of attack at the faster, outer parts of the blade. It worked nicely, but the military customer did not go with the design, because — as pzatchok pointed out — composites do not handle damage (e.g. anti aircraft attack) as well as the homogenous metals do. A hole shot into a metal blade is just a hole, but tests demonstrated that a hole in a composite blade can result in the blade ripping itself apart in a catastrophic way.

    Scaled Composites may be using this cross-coupling in order to add strength to the structure or to counter the twisting forces that have you, Gealon, so concerned.

    Although my preference in design is similar to yours, in that I would prefer to use the geometry of the design to assure structural strength and reliability, I would hate to second guess Scaled Composites, as they have a large amount of experience in the design of composite aircraft in general and some amount of experience in the similarly designed White Knight and White Knight Two. It could be that Scaled Composites believes that they save more weight and get sufficient strength through creative composite design than through using the rectangular frame geometry that you and I would prefer.

    pzatchok wrote: “Its a fully metal wing and they still watch for stress fractures even after 50 years of proven flight time.

    At this point, they are looking for fatigue fracturing in the B-52, rather than simple stress fracturing. I once had a window seat in a Boeing 707, and its wingtips also flapped throughout the entire flight. I would guess that designs of large jets, in those early days, did not include stiff wings.

    Stiffness of composite structures is less dependent upon thickness and weight, as metal structures are, but are more dependent upon the design of the layup. An advantage of composites is lighter weight for equivalent strength, but a disadvantage of less survivability due to small damages or construction flaws. A small flaw in the construction of the X-33 fuel tank resulted in delamination during test, which resulted in the demise of that project.

    Apparently, funding has similarities to composites in that it, too, is especially susceptible to failure.

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