Why doesn't the Scaled Composites Stratolaunch have Swept Wings?

[shynung]

21 hours ago, Northstar1989 said:

Quit it Shymung.

No way. This is more fun than my actual job.

Oh, and it's Shynung.

21 hours ago, Northstar1989 said:

I never said anything, anything at all about the reason for the extra speed being to decrease the speed increase needed to attain orbit.  I have in fact repeatedly said just the opposite- that the reason for it would be to allow the Roc to climb to a higher *altitude* before releasing its payload, and no other reason.

How much higher will that extra Mach .2 allow the launching aircraft fly? Wiki says the SC design can go up to 35K feet.

More importantly, does this extra altitude a higher-speed design allows justify the additional R&D costs? Remember, they're stuck with the 747's original wing mounts; building a swept wing is likely to mean a redesign of the mounts, which include gutting the structural parts of the original fuselage. If they are willing to do that, they might as well scrap the 747 they bought and built new fuselages from scratch.

Also, as everybody else pointed out, this is Burt Rutan's company we're talking about. They have experience in building high-endurance subsonic aircraft. Wouldn't building a swept wing design to squeeze out some more speed be outside their current specialty? Sure, they can do that, but that'd meant they have to throw away almost all their experience and learn entirely new things.

Edited February 17, 2017 by shynung

[magnemoe]

3 hours ago, shynung said:

How much higher will that extra Mach .2 allow the launching aircraft fly? Wiki says the SC design can go up to 35K feet.

More importantly, does this extra altitude a higher-speed design allows justify the additional R&D costs? Remember, they're stuck with the 747's original wing mounts; building a swept wing is likely to mean a redesign of the mounts, which include gutting the structural parts of the original fuselage. If they are willing to do that, they might as well scrap the 747 they bought and built new fuselages from scratch.

Also, as everybody else pointed out, this is Burt Rutan's company we're talking about. They have experience in building high-endurance subsonic aircraft. Wouldn't building a swept wing design to squeeze out some more speed be outside their current specialty? Sure, they can do that, but that'd meant they have to throw away almost all their experience and learn entirely new things.

747 tend to have swept wings then I fly with them
Else I agree with you speed is not primary here. 

[mikegarrison]

Despite all the fanciful reasons that Northstar has dreamed up, wings are swept for basically only one reason. Speed. Sweeping wings delays Mach buffet as the plane approaches transonic speed. And for supersonic airplanes, it puts the wings inside the Mach cone.

So if your airplane is not intended to fly at transonic or supersonic speeds, you don't sweep the wing. That's the answer Northstar just refuses to accept.

[Northstar1989]

12 hours ago, mikegarrison said:

Despite all the fanciful reasons that Northstar has dreamed up, wings are swept for basically only one reason. Speed. Sweeping wings delays Mach buffet as the plane approaches transonic speed. And for supersonic airplanes, it puts the wings inside the Mach cone.

So if your airplane is not intended to fly at transonic or supersonic speeds, you don't sweep the wing. That's the answer Northstar just refuses to accept.

Nobody knows how fast this plane is designed to fly, so your criticisms are baseless.  <snip>

 

Regardless of the speed the plane is designed to fly at, it's designed for altitude and payload, not fuel-efficiency, so one also wonders why they don't increase the wing chord (which would hurt subsonic Lift:Drag ratio but increase total Lift) and add the remaining 2 engines they have to the design.

The 747's actually *FLEW* to the Mojave location, so presumably their engines were all functional- and if they added additional engines and wing chord, they could attain additional surplus power at the same max speed and altitude- meaning either a higher climb-rate and altitude-ceiling, or a higher payload-capacity (or alternatively, more generous saftey and manufacturing margins, and a cheaper, more rugged/reliable airframe), in all cases at the expense of range...

 

Regards,

Northstar

Edited February 17, 2017 by Frybert

[Green Baron]

Nonsense.

if the profile is not completely messed up straight wings are more efficient than swept ones. See top gliders e.g. Swept wings need helpers like flaps to keep a plane flying at low speed.

From the website:

"The Stratolaunch Carrier Aircraft is constructed using state-of-the-art composite materials in order to be light, strong, and fuel efficient. Once complete, Stratolaunch will have the largest wing span of any airplane ever built and will be the world's largest composite aircraft."

underline mine

I mean, that is exactly what it looks like, isn't it ?

Edited February 17, 2017 by Green Baron

[DerekL1963]

1 hour ago, Northstar1989 said:

Nobody knows how fast this plane is designed to fly, so your criticisms are baseless.

Says the guy who has spent this entire thread insisting the aircraft just has to fly fast - despite ever accumulating evidence otherwise.  And that's the thing, we can make educated guesses as to the speed range it's meant to operate in based on available information.
 

1 hour ago, Northstar1989 said:

Regardless of the speed the plane is designed to fly at, it's designed for altitude and payload, not fuel-efficiency, so one also wonders why they don't increase the wing chord (which would hurt subsonic Lift:Drag ratio but increase total Lift) and add the remaining 2 engines they have to the design.

In addition to the information posted by Green Baron, it stands to reason that some degree of fuel efficiency is factored into the design.  It does have to ferry, and it does have to be able to reach launch sites well off the coast - while carrying a significant (and draggy) payload...  and fuel is going to be a major part of their operational costs.  That also works into understanding why they don't further increase drag, compounding their existing problem.

[Northstar1989]

23 hours ago, DerekL1963 said:

In addition to the information posted by Green Baron, it stands to reason that some degree of fuel efficiency is factored into the design.  It does have to ferry, and it does have to be able to reach launch sites well off the coast - while carrying a significant (and draggy) payload...  and fuel is going to be a major part of their operational costs.  That also works into understanding why they don't further increase drag, compounding their existing problem.

The fact that it's already carrying a draggy payload reduces the penalty of the wing chord.  Since the Lift:Drag ratio of the plane as a whole is determined by adding up a weighted average of ALL the components of the design, having an already low average means that there is less of a penalty for lowering the Lift:Drag ratio of the main wing.  Indeed, by increasing the absolute magnitude of the wing's Lift and Drag doing so may actually INCREASE the plane's overall Lift:Drag ratio (similar to this problem- which is greater: "9+4+1+2 / 4" or "7+7+4+1+2 / 5" ?)

What adding extra engines and wing chord actually does is increase excess power at a given altitude and speed.  This power can then be divided up among a number of competing priorities, including payload, structure (safety/manufacturing margins), altitude-ceiling and climb rate (basically, not adding any extra mass and instead letting the plane climb higher), or fuel-capacity.  And it's possible to split the difference between one or more of these, for instance adding a bit of fuel and a bit of payload, or reinforcing the structure a bit but not enough to prevent an overall increase in altitude-ceiling...

The Stratolaunch 351 design has, according to information provided by its designers to PopularMechanics, an intended round-trip range of 1500 miles (that is, it can fly 1500 miles out to sea, release its payload, and then fly/glide without the rocket to a runway 1500 miles back).  Which is already quite impressive, considering the size/weight if its payload- and, I would argue, a bit higher than necessary.

It should certainly be possible to trade off some of that range capability for extra payload capability or better safety margins (low safety and manufacturing margins are one of the primary drivers of aircraft construction and maintenance costs, after all.  When the aircraft parts have to be built to very high precision for the design as a whole to function, this adds a lot to the costs if production and requires more frequent maintenancw to maintain the aircraft within those narrow limits...) through an increase in the number of engines from 6 to 8 and an increase in the wing-area (mostly through wing-chord, as I assume they've already reached the practical/material limits for wingspan) as well.  It would drive up fuel-costs to be sure, but in anything to do with space exploration fuel is only a tiny fraction of the overall cost of a mission...

Finally, note that payload, range, and altitude-ceiling are, to some degree, interchangeable.  That is, the Stratolaunch 351 could launch with a heavier payload on some days and climb to a lower altitude, and on others launch with a much lighter payload but launch it from a substantially higher altitude or further downrange.  Thus, the extra power that adding engines and wing-area provides should in almost all cases be beneficial, and allow the Roc to carry a wider range of payload masses while benefitting all payloads...

 

Regards,

Northstar

Edited February 18, 2017 by Northstar1989

[Nibb31]

1 hour ago, Northstar1989 said:

Thus, the extra power that adding engines and wing-area provides should in almost all cases be beneficial, and allow the Roc to carry a wider range of payload masses while benefitting all payloads...

So why did they design it the way they did instead of the way you suggest? 

[Northstar1989]

2 hours ago, Nibb31 said:

So why did they design it the way they did instead of the way you suggest? 

That's a good question- and why I started this thread: to try and figure out why Scaled Composites designed the Roc the way they did...

The more I look at it, the more I am starting to think they just took earlier designs like that of the White Knight carrier plane- and just scaled them up.

Otherwise, if they were going for an efficient airframe layout, why not connect the tailplanes together P-38 style, for instance?  A simple comparison of it and the P-38 will show those tail booms are just BEGGING to be connected.  Doing so would also allow them to REDUCE the chord of their main wing a bit while maintaining the same overall wing-area: leading to a higher Aspect Ratio and thus more Lift for the same Drag- as well as the structural strength benefits doing this would convey...

Stratolaunch 351:

https://goo.gl/images/rwK0St

P-38 "Lightning"

https://goo.gl/images/OxSSXV

 

Let's also not forget that moving more of the wing to the rear like that would also make the Stratolaunch 351 more inherently aerodynamically stable- and provide a large area to attach elevators to the rear edge of the tailplane...

 

Regards,

Northstar

Edited February 19, 2017 by Northstar1989

[wumpus]

48 minutes ago, Northstar1989 said:

Otherwise, if they were going for an efficient airframe layout, why not connect the tailplanes together P-38 style, for instance?  A simple comparison of it and the P-38 will show those tail booms are just BEGGING to be connected.  Doing so would also allow them to REDUCE the chord of their main wing a bit while maintaining the same overall wing-area: leading to a higher Aspect Ratio and thus more Lift for the same Drag- as well as the structural strength benefits doing this would convey...

I'd guess it has to do with trying to maintain rigidity or avoiding harmonic vibrations to travel around the loop.  I'm not certain how much was understood about the P-38 when it was designed, I know it had issues related to transonic flight that were completely unknown until supersonic flight was more understood.  I'd hardly assume that everything about that plane was "better".  Of course, this is more a wild guess (I have almost no training (formal or otherwise) in structures and just ksp-level understanding of aerodynamics).

I think the main point is that if you want more lift (and higher ceiling), increasing speed increases drag by the square of velocity, while increasing wingspan increases drag linearly.  Not only that, but the paired fuselages work to help extend wingspan, as the White Knight and Voyager show.  I'm guessing that White Knight simply didn't need that much tail (and extending it wouldn't make sense), and I wouldn't be too surprised if the entire length of the Stratolaunch plane is shorter than the distance between the fuselages.

[Northstar1989]

1 hour ago, wumpus said:

I'd guess it has to do with trying to maintain rigidity or avoiding harmonic vibrations to travel around the loop.  I'm not certain how much was understood about the P-38 when it was designed, I know it had issues related to transonic flight that were completely unknown until supersonic flight was more understood.  I'd hardly assume that everything about that plane was "better".  Of course, this is more a wild guess (I have almost no training (formal or otherwise) in structures and just ksp-level understanding of aerodynamics).

The Stratolaunch doesn't fly supersonic, not anywhere close- in fact the entire past 3 pages of discussion have been largely about how fast it DOES fly, because that determines whether the aerodynamic advantage from swept wings is worth the structural stability issues they produce...

The P-38 flew beautifully in subsonic regimes- so much so, in fact, that it was probably the most deadly mainstream  (non-experimental, so early jets don't count) fighter in WW2.  The top 3 American "Ace" pilots in WW2 all flew P-38's.

Aerodynamics don't miraculously just change because a few decades have passed- and there is no reason to think connecting the tailplanes together on the Stratolaunch 351 wouldn't provide the same structural and aerodynamic stability benefits that doing so provided the P-38, as well as improved Lift:Drag ratio by raising the average Aspect Ratio of the plane's wings...

Unlike the main wings, the tailplane wing sections would be relatively short, and so don't face the same limitations from the structural mass needing to increase so much along the rest of the wing length (to counteract bending) as to make extra wingspan no longer beneficial.  And if increasing the tailplane wingspan and connecting the tailplanes together made the Stratolaunch 351 *too stable* they could always move the main wings back a little to compensate (which would also have the useful side-benefits of reducing the stresses on the fuselages, and reducing the shift in Center of Gravity when the payload is released...)

Once again, here are the images of the Roc and P-38 so everybody can see and try to visualize what I'm talking about...

Stratolaunch 351:

https://goo.gl/images/rwK0St

P-38 "Lightning"

https://goo.gl/images/OxSSXV

 

Regards,

Northstar

Edited February 19, 2017 by Northstar1989

[Bill Phil]

14 minutes ago, Northstar1989 said:

The Stratolaunch doesn't fly supersonic, not anywhere close- in fact the entire past 3 pages of discussion have been largely about how fast it DOES fly, because that determines whether the aerodynamic advantage from swept wings is worth the structural stability issues they produce...

The P-38 flew beautifully in subsonic regimes- so much so, in fact, that it was probably the most deadly mainstream  (non-experimental, so early jets don't count) fighter in WW2.  The top 3 American "Ace" pilots in WW2 all flew P-38's.

Aerodynamics don't miraculously just change because a few decades have passed- and there is no reason to think connecting the tailplanes together on the Stratolaunch 351 wouldn't provide the same structural and aerodynamic stability benefits that doing so provided the P-38, as well as improved Lift:Drag ratio by raising the average Aspect Ratio of the plane's wings...

Unlike the main wings, the tailplane wing sections would be relatively short, and so don't face the same limitations from the structural mass needing to increase so much along the rest of the wing length (to counteract bending) as to make extra wingspan no longer beneficial.  And if increasing the tailplane wingspan and connecting the tailplanes together made the Stratolaunch 351 *too stable* they could always move the main wings back a little to compensate (which would also have the useful side-benefits of reducing the stresses on the fuselages, and reducing the shift in Center of Gravity when the payload is released...)

Once again, here are the images of the Roc and P-38 so everybody can see and try to visualize what I'm talking about...

Stratolaunch 351:

https://goo.gl/images/rwK0St

P-38 "Lightning"

https://goo.gl/images/OxSSXV

 

Regards,

Northstar

The P-38 actually flew relatively poorly in transonic regimes. This usually only occurred when diving from very high altitude at a steep angle. As did most aircraft of the period. This aircraft doesn't need to go fast, either.

As to its deadliness... The top aces of the entire war were German pilots, with hundreds of confirmed kills.

The design requirements for the two planes are inherently different. The P-38 only had twin booms because it was Lockheed's answer to a fighter competition for a twin-boom aircraft. They may have connected the tail-plane for more area to add more control authority or balance out the forces acting on the craft. The people designing this don't need to connect the tailplane, because control authority is a minor issue. (it's not a fighter).

But even if the reason was structural for the Lightning, there is something major that has changed since the late 30s/40s. Materials. We have better materials. Stronger materials. The structure is stronger by just using better materials (usually...).

Remember, these engineers are very likely to be smarter than most of us. But we have to remember that they also be forced to design and construct within given requirements. As was the P-38, and all aircraft, generally.

[DerekL1963]

2 hours ago, Northstar1989 said:

Aerodynamics don't miraculously just change because a few decades have passed- and there is no reason to think connecting the tailplanes together on the Stratolaunch 351 wouldn't provide the same structural and aerodynamic stability benefits that doing so provided the P-38, as well as improved Lift:Drag ratio by raising the average Aspect Ratio of the plane's wings...

Need I remind you that Stratolaunch is an entirely different aircraft designed for an entirely different mission with entirely different structural loads due it's mission?

Sheesh.

[shynung]

4 hours ago, Northstar1989 said:

That's a good question- and why I started this thread: to try and figure out why Scaled Composites designed the Roc the way they did..

 

I believe we've offered quite a few answers already. Avoiding overstressing the wing mounts, design requirements calling for altitude rather than speed, ability to treat a straight wing as a single piece that the rest of the plane hangs off of. I'm sure there are plenty of others.

2 hours ago, Northstar1989 said:

The more I look at it, the more I am starting to think they just took earlier designs like that of the White Knight carrier plane- and just scaled them up.

There you have it.

Scaled Composites is experienced in designing efficient high altitude flyers. Building the Stratolaunch as another efficient high altitude flyer allows them to tap to their expertise and save on design costs. Their decision to buy old 747 is also due to cost, so that they don't have to design a new fuselage, avionics, pressurized cabin, engines, and so on.

Northstar, you have to understand, their engineers have a budget constraint. They can't just design the best-performing plane without considering costs. They're working with a lot of compromises to create something Scaled Composites are able to afford that is able to do the job, and little more. The changes of the wing you proposed would cost them either a major rework of the 747 fuselages they have, or 2 brand-new fuselages.

[Northstar1989]

2 hours ago, Bill Phil said:

The P-38 actually flew relatively poorly in transonic regimes. This usually only occurred when diving from very high altitude at a steep angle. As did most aircraft of the period. This aircraft doesn't need to go fast, either.

As to its deadliness... The top aces of the entire war were German pilots, with hundreds of confirmed kills.

The design requirements for the two planes are inherently different. The P-38 only had twin booms because it was Lockheed's answer to a fighter competition for a twin-boom aircraft. They may have connected the tail-plane for more area to add more control authority or balance out the forces acting on the craft. The people designing this don't need to connect the tailplane, because control authority is a minor issue. (it's not a fighter).

But even if the reason was structural for the Lightning, there is something major that has changed since the late 30s/40s. Materials. We have better materials. Stronger materials. The structure is stronger by just using better materials (usually...).

Remember, these engineers are very likely to be smarter than most of us. But we have to remember that they also be forced to design and construct within given requirements. As was the P-38, and all aircraft, generally.

Transonic performance isn't what we"re discussing here, so there's no need to bring that up.

There was nothing in the P-38 design that called for twin fuselages- that's actually completely factually incorrect.  In fact I just watched a 1-hour documentary on the P-38 last night that extensively discussed the reason it was designed as it was... The design requirements called for a specific top speed and altitude-ceiling, and Lockheed determined the only way to achieve the desired requirements was to use two engines.  Having two fuselages to hold those two propellers just evolved naturally from the design requirements as a result.

As for the connected tailplanes, it provided extra structural strength on the P-38 just as it would on the Stratolaunch 351.  Today's matetials may be stronger, but that's no excuse to avoid making use of optimal design solutions where possible to increase structural strengthband allow use of less/thinner structural reinforcement to bring down structural mass...

Having extra space to attach control surfaces doesn't mean that designers have to do so- but the same control surface attached further towards the rear of the plane has a longer lever-arm to the Center of Geavity, and thus you can get away with less total control-surface area.  This reduces drag, mass, and cost- so it's certainly not a bad thing even with civilian aircraft...

Simply saying the engineers are "smarter than us" are how we ended up with the incredibly-inefficient Shuttle.  It's how we end up with drugs thst kill people and medical treatments that don't work (to name something closer to my own area of expertise- Biology).  The lay public can and should be interested in, and yes even curiously critical of, scientific and engineering issues rather than professing ignorance about them- it's in everyone's best interest, because even engineers get lazy, stubborn, have ulterior motives, or make mistakes...

13 minutes ago, shynung said:

I believe we've offered quite a few answers already. Avoiding overstressing the wing mounts, design requirements calling for altitude rather than speed, ability to treat a straight wing as a single piece that the rest of the plane hangs off of. I'm sure there are plenty of others.

There you have it.

Scaled Composites is experienced in designing efficient high altitude flyers. Building the Stratolaunch as another efficient high altitude flyer allows them to tap to their expertise and save on design costs. Their decision to buy old 747 is also due to cost, so that they don't have to design a new fuselage, avionics, pressurized cabin, engines, and so on.

Northstar, you have to understand, their engineers have a budget constraint. They can't just design the best-performing plane without considering costs. They're working with a lot of compromises to create something Scaled Composites are able to afford that is able to do the job, and little more. The changes of the wing you proposed would cost them either a major rework of the 747 fuselages they have, or 2 brand-new fuselages.

I wasn't discussing the swept wings right at this moment, so your answer is completely off-mark.

Though I don't think you were writing with this in kind, just in case, how would connecting the tailplanes together require any rework of the fuselages?  They already have horizontal wing sections protruding from each tail boom- essentially all they would have to do is increase their span in the design until they had one single tail-wing (which should be stronger and simpler for the same reasons straight wings are compared to swept ones in this design) which they could slot through both tail booms instead of two seperate wings.

Edited February 19, 2017 by Northstar1989

[shynung]

30 minutes ago, Northstar1989 said:

I wasn't discussing the swept wings right at this moment, so your answer is completely off-mark.

Though I don't think you were writing with this in kind, just in case, how would connecting the tailplanes together require any rework of the fuselages?  They already have horizontal wing sections protruding from each tail boom- essentially all they would have to do is increase their span in the design until they had one single tail-wing (which should be stronger and simpler for the same reasons straight wings are compared to swept ones in this design) which they could slot through both tail booms instead of two seperate wings.

I'm not just talking about swept wings, though. Any design which requires a major rework of the fuselage is basically off budget at this point. Bigger wings, though, I think is still feasible.

Merging the tailplanes would need quite some rework on the tail section, as you'd expect. The tail of each fuselage, if using a merging tailplane, now applies a torque on each other whenever there is a slight imbalance in lift, since they're now connected. Original 747 fuselage design is likely to not consider this effect; P-38's certainly does, as it was designed to have a merged tailplane from the start. The obvious solution would be to stiffen the tail torsionally, but this would add weight.

Even then, we still have to consider if the merged tailplane gets tilted due to lift imbalances, the roll effect this would induce, and whether the main wing can counteract this induced roll. There's also the question of how much torque stress the tail section can withstand, and for how long, before it eventually wears enough of the airframe to render it unsafe to fly.

[wumpus]

12 hours ago, Northstar1989 said:

Transonic performance isn't what we"re discussing here, so there's no need to bring that up.

You're the one insisting that not having swept wings slows down the jet powered Stratolaunch too much.  Wiki claims that the P-38's problems started at mach .68 (it was a propeller craft, remember).

On 2/14/2017 at 3:39 PM, Northstar1989 said:

Simply sweeping the wings a bit beyond the twin fuselages and engine pods should reduce their drag in high-subsonic/transonic flight (the engines are cannabilized from 747's, so it's a good bet the Stratolaunch Model 351 is also designed to fly at around Mach 0.86) provided they are held to the same wing-area and wingspan to maintain the same aspect-ratio... (increasing wing-sweep doesn't *necessarily* increase wing area, only if you fail to reduce front-to-back wing width to maintain the same total wing-area.  If wingspan and wing area are held constant, aspect ratio necessarily remains the same.)

If you've given up high-speed flight, there is absolutely zero reason to sweep the wings.  I also suspect that the tail-wings were simply inherited from the original proposal to use actual 747s as the twin fuselages and that the rest of the plane had already been designed for those tails.

On 2/15/2017 at 1:21 AM, Northstar1989 said:

The suborbital plane?  YNM, the Stratolaunch 351 is a plane designed for releasing *ORBITAL ROCKETS* that carry satellites all the way to Low Earth Orbit, not smaller suborbital planes.  They haul the rocket up above the densest part of the atmosphere so it doesn't have to climb as far and its engines become more powerful/efficient due to lower ambient pressure.  That, and the ability to fly to different lattitudes before release are literally the main reasons for air-launch in the first place- so the ability to fly higher (due to increased speed) would be an undeniable advantage.

I'm fairly sure your reasoning is entirely backwards.  The advantages of arbitrary launch (although not *quite* as impressive as they sound due to the funky "go well past GSO, adjust attitude, then circularize" that GSO use) is more important than not needing a nozzle that operates at sea level and "climb as far" is so amazingly trivial I laughed when I read it.

[Northstar1989]

1 hour ago, wumpus said:

You're the one insisting that not having swept wings slows down the jet powered Stratolaunch too much.  Wiki claims that the P-38's problems started at mach .68 (it was a propeller craft, remember).

If you've given up high-speed flight, there is absolutely zero reason to sweep the wings.  I also suspect that the tail-wings were simply inherited from the original proposal to use actual 747s as the twin fuselages and that the rest of the plane had already been designed for those tails.

I'm fairly sure your reasoning is entirely backwards.  The advantages of arbitrary launch (although not *quite* as impressive as they sound due to the funky "go well past GSO, adjust attitude, then circularize" that GSO use) is more important than not needing a nozzle that operates at sea level and "climb as far" is so amazingly trivial I laughed when I read it.

Don't select a mish-mash of out-of-context quotes, misrepresent that context, and think you have a point.  You don't.  For starters, I dropped any argument that the Stratolaunch flies transonic some time ago...You'll also have to cite that claim that the P-38 had problems at Mach 0.68 (as in an actual Wikipedia page/paragraph if it cones from Wiki), because that is a completely nonsensical statement.  Teansonic regime starts *nowhere near* Mach 0.68, and the known aero problems of the P-38 were mainly in steep transonic dives...

[Northstar1989]

14 hours ago, shynung said:

I'm not just talking about swept wings, though. Any design which requires a major rework of the fuselage is basically off budget at this point. Bigger wings, though, I think is still feasible.

Merging the tailplanes would need quite some rework on the tail section, as you'd expect. The tail of each fuselage, if using a merging tailplane, now applies a torque on each other whenever there is a slight imbalance in lift, since they're now connected. Original 747 fuselage design is likely to not consider this effect; P-38's certainly does, as it was designed to have a merged tailplane from the start. The obvious solution would be to stiffen the tail torsionally, but this would add weight.

Even then, we still have to consider if the merged tailplane gets tilted due to lift imbalances, the roll effect this would induce, and whether the main wing can counteract this induced roll. There's also the question of how much torque stress the tail section can withstand, and for how long, before it eventually wears enough of the airframe to render it unsafe to fly.

First of all Shynung, the tailplane is always aligned parallel to the main wing based on the way it is mounted, so any contradictory roll effects between the two wings is impossible, short of highly-localized lift imbalances that affect the front and rear wings very differently...

And, while torques are possible if lift imbalances occur between the two sides of the wing, any torques placed on the tailplane are shared with the main wing- meaning the total level of necessary structural reinforcement required to deal with roll torques between the two remains more or less constant (not to mention it eliminates any possibility of the fuselages pitching differently, which can place some fairly serious stresses on the main wing attachments...)

Structurally-speaking, you go from two weights suspended below a level board, to a box-shape, which is much stronger (especially in resisting any unequal torques in the yaw direction) and requires less structural mass for some of the same reasons that biplane wings were stronger than early monoplane wings, and allowed greater total wingspan and wing-loading...

There's also the secondary or even tertiary advantage that merging the tailplane puts more Lift in the rear of the plane, meaning mass towards the rear of the plane is suspended from a closer point, and the fuselage needs less structural reinforcement to avoid sagging like a wet noodle...

 

Ultimately it wouldn't matter what I say Shynung.  I could say independent tailplanes were better and you would say they should be merged.  I could state that straight wings had an advantage and you would insist on arguing in favor of swept wings.  You're just arguing against whatever I suggest, and to a lesser degree in favor of whatever the current design utilizes, regardless of the facts.  I find it quite disrespectful, and wish you would stop.

 

Regards,

Northstar

Edited February 19, 2017 by Northstar1989

[KSK]

15 hours ago, Northstar1989 said:

There was nothing in the P-38 design that called for twin fuselages- that's actually completely factually incorrect.  In fact I just watched a 1-hour documentary on the P-38 last night that extensively discussed the reason it was designed as it was... The design requirements called for a specific top speed and altitude-ceiling, and Lockheed determined the only way to achieve the desired requirements was to use two engines.  Having two fuselages to hold those two propellers just evolved naturally from the design requirements as a result.

OK, slightly rhetorical question. Why didn't the P-38 designers go for a trimotor configuration with a third engine on the centre fuselage? Using your previous argument, that third engine would have made for a better aircraft since it would have allowed for increased excess power at a given altitude and speed, which could then be divided up amongst competing priorities. At the time there was also no shortage of trimotor designs that had been buillt and flown, so three engine configurations weren't something new and untried.

I would suggest that you've answered the question above. The P38 design called by a given top speed and altitude ceiling and Lockheed designed their aircraft accordingly. I'm guessing (like yourself, aeronautical engineering is not my field of expertise) that the same was true for Scaled Composites with Stratolauncher. Once they'd identified their required top speed and altitude, they could design an appropriate airframe capable of tolerating (with an appropriate safety margin) the associated aerodynamic stresses and choose an appropriate number and type of engines to propel the aircraft to that speed and altitude. After that, adding any further engines would add weight, complexity, fuel consumption, manufacturing expense and servicing complexity. Which suggests to me, that adding excess power for the sake of it, probably isnt worth it.

Edited February 19, 2017 by KSK

[kunok]

@Northstar1989 What background you have in engineering? You claim yourself to be a biologist, not an engineer.

And in real design (in contraposition to just have lectured yourself in engineering)?

In real design you try to get some given specifications (including margins), not any more, not any less, using the less time and resources possible. And that's usually not the best possible design. If you go for a better than needed design you get usually an increase in times and costs,  usually exponential, that will have little or no benefit.

And seriously the central fuselage of the P-38 and an undefined rocket will have different aerodynamic properties and thus their aerodynamic effects in a merged tail. Or maybe is just that two tails just works fine and are easier/cheaper.

 

[Steel]

These last two posts have it exactly: There's never one, perfect design. There is always more than one way to meet a design brief.

 

The design that Scaled Composites have come up with satisfies their brief, and does it while drawing upon their existing expertise. Yes it could be changed as @Northstar1989 has said, but if the design already fits within all their targets, why bother?

 

Let's put it this way, in an alternate universe where the designers have designed the carrier with all the changes that have been talked about in this thread, there would probably be a discussion on an internet forum about how they should have used straight wings instead of swept ones and how they could have only used 6 engines rather than all 8.

[wumpus]

3 hours ago, Northstar1989 said:

Don't select a mish-mash of out-of-context quotes, misrepresent that context, and think you have a point.  You don't.  For starters, I dropped any argument that the Stratolaunch flies transonic some time ago...You'll also have to cite that claim that the P-38 had problems at Mach 0.68 (as in an actual Wikipedia page/paragraph if it cones from Wiki), because that is a completely nonsensical statement.  Teansonic regime starts *nowhere near* Mach 0.68, and the known aero problems of the P-38 were mainly in steep transonic dives...

The point is that you are complaining that Scaled Composites made design decisions differently that your pet ideas, then use an example plane for high speeds that was an absolute disaster anywhere near the speeds you suggest.  You are taking a proven design for high altitude flight (White Knight, also the U2 didn't have swept wings) and trying to force it to fit your pet ideas.  And bringing up the P-38 as a good example for high speed flight just doesn't work at all. I'd also really like to know how you could possibly suggest that your entire original point wasn't that you thought that the stratolaunch should have swept wings to hit mach .86.

From https://airandspace.si.edu/collection-objects/lockheed-p-38j-10-lo-lightning

Quote

The most vexing difficulty was the loss of control in a dive caused by aerodynamic compressibility. During late spring 1941, Air Corps Major Signa A. Gilke encountered serious trouble while diving his Lightning at high-speed from an altitude of 9,120 m (30,000 ft). When he reached an indicated airspeed of about 515 kph (320 mph), the airplane's tail began to shake violently and the nose dropped until the dive was almost vertical. Signa recovered and landed safely and the tail buffet problem was soon resolved after Lockheed installed new fillets to improve airflow where the cockpit gondola joined the wing center section. Seventeen months passed before engineers began to determine what caused the Lightning's nose to drop. They tested a scale model P-38 in the Ames Laboratory wind tunnel operated by the NACA (National Advisory Committee for Aeronautics) and found that shock waves formed when airflow over the wing leading edges reached transonic speeds. The nose drop and loss of control was never fully remedied but Lockheed installed dive recovery flaps under each wing in 1944. These devices slowed the P-38 enough to allow the pilot to maintain control when diving at high-speed.

You'd be surprised how soon transonic problems happen.  It doesn't matter if the airplane is traveling at transonic speeds or not, once *any* airflow gets there the problems start.

[Bill Phil]

19 hours ago, Northstar1989 said:

Transonic performance isn't what we"re discussing here, so there's no need to bring that up.

There was nothing in the P-38 design that called for twin fuselages- that's actually completely factually incorrect.  In fact I just watched a 1-hour documentary on the P-38 last night that extensively discussed the reason it was designed as it was... The design requirements called for a specific top speed and altitude-ceiling, and Lockheed determined the only way to achieve the desired requirements was to use two engines.  Having two fuselages to hold those two propellers just evolved naturally from the design requirements as a result.

As for the connected tailplanes, it provided extra structural strength on the P-38 just as it would on the Stratolaunch 351.  Today's matetials may be stronger, but that's no excuse to avoid making use of optimal design solutions where possible to increase structural strengthband allow use of less/thinner structural reinforcement to bring down structural mass...

Having extra space to attach control surfaces doesn't mean that designers have to do so- but the same control surface attached further towards the rear of the plane has a longer lever-arm to the Center of Geavity, and thus you can get away with less total control-surface area.  This reduces drag, mass, and cost- so it's certainly not a bad thing even with civilian aircraft...

Simply saying the engineers are "smarter than us" are how we ended up with the incredibly-inefficient Shuttle.  It's how we end up with drugs thst kill people and medical treatments that don't work (to name something closer to my own area of expertise- Biology).  The lay public can and should be interested in, and yes even curiously critical of, scientific and engineering issues rather than professing ignorance about them- it's in everyone's best interest, because even engineers get lazy, stubborn, have ulterior motives, or make mistakes...

You were discussing transonic performance earlier.

Lockheed was told to use two engines. Sure, it wasn't necessarily in a twin boom configuration, but that is the simplest method.

Who ever said it was optimal? Remember, what's optimal for a fighter plane isn't inherently optimal for another type of airplane that's completely different. Like when you're deploying payload mid-air that's very likely to go backwards.

Increasing the lever arm hurts angular acceleration, and thus performance, given the same torque. This is due to the moment of inertia. Torque = moment of inertia * angular acceleration. Moment of inertia is, generally, proportional to mass * radius^2. Increase the radius, you increase the moment of inertia by a faster amount than lowering the mass lowers it. Increase the moment of inertia without changing torque, or by increasing torque too slowly, and you lower your angular acceleration. Remember, the P-38 was a fighter aircraft. This is pretty important.

I'm saying the engineers are smarter than us, not the management. The engineers usually have to contend with performance requirements which necessitate trade-offs and certain design parameters. Heck, maybe they don't want the rocket to collide with the tail-plane? Makes a good chunk of sense.

The Shuttle is not what we're talking about. Heck, it was actually a good design, the main problem was that it was too big and the administration wasn't committed to high flight rates. The engineers knew pretty early that it wasn't going to lower launch costs. Remember, the Shuttle was designed the way it was because they were required to bring stuff down to the surface as well as up. This means you need a low-g reentry, only really possible with wings. Which added a whole host of problems.

Drugs that kill people come from inadequate testing, which usually falls outside of the engineer's responsibilities.

[shynung]

4 hours ago, Northstar1989 said:

First of all Shynung, the tailplane is always aligned parallel to the main wing based on the way it is mounted, so any contradictory roll effects between the two wings is impossible, short of highly-localized lift imbalances that affect the front and rear wings very differently...

And, while torques are possible if lift imbalances occur between the two sides of the wing, any torques placed on the tailplane are shared with the main wing- meaning the total level of necessary structural reinforcement required to deal with roll torques between the two remains more or less constant (not to mention it eliminates any possibility of the fuselages pitching differently, which can place some fairly serious stresses on the main wing attachments...)

Structurally-speaking, you go from two weights suspended below a level board, to a box-shape, which is much stronger (especially in resisting any unequal torques in the yaw direction) and requires less structural mass for some of the same reasons that biplane wings were stronger than early monoplane wings, and allowed greater total wingspan and wing-loading

I was thinking about highly localized gusts. Though, as you said, chances are small; this is the kind of thing you'd encounter while flying into really bad weather. Hopefully, SC isn't going to act like Russians and launch in blizzards.

Also, I missed that point. It'll definitely be stiffer when the tailplane and the main wings

4 hours ago, Northstar1989 said:

There's also the secondary or even tertiary advantage that merging the tailplane puts more Lift in the rear of the plane, meaning mass towards the rear of the plane is suspended from a closer point, and the fuselage needs less structural reinforcement to avoid sagging like a wet noodle

I think the tail section is supposed to be able to withstand its own weight while clinging to the rest of the airframe. From what I see, the payload rocket only attaches to the main wing, and doesn't apply any forces on the tail region. Any lift there would be helpful, but might induce a pitch-down tendency.

 

4 hours ago, Northstar1989 said:

Ultimately it wouldn't matter what I say Shynung.  I could say independent tailplanes were better and you would say they should be merged.  I could state that straight wings had an advantage and you would insist on arguing in favor of swept wings.  You're just arguing against whatever I suggest, and to a lesser degree in favor of whatever the current design utilizes, regardless of the facts.  I find it quite disrespectful, and wish you would stop.

Will answer via PM.