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

[Northstar1989]

A fairly simple question here- I was looking at info on the Scaled Composites Stratolaunch (which work has continued steadily on through the present despite the lack of many news releases) and wondering -why are its wings straight?

https://goo.gl/images/rwK0St

https://en.m.wikipedia.org/wiki/Scaled_Composites_Stratolaunch

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.)

What I am wondering here is- why they don't do something similar to a raked-wingtip concept, where sweep-angle increases towards the ends of the wing- in this case only once you get beyond the engine-pods?

 

Regards,

Northstar

Edited February 19, 2017 by Northstar1989

[DerekL1963]

IMO It's expensive and structurally and aerodynamically complicated enough already - swept wings just bump that up a notch without adding significant benefit.

[Steel]

My guess would be that their planned separation velocity is lower than you think (ejecting things at transonic velocities is not the easiest thing to do in the world, so they've probably kept it to Mach 0.6 or 0.7) and they've designed the aircraft for stability around that speed.

[Bill Phil]

If you're not going supersonic/transonic, there's little advantage.

[Shpaget]

High aspect ratio (long thin straight) wings are more efficient (better lift to drag ratio).

[Nibb31]

For the same reason the U-2 has straight wings. It is optimized for high altitude subsonic flight. 

[Northstar1989]

2 hours ago, Shpaget said:

High aspect ratio (long thin straight) wings are more efficient (better lift to drag ratio).

Sweeping the wings back doesn't affect aspect-ratio, as long as you keep the wing-area and wingspan constant.  I pointed as much out in the OP already, and shouldn't need to reiterate it...

1 hour ago, Nibb31 said:

For the same reason the U-2 has straight wings. It is optimized for high altitude subsonic flight. 

The U-2 had a top speed of Mach 0.67 Nibb.  Swept wings aren't useful on a plane that flies that slow, except for improving aerodynamic stability by moving the CoL back...

They also make it harder to take off without bashing your wingtips on the runway, and require more ground clearance.  The Stratolaunch Model 351 doesn't have to worry about this, however, as there is already massive desigmed-in runway clearance for the wings, due to the need to suspend the rocket and fuselages below the main wing...

2 hours ago, Bill Phil said:

If you're not going supersonic/transonic, there's little advantage.

But the Stratolaunch uses 747 engines, so it's likely they designed it to fly at Mach 0.86 (where swept wings are beneficial by pushing transonic stability and drag problems to only occur at higher speeds) to get the most altitude out of its airframe...  Therefore, swept wingtips ought to be useful...

Edited February 14, 2017 by Northstar1989

[Northstar1989]

3 hours ago, DerekL1963 said:

IMO It's expensive and structurally and aerodynamically complicated enough already - swept wings just bump that up a notch without adding significant benefit.

It's only supposed to come in at a bit more than $300 million for the carrier plane development.  That's a *bargain* as far as aircraft development goes...  The Airbus A380 cost 14.4 Billion USD, and even the 747 off which many parts of the Stratolaunch are based came in at around 3.7 Billion USD according to Hofstra University...

https://people.hofstra.edu/geotrans/eng/ch3en/conc3en/table_aircraftdevcosts.html

 

Edited February 15, 2017 by Northstar1989

[Bill Phil]

8 minutes ago, Northstar1989 said:

But the Stratolaunch uses 747 engines, so it's likely they designed it to fly at Mach 0.86 (where swept wings are beneficial by pushing transonic stability and drag problems to only occur at higher speeds) to get the most altitude out of its airframe...  Therefore, swept wingtips ought to be useful...

There's no reason to assume that it will fly at Mach 0.86. Sure, it has 747-400 engines, but it has a wholly different purpose.

[Northstar1989]

2 minutes ago, Bill Phil said:

There's no reason to assume that it will fly at Mach 0.86. Sure, it has 747-400 engines, but it has a wholly different purpose.

The engines are designed to perform well at Mach 0.86 and they have access to up to 8 of them from the two 747's they cannabilized- yet only choose to make use of 6 of them on the Roc.  If it was a matter of needing more thrust to fly faster, they could just strap on a couple more of the engines.  That they don't indicates they provably already have enough thrust to fly at the engines' designed top speed...

[Steel]

2 minutes ago, Northstar1989 said:

The engines are designed to perform well at Mach 0.86 and they have access to up to 8 of them from the two 747's they cannabilized- yet only choose to make use of 6 of them on the Roc.  If it was a matter of needing more thrust to fly faster, they could just strap on a couple more of the engines.  That they don't indicates they provably already have enough thrust to fly at the engines' designed top speed...

Or, as is more likely, that they do not want to fly at the engine's top speed.

[DerekL1963]

2 minutes ago, Northstar1989 said:

The engines are designed to perform well at Mach 0.86 and they have access to up to 8 of them from the two 747's they cannabilized- yet only choose to make use of 6 of them on the Roc.  If it was a matter of needing more thrust to fly faster, they could just strap on a couple more of the engines.  That they don't indicates they provably already have enough thrust to fly at the engines' designed top speed...

Or that the top speed is considerably slower than you've presumed it to be.  Speed is no particular advantage for Stratolaunch...    They're not trying to maximize the number of passenger miles per day, and it's extraordinarily unlikely they'll have a launch off of Virginia on Tuesday and one off of Hawaii on Wednesday.   Nor does a few extra tenths of Mach make much difference in launch performance.
 

32 minutes ago, Northstar1989 said:

The U-2 had a top speed of Mach 0.67 Nibb.  Swept wings aren't useful on a plane that flies that slow, except for improving aerodynamic stability by moving the CoL back...

So, since Stratolaunch doesn't have swept swings, that suggests to me that they don't intend to fly in the speed range where they'd be useful.   Sometimes the easiest answer is the simplest - they're using 747 nacelles because they came with the 747's they bought with the overt intention of stripping for parts, not because they intend to fly in the upper range of the 747's performance envelope.

[softweir]

Don't get hung up on the high-speed performance of the engines. Scaled Composites didn't choose them for their high speed capability. They chose them for their power at takeoff, their capability at high altitude, their fuel efficiency, and the fact that they were already bolted to the fuselages they purchased.

There are two main challenges facing the Stratolauncher. The first is its requirement for very high power at takeoff - two large fuselages, the orbital vehicle and payload will make it amongst the heaviest aircraft ever to be launched, and the wing it needs for high altitude flight will create a large amount of drag. To get that moving on the runway without silly bolt-ons like JATOs demands a heck of a lot of low-speed power, and that is something those engines provide.

The other problem is altitude. They intend to take the orbiter up to near stratospheric altitudes before launch. The engines they have chosen are proven high-altitude engines.

They might have ended up with other engines, had the 747s they purchased been fitted differently. But the two they bought happened to have Pratt and Whitney 4056 engines - they might have ended up with engines from General Electric or Rolls Royce. It doesn't really matter - they all have similar power on takeoff, efficiency and high-altitude capability.

[Northstar1989]

1 hour ago, DerekL1963 said:

Or that the top speed is considerably slower than you've presumed it to be.  Speed is no particular advantage for Stratolaunch...    They're not trying to maximize the number of passenger miles per day, and it's extraordinarily unlikely they'll have a launch off of Virginia on Tuesday and one off of Hawaii on Wednesday.   Nor does a few extra tenths of Mach make much difference in launch performance.
 

So, since Stratolaunch doesn't have swept swings, that suggests to me that they don't intend to fly in the speed range where they'd be useful.   Sometimes the easiest answer is the simplest - they're using 747 nacelles because they came with the 747's they bought with the overt intention of stripping for parts, not because they intend to fly in the upper range of the 747's performance envelope.

The *engine* top speed Derek.  That is clearly at least Mach 0.86 as a 747 flies at Mach 0.86, regardless of the capabilities if the airframe.  You're wrong in that sense, and I'd like you to admit it.

The point of flying faster, on the other hand, would be to fly higher.  If you fly faster with only a small increase in drag and mass due to an extra pair of engines, you can fly higher for the same wing-area.  Flying higher and faster not only means reduced Delta-V to orbit, it means higher thrust and USP for the rocket engines due to reduced ambient pressure.  Every little bit helps with spaceflight, and even a 1% increase in payload capacity relative to what it was before (say 25250 kg instead of 25000 kg) can be worth hundreds of thousands of dollars...

16 minutes ago, softweir said:

Don't get hung up on the high-speed performance of the engines. Scaled Composites didn't choose them for their high speed capability. They chose them for their power at takeoff, their capability at high altitude, their fuel efficiency, and the fact that they were already bolted to the fuselages they purchased.

There are two main challenges facing the Stratolauncher. The first is its requirement for very high power at takeoff - two large fuselages, the orbital vehicle and payload will make it amongst the heaviest aircraft ever to be launched, and the wing it needs for high altitude flight will create a large amount of drag. To get that moving on the runway without silly bolt-ons like JATOs demands a heck of a lot of low-speed power, and that is something those engines provide.

The other problem is altitude. They intend to take the orbiter up to near stratospheric altitudes before launch. The engines they have chosen are proven high-altitude engines.

They might have ended up with other engines, had the 747s they purchased been fitted differently. But the two they bought happened to have Pratt and Whitney 4056 engines - they might have ended up with engines from General Electric or Rolls Royce. It doesn't really matter - they all have similar power on takeoff, efficiency and high-altitude capability.

If they need as much power on the runway as possible (which I don't doubt- it IS a massive aircraft) then why use only 6 of the engines instead of all 8?

Altitude isn't such a big problem on the other hand.  They intend to release the payload at 30,000 feet- which is LESS than the roughly 45,000 foot altitude-ceiling of the 747's they were cannibalizing.  Plus, as I've stated *repeatedly*, the faster you fly the higher you can fly with the same wings...

Edited February 15, 2017 by Northstar1989

[Nuke]

its likely all about stability. this things going to fly up to altitude and drop a large fraction of its mass of there. it cant loose control during this change in geometry. so you keep the geometry as simple as possible.

[Northstar1989]

3 minutes ago, Nuke said:

its likely all about stability. this things going to fly up to altitude and drop a large fraction of its mass of there. it cant loose control during this change in geometry. so you keep the geometry as simple as possible.

If it was about stability then sweeping the wingtips would be even MORE important, since it moves the Center of Lift back.  There has to be more to it than that...

Edited February 15, 2017 by Northstar1989

[Bill Phil]

@Northstar1989

Just because the engines are capable doesn't mean that they intend to fly it up that fast. You don't really need speed here. You need lift. Lots of lift. That comes easier with speed, but you don't want to hit wave drag. This means that they will likely use straight wings so as to have more lift to drag at low speeds, to get altitude with relative speed. Planes have this weird property where, even if you fly them straight, they can gain altitude (wing AoA may be offset, for example). Using an already existing, proven, engine which can go beyond your performance requirement gives you margin. And that's always good to have (unless it costs you too much money). Even more important is that this is not a 747-400. It's fuselage is fundamentally different. It's designed for air launch. So, even though it might have the power to go to a certain speed, it may not be desirable to do so, since they could potentially cause harm to the launch systems, if not the whole airframe.

[DerekL1963]

3 hours ago, Northstar1989 said:

The *engine* top speed Derek.  That is clearly at least Mach 0.86 as a 747 flies at Mach 0.86, regardless of the capabilities if the airframe.  You're wrong in that sense, and I'd like you to admit it.

I can't be wrong about something I never addressed despite you being so hung up about it.  Heck, it's not even mentioned in what you quoted.
 

3 hours ago, Northstar1989 said:

If they need as much power on the runway as possible (which I don't doubt- it IS a massive aircraft) then why use only 6 of the engines instead of all 8?

Real world engineering is all about compromises.  Eight engines increase the complexity of all associated systems and structures.  So I suspect they compromised.  (And gained two spare engines already paid for out of the deal.)

Seriously, all the evidence points in a direction that answers your question - they don't have swept wings because they're not going to go fast.

[Northstar1989]

49 minutes ago, DerekL1963 said:

I can't be wrong about something I never addressed despite you being so hung up about it.  Heck, it's not even mentioned in what you quoted.
 

Real world engineering is all about compromises.  Eight engines increase the complexity of all associated systems and structures.  So I suspect they compromised.  (And gained two spare engines already paid for out of the deal.)

Seriously, all the evidence points in a direction that answers your question - they don't have swept wings because they're not going to go fast.

No Derek, it's 100% clear exactly what we were discussing.  I said the engine ate designed to operate up to at least Mach 0.86, citing that they fly this fast on the 747, and you said "Or that the top speed is considerably slower than you've presumed it to be."  You were wrong, the engines clearly DO operate up to those speeds, and you should admit it.  I strongly dislike your insistence that you are never wrong about anything, and if you really must follow me around posting on all my threads (I can't stop you- though you've been doing it for months now) then you really ought to admit you're wrong now and then...

As for the speed the aircraft actually flies at, we literally know nothing about it.  But swept wings are an advantage in pretty much any design where runway clearance is ample and getting shot at is not a concern (so you don't need to worry about how overall shape affects how easy your plane is to shoot), as it provides a further-back Center of Lift and more rear edge space for trailing edge control surfaces to attach to with literally no penalty to drag (which is determined by aspect ratio and frontal area- both of which are unaffected by wing-sweep as long as you keep wing-area and span constant).

I'm not talking large delta wings with a high chord here- I'm talking relatively thin wing tips with a low chord.  The only reason I can think of not to do this is that the bending moment is too high this way (swept wings increase how much torque is placed on wings as they are further from the attachment point at the ends...)  Then again, maybe I just answered my own question.

Alternatively, since Scaled Composites is building most of the airframe out of identical carbon fiber panels, maybe adding wing-sweep would have required a new type of panel, driving up costs.  Think of needing a new wing part type in KSP to build a desired shape- if you don't already have it unlocked in Career Mode, getting it is going to cost you money...

 

Edited February 15, 2017 by Northstar1989

[YNM]

I suppose they're not chasing speed when releasing the suborbital plane. I mean, the SCA was limited to lower speed when carrying the Shuttle, no ? Also, swept wings aren't the only answer to transonic capabilities - airfoil also matters.

Edited February 15, 2017 by YNM

[Northstar1989]

3 hours ago, Bill Phil said:

@Northstar1989

Just because the engines are capable doesn't mean that they intend to fly it up that fast. You don't really need speed here. You need lift. Lots of lift. That comes easier with speed, but you don't want to hit wave drag. This means that they will likely use straight wings so as to have more lift to drag at low speeds, to get altitude with relative speed. Planes have this weird property where, even if you fly them straight, they can gain altitude (wing AoA may be offset, for example). Using an already existing, proven, engine which can go beyond your performance requirement gives you margin. And that's always good to have (unless it costs you too much money). Even more important is that this is not a 747-400. It's fuselage is fundamentally different. It's designed for air launch. So, even though it might have the power to go to a certain speed, it may not be desirable to do so, since they could potentially cause harm to the launch systems, if not the whole airframe.

Lift increases with airspeed.  That's a big part of why supersonic aircraft can fly so much higher than their subsonic counterparts with similar airframes- for example the Concorde flew much higher than the 747.  The Stratolaunch 351 is supposedly only going to fly up to 30,000 feet, whereas the 747-100 could reach over 45,000 in level flight- so I don't see why they couldn't add on their two extra engines so they can climb higher/faster, and sweep their wingtips to reduce the drag.  It would also make just getting off the runway easier.

The other components, such as avionics, mostly come from the 747's as well.  Even the landing gear are just the 747 gears.  Literally the only parts that are new are the airframe-which are built of carbon fiber to save weight.  However with 2 extra engines they would have wider performance margins which would mean there'd be less need for weight-savings in the first place.  Even a wing made of lead will fly if you move it fast enough- and if they're really not moving fast enough that they need to seeep the wings with 6 engines (which I find a doubtful explanation) then 8 engines certainly won't push them supersonic...

Another possible explanation I thought of (why am I the only one pushing actual reasonable explanations here?  I came here looking for ideas better than my own- or better yet somebody who actually KNOWS the reason and has sources...) is that perhaps they kept the wings straight because they want to make the aircraft easier to fit in a hanger, and bring out of it...

 

8 minutes ago, YNM said:

I suppose they're not chasing speed when releasing the suborbital plane. I mean, the SCA was limited to lower speed when carrying the Shuttle, no ? Also, swept wings aren't the only answer to transonic capabilities - airfoil also matters.

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.

Edited February 15, 2017 by Northstar1989

[shynung]

15 minutes ago, Northstar1989 said:

I'm not talking large delta wings with a high chord here- I'm talking relatively thin wing tips with a low chord.  The only reason I can think of not to do this is that the bending moment is too high this way (swept wings increase how much torque is placed on wings as they are further from the attachment point at the ends...)  Then again, maybe I just answered my own question.

Bingo.

The original Boeing 747 wings are meant to support one fuselage. The engineers at Boeing designed those wings' construction parameters with that in mind, and came up with the swept wings the 747 came with from the factory.

The Stratolaunch's wings are meant to support not one, but three fuselages. Designing a swept wing big enough to handle the load would necessitate rebuilding the wing mounting points, which means reshaping the fuselages. At that point, the costs would be so high, they might as well design a brand new fuselage.

So the guys at SC have to come up with a wing design that can support the weight of 2 additional fuselages, yet fits into 747's original wing mounts and won't overstress it. And they came up with the straight wing design, to minimize torsional loads on the mounts, and get maximum lift out of a limited wing surface area.

[Nibb31]

They picked 6 engines because:

• 6 was enough for their application.
• By buying a pair of 747s, they got the opportunity to pick or refurb the 6 best engines and have 2 spares.

They picked a straight wing configuration because:

• Scaled has a lot of experience with high-altitude straight wing aircraft (WK1, WK2, Proteus, Global Flyer, etc...)
• It is the best for their flight profile: high and slow, like the U2 and the aforementioned planes from Scaled.

I don't know why you insist on discussing speed. They don't care about speed. 0.2 Mach won't make a difference for a rocket that has to get to Mach 24.

The engineers at Scaled aren't idiots (I wouldn't necessarily say the same about the folks pushing Stratolaunch). If they had needed a swept-wing configuration, they could have done it. The reason they didn't is because it wasn't needed.

Edited February 15, 2017 by Nibb31

[Northstar1989]

1 hour ago, Nibb31 said:

They picked 6 engines because:

• 6 was enough for their application.
• By buying a pair of 747s, they got the opportunity to pick or refurb the 6 best engines and have 2 spares.

They picked a straight wing configuration because:

• Scaled has a lot of experience with high-altitude straight wing aircraft (WK1, WK2, Proteus, Global Flyer, etc...)
• It is the best for their flight profile: high and slow, like the U2 and the aforementioned planes from Scaled.

I don't know why you insist on discussing speed. They don't care about speed. 0.2 Mach won't make a difference for a rocket that has to get to Mach 24.

The engineers at Scaled aren't idiots (I wouldn't necessarily say the same about the folks pushing Stratolaunch). If they had needed a swept-wing configuration, they could have done it. The reason they didn't is because it wasn't needed.

Don't put words in my mouth.  I've repeatedly said that the benefit of the additional speed would be in allowing the carrier plane to fly higher- so it could release its payload at, say, 36000 feet instead of 30000 feet.  0.2 Mach is a roughly 30% increase in speed from Mach 0.66 to 0.86, so that represents a significant increase in lift for the same altitude.

Saying I've said *anything* about the speed being important in and of itself is incredibly misleading, and I don't appreciate the Straw Man argument...

Anyways, the entire PURPOSE of the carrier plane is to get the rocket as high as possible before release.  Altitude makes a significant difference because it improves the ISP of the rocket engines and allows them to start firing more horizontally sooner in the ascent to orbit, and 6000 feet, for example, would represent a 20% in release altitude...

 

Edited February 15, 2017 by Northstar1989

[Nibb31]

1 hour ago, Northstar1989 said:

Don't put words in my mouth.  I've repeatedly said that the benefit of the additional speed would be in allowing the carrier plane to fly higher- so it could release its payload at, say, 36000 feet instead of 30000 feet.  0.2 Mach is a roughly 30% increase in speed from Mach 0.66 to 0.86, so that represents a significant increase in lift for the same altitude.

Designing an aircraft is a game of compromises. The primary requirements are altitude and payload capacity. Speed is secondary. They can compensate speed by designing wings that provide more lift, which is precisely what Scale does on all of their high-altitude aircraft. Swept wings would allow the plane to go faster, which might provide more lift, but would need a stronger and heavier structure which would reduce payload capacity.

As you said, the U-2 was capable of flying at a high altitude at low speed. Same as the White Knights, Proteus, and Global Flyer, etc... Roc will have a similar flight envelope: high and slow, which is why it has a similar wing design.

At any rate, there are many problems with the Stratolaunch concept. The design of the carrier aircraft is really the least of them.

 

 

Edited February 15, 2017 by Nibb31