YNM Posted February 15, 2017 Share Posted February 15, 2017 (edited) 3 hours ago, 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. Ah, I missed the news again then. Thought of this (or this), turns out to be the other one. Still, the requirement for transonic or high-lift isn't just swept wings. I can see that they need to make a pretty solid structural bridge between the two fuselage - having the wing swept, even only at the outside, won't help that. Also, even at speeds as fast as any subsonic (or transonic) aircraft can go during the release and launch, most of the work will still be completed by the rocket. (unless you're trying to bring this back to life that is.) EDIT : Worth to note as well that Burt Rutan's designs seems to have stemmed from endurance aircraft. You don't expect him to make Concorde, you expect him to make these things. Edited February 15, 2017 by YNM Link to comment Share on other sites More sharing options...
p1t1o Posted February 15, 2017 Share Posted February 15, 2017 If you're not going to accept any answers, why ask? Link to comment Share on other sites More sharing options...
shynung Posted February 15, 2017 Share Posted February 15, 2017 4 hours ago, Northstar1989 said: 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 Rocket's final speed is about Mach 24. Starting from Mach .86 instead of Mach .66 doesn't reduce enough chunk of the 2nd stage rocket's deltaV requirement to justify building two entirely new fuselages and wings just to take advantages of the PW4056's maximum speed. Link to comment Share on other sites More sharing options...
Bill Phil Posted February 15, 2017 Share Posted February 15, 2017 Transonic aircraft experience something called wave drag. This increases the drag enormously, until you either slow down or speed up past the transonic region. But lift to drag still suffers. Supersonic aircraft generally have more than enough power to overcome this, of course. I'm pretty sure Concorde flew higher so as to reduce the sonic boom (it only flew supersonic over the ocean, but i guess they didn't want to hurt sailors?) If they don't have sweeped wings, it's likely for a good reason. They're probably smarter than many of us when it comes to their own aircraft. Sweeped wings have advantages once you're going very fast. Mainly to reduce exposed area, which helps the area rule. Link to comment Share on other sites More sharing options...
mikegarrison Posted February 15, 2017 Share Posted February 15, 2017 16 hours ago, Bill Phil said: If you're not going supersonic/transonic, there's little advantage. More than that, there are many disadvantages. 14 hours 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... The engines are designed (obviously) to function anywhere between sea level static and near-sonic at flight level 410 or so. It's not like the SC airplane has to be so efficient it can fly from LAX to SYD. Link to comment Share on other sites More sharing options...
mikegarrison Posted February 15, 2017 Share Posted February 15, 2017 33 minutes ago, Bill Phil said: I'm pretty sure Concorde flew higher so as to reduce the sonic boom (it only flew supersonic over the ocean, but i guess they didn't want to hurt sailors?) Nope. SSTs fly high just so they can get less drag. Link to comment Share on other sites More sharing options...
Green Baron Posted February 15, 2017 Share Posted February 15, 2017 (edited) 1 hour ago, Bill Phil said: I'm pretty sure Concorde flew higher so as to reduce the sonic boom (it only flew supersonic over the ocean, but i guess they didn't want to hurt sailors?) Concorde flew higher (50.000ft ?) because of drag (less density, less drag, less thrust needed). It only flew supersonic after crossing the coastline because of regulations. Sketchy spoken: besides improved behaviour during trans- und supersonic flight swept wings are a constructive feature that provides stability, mainly pitch. For an airliner this is a reassuring feature, in conjunction with a v-shape and a torsion in profile. A plane with straight wings can be more responsive and can have better gliding ratios. A multihull design seems not to be in favour of swept wings. Though, it's Burt Rutan, you never know ... :-) Judging the design, Stratolaunch is or will be a slow flying, high altitude plane with transport capacity. Edited February 15, 2017 by Green Baron Link to comment Share on other sites More sharing options...
KSK Posted February 15, 2017 Share Posted February 15, 2017 10 hours ago, Northstar1989 said: 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... You're not the only one pushing reasonable explanations. It's just that everybody else's reasonable explanations don't appear to fit your preconceptions. If that's what you were looking for, then I suggest posting a more specific question next time, rather than posting a fairly open-ended one and then loudly disagreeing with the answers you get. 15 hours ago, Northstar1989 said: 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... Unless we also know where the centre-of-mass is, then I don't see how we can decide whether moving the centre of lift back will improve stability or not. Then again, I'm only relying on what I've picked up from KSP, which is probably even more dangerous for aeronautical engineering than it is for rocket engineering. Link to comment Share on other sites More sharing options...
Green Baron Posted February 15, 2017 Share Posted February 15, 2017 (edited) 1 hour ago, KSK said: Unless we also know where the centre-of-mass is, then I don't see how we can decide whether moving the centre of lift back will improve stability or not. Then again, I'm only relying on what I've picked up from KSP, which is probably even more dangerous for aeronautical engineering than it is for rocket engineering. Yeah, well a possible "moving the center of lift back" is of course compensated by other means, like moving the root of the wing forward. I can speculate about many reasons while the designer built/will build this thing like he did/will do. This one looks pretty straightforward for me, except for the multi-hull. Wingsweep has advantages and disadvantages and aircrafts are designed to serve a purpose. I am sure that all criteria like lift, drag, structural considerations, stability, etc. were taken into account, so the initial question is somewhat obsolete. I mean, we know that Rutan's contraptions actually do fly ... If you really want to know then ask Rutan. Write a friendly email, like interested, lay, model maker, whatever. Worst case: you don't get an answer. I tried something similar once with the making of a model of a historic sailing boat, wrote to the national museum in Paris, France and actually got copies of historic plans ! :-) Great service. Edited February 15, 2017 by Green Baron Link to comment Share on other sites More sharing options...
DerekL1963 Posted February 15, 2017 Share Posted February 15, 2017 (edited) 9 hours ago, Nibb31 said: 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. Exactly this. Northstar keeps harping on the advantages that speed provides the rocket (in terms of payload and performance) - but doesn't seem to grasp that Roc is part of the booster system too and the impacts that speed would have there need to factored in as well. Increasing speed during the airbreathing phase actually provides little benefit to the rocket stage but has considerable impacts on the airbreathing stage in terms of complexity (design and manufacture), weight, operations, and operating costs. Increasing speed also has negative impacts on the design of the rocket stage as well, most notably the mounts will have the be stronger (read: heavier) - increasing the already significant impact that horizontal carry imposes. Altitude during the airbreathing phase is the real key (and one of the few benefits brought to the table). And all the evidence points to the obvious - Scaled is playing to their strengths by going with a (relatively slow) straight winged airbreathing stage designed for high altitude. The same aircraft they've designed and built before, only turned up to 11. This reduces the dimensions of the engineering risk considerably as well as reducing the engineering costs. It's not as obvious (because rocketry fans aren't used to thinking like actual engineers, let alone like accountants), but reducing the costs of the re-useable airbreathing stage has significant benefits. You reduce the per-flight amortized costs, and you considerably reduce the payback period... vital considerations when you're talking about a system that will likely have a fairly low flight rate the begin with. 8 minutes ago, Green Baron said: If you really want to know then ask Rutan. Write a friendly email, like interested, lay, model maker, whatever. Worst case: you don't get an answer. I tried something similar once with the making of a model of a historic sailing boat, wrote to the national museum in Paris, France and actually got copies of historic plans ! :-) Great service. Did that once (with Burt). Only got a hand wavy answer, but I also got a pass to attend a family-and-friends-and-select-others day at the factory. (Couldn't take advantage of it because it was the next day and I didn't have the budget to fly to CA on short notice, but it was still cool.) But Burt is retired, and Scaled is now corporate... Edited February 15, 2017 by DerekL1963 Link to comment Share on other sites More sharing options...
Green Baron Posted February 15, 2017 Share Posted February 15, 2017 Well, what i should have said is that the probabilitiy for a profound answer is higher at the source than in a computer game forum :-) Link to comment Share on other sites More sharing options...
mikegarrison Posted February 15, 2017 Share Posted February 15, 2017 It's kind of funny how often people fall in love with particular design solutions and won't listen to any explanations or evidence that said solution is not the best answer to everything. Link to comment Share on other sites More sharing options...
Northstar1989 Posted February 16, 2017 Author Share Posted February 16, 2017 (edited) 21 hours ago, Nibb31 said: 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. 30,000 feet isn't high compared to any of those aircraft. And one cannot simply "design wings to provide more lift". Wings aren't some kind of magic where you wave a wand and get Lift- you have to purchase it with Drag. True, you can design higher aspect-ratio wings to provide better Lift:Drag ratio, but the only way to improve aspect ratio without compromising payload capacity is to increase wingspan. Since their wingspan is already at the limits of sanity (greater than any other aircraft in history) that's not really an option for a number of reasons including ground facility limitations and the need to make stronger/heavier wings to resist bending if they make them longer... Thus the logical way to gain additional altitude-ceiling for payload deployment would seem to be to add additional engines so they fly faster, and reduce Drag by sweeping/raking the wingtips back (which will also improve aerodynamic stability). They already do this with the wingtips on some commercial jets, there's no reason for you to treat it as something so extraordinary... For that matter, since they ought to be optimizing for payload and altitude, it would make perfect sense to just add on the 2 extra 747 engines they have access to and increase the wing chord if they're really going to fly slow enough that wing sweep doesn't matter. Decreasing the aspect ratio like this by thickening the wing chord would of course hurt the Lift:Drag ratio, but it would also provide more raw Lift for any given speed and altitude- meaning that with the extra Thrust the additional 2 engines would provide, they could "purchase" enough extra Lift to significantly improve the overall altitude-ceiling at the expense of fuel-efficiency and range... In the final analysis, having a big, heavy plane that's over-built is a good thing. It means that they can build and maintain the aircraft to looser engineering margins without increased risk of failure, bringing down overall costs. Given the purpose of the Stratolaunch 351 to merely serve as a launch platform, they should probably be aiming for big, rugged, and overbuilt- as really all the Roc needs to do is act as a super-heavy cargo plane of a sort (except one that releases its cargo midair...) Further, the heavier the plane is the more easily it should remain aerodynamically stable after releasing its rocket, as the rocket will be relatively lighter and its release will cause less overall shift in the Center of Mass... Thus the extra mass required to reinforce the wingtips enough to rake them back without causing excessive bending really wouldn't be such a bad thing. And it would actually *increase* maximum takeoff weight by more than the extra mass required, just like it does with commercial jetliners, if they stuck on enough 747 engines to ensure it could fly in a comparable speed range... 18 hours ago, shynung said: Rocket's final speed is about Mach 24. Starting from Mach .86 instead of Mach .66 doesn't reduce enough chunk of the 2nd stage rocket's deltaV requirement to justify building two entirely new fuselages and wings just to take advantages of the PW4056's maximum speed. Quit it Shymung. I have asked you politely now several times- drop that Straw Man argument right there. 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. Edited February 16, 2017 by Northstar1989 Link to comment Share on other sites More sharing options...
Northstar1989 Posted February 16, 2017 Author Share Posted February 16, 2017 (edited) 16 hours ago, mikegarrison said: More than that, there are many disadvantages. Such as? A few of you keep repeating that mantra, but provide absolutely no specifics to back it up. It"s not true *just* because somebody said it earlier. Runway clearance is a non-issue with the wings of the Stratolaunch 351 as the wings are already positioned so far above (nearly 40 feet!) the landing gear. And the extra mass required to structurally reinforce the wings to resist the extra bending moment created by sweeping the wings is only one (relatively minor) isdue, and has been discussed ad nauseum... Edited February 16, 2017 by Northstar1989 Link to comment Share on other sites More sharing options...
mikegarrison Posted February 16, 2017 Share Posted February 16, 2017 (edited) 9 minutes ago, Northstar1989 said: Such as? A few of you keep repeating that mantra, but provide absolutely no specifics to back it up. It"s not true *just* because somebody said it earlier. Runway clearance is a non-issue with the wings of the Stratolaunch 351 as the wings are already positioned so far above (nearly 40 feet!) the landing gear. And the extra mass required to structurally reinforce the wings to resist the extra bending moment created by sweeping the wings is only one (relatively minor) isdue, and has been discussed ad nauseum... Dutch roll. Spanwise flow. Lower effective aspect ratio. More flutter concerns. More shift in cg (if it's a wet wing). Planes that are designed to fly high and slow almost always have long, straight, high aspect ratio wings. Like the U2, or gliders, or most long-endurance drones. Edited February 16, 2017 by mikegarrison Link to comment Share on other sites More sharing options...
Northstar1989 Posted February 16, 2017 Author Share Posted February 16, 2017 (edited) 19 minutes ago, mikegarrison said: Dutch roll. Spanwise flow. Lower effective aspect ratio. More flutter concerns. More shift in cg (if it's a wet wing). Spanwise flow is actually a GOOD thing in certain circumstances- it reduces drag in transonic conditions- it's disingenuous to list it as a negative. It DOES lead to vortices becoming a problem if you don't design the wingtips properly, but in fact the entire reason wingtips are often raked is to prevent this from becoming a problem by moving the vortices away from the main wing, and rectangular non-swept wings experience STRONGER vortices than wings with swept wingtips... Dutch roll could of course be a problem. But do please elaborate on how sweeping the wings back would lead to it, as it's one of the concepts I've always struggled to wrap my head around the causes of, and my plane designs in FAR actually often struggle not to suffer from it... "Lower effective aspect ratio" is also a disingenuous answer. If you proportionally reduce wing chord such that the wing-area of the wingtips remains unchanged then it's exactly the same... So, two bad answers that are outright wrong (aspect ratio *doesn't* decrease if wing area and span are held constant, and wingtip vortices caused by spanwise flow are actually STRONGER with straight, rectangular wings than tapered ones) and one- about Dutch Roll- that I don't understand as well as I would like to and would like to know more about before I make up my mind about one way or another, in short... Edited February 16, 2017 by Northstar1989 Link to comment Share on other sites More sharing options...
YNM Posted February 16, 2017 Share Posted February 16, 2017 I think I see one advantage of having a long "rod" instead of some bizzare swept-straight-swept wing here : structural. A single piece rod would have better properties at resisting bending rather than an already bent one. Not to mention you need to connect between the bent and straight section. An aircraft is suspended in the air solely thanks to lift from the wing. A way to see this is having a similar shaped contraption but with tyres on each end of what was the wing (think as a chassis or something). Having a straight rod chassis would be a better option than having a bent one. This is also why airplanes with swept wings more or less treat each wing as a separate section - there's no way to make a rigid connection between the two wings. Better to just have a quite solid anchor point on the fuselage. TL;DR don't see this wing design as three separate wings. See it as a single wing. Link to comment Share on other sites More sharing options...
Northstar1989 Posted February 16, 2017 Author Share Posted February 16, 2017 (edited) Since certain individuals keep insisting that raking the wingtips *reduces* the effective aspect-ratio if a wing, I *STRONGLY* recommend reading the Wikipedia section on raked wingtips. Raking the wingtips of a plane in fact *increases* effective aspect ratio, which is why raked wingtips are used: https://en.m.wikipedia.org/wiki/Wingtip_device 38 minutes ago, YNM said: I think I see one advantage of having a long "rod" instead of some bizzare swept-straight-swept wing here : structural. A single piece rod would have better properties at resisting bending rather than an already bent one. Not to mention you need to connect between the bent and straight section. An aircraft is suspended in the air solely thanks to lift from the wing. A way to see this is having a similar shaped contraption but with tyres on each end of what was the wing (think as a chassis or something). Having a straight rod chassis would be a better option than having a bent one. This is also why airplanes with swept wings more or less treat each wing as a separate section - there's no way to make a rigid connection between the two wings. Better to just have a quite solid anchor point on the fuselage. TL;DR don't see this wing design as three separate wings. See it as a single wing. So the main reason for the straight wings is so you can run the wings right through the two fuselages and treat it as a single section, structurally speaking? If that's even possible with this design (to run the wing through the two fuselages of the Roc), then I buy that. That *WOULD* pose a fairly large structural strength penalty on swept wings, far in excess of what the swept wings alone might pose. And considering they're already making the wings out if fairly flimsy carbon fiber panels... I guess that answers my question- as that's actually a highly plausible reason for the use of straight wings despite their inherent aerodynamic inefficiency (stronger wingtip vortices, reduced effective aspect ratio vs. raked wingtips, greatly reduced transonic stability, and a further-forward Center of Lift). Edited February 16, 2017 by Northstar1989 Link to comment Share on other sites More sharing options...
Northstar1989 Posted February 16, 2017 Author Share Posted February 16, 2017 45 minutes ago, mikegarrison said: Dutch roll. Spanwise flow. Lower effective aspect ratio. More flutter concerns. More shift in cg (if it's a wet wing). Planes that are designed to fly high and slow almost always have long, straight, high aspect ratio wings. Like the U2, or gliders, or most long-endurance drones. Flutter concerns are addressed by reinforcing the wing (which is heavier- but we already discussed this). Gliders and drones actually fly LOW, in relative terms- so you're defeating your own point there. And the Roc doesn't have a wet wing, as far as I know. Anyways, YNM already suggested a credible explanation regarding the ability to run the wing through the fuselages with a straight wing, but not with a swept wing- and since the Roc needs to carty an incredibly heavy payload below its center section, that actually makes a lot of sense (it would be much easier to carry the payload below a wing that ran through the fuselages than one thst was anchored externally). So I'm going to drop this here (provided nobody else posts anything I feel the need to reply to), as it looks like I have a good answer from YNM. Link to comment Share on other sites More sharing options...
YNM Posted February 16, 2017 Share Posted February 16, 2017 (edited) On 2/16/2017 at 2:33 PM, Northstar1989 said: So the main reason for the straight wings is so you can run the wings right through the two fuselages and treat it as a single section, structurally speaking? If that's even possible with this design (to run the wing through the two fuselages of the Roc), then I buy that. Yes, precisely. There's no other not much way around - if you haven't noticed, the only thing that's connecting the two fuselages and the payload is nothing else but that colossal wing. The engines, fuselages, and payload(s?) are just another attachment to the wing. It's not like some of the other planes where the horizontal stabilizer also plays role in structural strength (example). Also, letting a straight wing to bend (usually "up") is easier than letting swept wing to do one - that's why most swept wings are made bent the "other way" (ie. "down") (example, also showing what I referred as "anchor points"), and already having a positive dihedral, so that when in-flight bending occurs (bend "up"), this is easily taken by their design. I suppose letting the wings to really bend "up" on swept wings would be a disaster, unlike on straight wings. Having to add up swept to straight to swept, and joining three different dihedral angles... sounds like a recipe for over-engineer or disaster; that's why they settled for a simple, honest, straight wing. God I have problems with the correct terms here. Also, please take note that all our discussion here is based on the current images (or artistic impressions) of the plane. If it turns out different, you don't know that in advance. Edited February 17, 2017 by YNM Link to comment Share on other sites More sharing options...
p1t1o Posted February 16, 2017 Share Posted February 16, 2017 @Northstar1989 Question: why do you *think* that none of your proposed design principles made it to final production? Link to comment Share on other sites More sharing options...
mikegarrison Posted February 16, 2017 Share Posted February 16, 2017 10 hours ago, Northstar1989 said: Spanwise flow is actually a GOOD thing in certain circumstances- it reduces drag in transonic conditions- it's disingenuous to list it as a negative. It DOES lead to vortices becoming a problem if you don't design the wingtips properly, but in fact the entire reason wingtips are often raked is to prevent this from becoming a problem by moving the vortices away from the main wing, and rectangular non-swept wings experience STRONGER vortices than wings with swept wingtips... Dutch roll could of course be a problem. But do please elaborate on how sweeping the wings back would lead to it, as it's one of the concepts I've always struggled to wrap my head around the causes of, and my plane designs in FAR actually often struggle not to suffer from it... "Lower effective aspect ratio" is also a disingenuous answer. If you proportionally reduce wing chord such that the wing-area of the wingtips remains unchanged then it's exactly the same... So, two bad answers that are outright wrong (aspect ratio *doesn't* decrease if wing area and span are held constant, and wingtip vortices caused by spanwise flow are actually STRONGER with straight, rectangular wings than tapered ones) and one- about Dutch Roll- that I don't understand as well as I would like to and would like to know more about before I make up my mind about one way or another, in short... LOL! Just because you don't understand the answer doesn't make it wrong. Link to comment Share on other sites More sharing options...
mikegarrison Posted February 16, 2017 Share Posted February 16, 2017 (edited) It's actually really simple why Dutch roll is linked to swept wings. If the plane yaws a little to one side, that makes the wing on that side more swept and the wing on the other side less swept. So the less-swept side has more lift. So the plane rolls. Thus, a back and forth oscillation in yaw is amplified into also being a back and forth oscillation in roll. That's Dutch roll. Wing sweep is not the only source of yaw-roll coupling, so Dutch roll can happen in unswept wings too. But wing sweep is a powerful amplifier to it, which is why swept wing airplanes now universally have yaw-damping systems to avoid it. Edited February 16, 2017 by mikegarrison Link to comment Share on other sites More sharing options...
CSE Posted February 16, 2017 Share Posted February 16, 2017 (edited) On 15/02/2017 at 4:49 PM, KSK said: Unless we also know where the centre-of-mass is, then I don't see how we can decide whether moving the centre of lift back will improve stability or not. I think the CoM is at the payload attach point, inside the (plan view of the) centre spar. 12 hours ago, YNM said: TL;DR don't see this wing design as three separate wings. See it as a single wing. These two make a lot of sense to me. I'd want to couple the payload to the wing near the payload's CoM to give the least twisting moment on the coupling. I'd want the payload CoM near the carrier's CoM so there would be no dramatic control change on releasing payload. And if the payload is heavy then the CoL needs to be not very far behind the CoM to avoid an excessive pitch-down moment with payload attached. Put these together and you get a straight main wing and relatively small tailplane. (Or KSP canards.) Edited February 16, 2017 by CSE Link to comment Share on other sites More sharing options...
YNM Posted February 17, 2017 Share Posted February 17, 2017 6 hours ago, CSE said: I'd want to couple the payload to the wing near the payload's CoM to give the least twisting moment on the coupling. I'd want the payload CoM near the carrier's CoM so there would be no dramatic control change on releasing payload. And if the payload is heavy then the CoL needs to be not very far behind the CoM to avoid an excessive pitch-down moment with payload attached. Or you can just shift the payload around. Thought that should be easier really. But structural... you're limited. (unless you want to make a circular wing ‽) Link to comment Share on other sites More sharing options...
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