Q&A: Pegasus Designer Dr. Antonio Elias

• #320 by Lee Jay on 04 Oct, 2007 16:08
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  antonioe - 4/10/2007  8:54 AM
  I know of three alternatives to the wing (there may be others):

  1. Drop vertically (with a 'chute).  You loose a lot of altitude and most, if not all, of the productive, horizontal velocity of the carrier aircraft.
  2. Have the carrier aircraft have sufficient excess thrust to pull up to a high flight path angle before release.  My trade studies show to me that you're better off using the mass that that excess thrust costs you on a heavier rocket.
  3. Fly at scary flight path angles.  Lossy and risky.

  The original (pre-wing) design for Pegasus did (3)

  
  Is a "zoom launch" not possible with the carrier aircraft?
• #321 by aero313 on 04 Oct, 2007 16:22
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  antonioe - 4/10/2007  10:54 AM
  
  

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  pad rat - 4/10/2007 9:25 AM Yeah, I was told by one of our flight controls geniuses that the wing's sole purpose is to serve as the fulcrum on which the tail fins act to pitch the vehicle up.

  Oh, nonononono.... the wing is there for one thing and one thing only; LIFT.  

  
  Come on, Antonio.  You and I both know that the wing is there so that every magazine article written after the maiden flight would start out "The distinctive delta-winged Pegasus..."  ;-)
• #322 by antonioe on 04 Oct, 2007 16:50

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  yinzer - 4/10/2007 3:01 AM Had no idea what chapman's equation was

  Did you find a suitable tutorial on aerodynamic heat modeling?  If not, I can probably locate one for you.

• #323 by Propforce on 04 Oct, 2007 17:24
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  antonioe - 4/10/2007  9:50 AM
  
  

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  yinzer - 4/10/2007 3:01 AM Had no idea what chapman's equation was

  Did you find a suitable tutorial on aerodynamic heat modeling?  If not, I can probably locate one for you.

  
  You mean... you don't use Eckert's equations?  
• #324 by Propforce on 04 Oct, 2007 17:28
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  antonioe - 4/10/2007 7:54 AM

  Oh, nonononono.... the wing is there for one thing and one thing only; LIFT.  As a matter of fact, as I mentioned earlier, the original design did NOT have one; it earned its way into the design!  That lift does two inordinately important things for Pegasus:

  It rotates the flight path angle up (or, conversely, it prevents it from going further negative after drop).  Otherwise, you would have to use rocket thrust to do that, and that would both consume a lot of precious ΔV ("turning loss") and would require a scary (over 45 degrees at significant dynamic pressure) angle of attack to accomplish it

   

  Antonio,

  Thanks for a detailed response.  It's much appreciated.

  How would your design (the Pegasus) be different if you have a supersonic carrier aircraft available?

   Also, why the L1011?  That looks like an awfully big carrier for Pegasus.  Can you do it with a smaller carrier aircraft? 

   

• #325 by antonioe on 04 Oct, 2007 19:46
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  aero313 - 4/10/2007 11:22 AM

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  antonioe - 4/10/2007 10:54 AM

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  pad rat - 4/10/2007 9:25 AM Yeah, I was told by one of our flight controls geniuses that the wing's sole purpose is to serve as the fulcrum on which the tail fins act to pitch the vehicle up.

  Oh, nonononono.... the wing is there for one thing and one thing only; LIFT.

  Come on, Antonio. You and I both know that the wing is there so that every magazine article written after the maiden flight would start out "The distinctive delta-winged Pegasus..." ;-)

  You remind me of a design meeting in the earlier days of X-34A when Scott Frazier, an ardent "Big Dumb Booster" proponent, accused DWT and me of wanting wings on X-34A just for its "sex appeal".  He then banged his fist on the table and proclaimed:

  "And we're engineers - we're not interested in sex"

• #326 by Propforce on 04 Oct, 2007 19:51
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  antonioe - 4/10/2007 12:46 PM

  You remind me of a design meeting in the earlier days of X-34A when Scott Fraser, an ardent "Big Dumb Booster" proponent, accused DWT and me of wanting wings on X-34A just for its "sex appeal".  He then banged his fist on the table and proclaimed:

  "And we're engineers - we're not interested in sex"

  I am from old school, if it doesn't look good, it probably wouldn't fly too well. 

  Also, what was the story on X-34?  I bet you have lots of good story there :cool:

   

• #327 by antonioe on 04 Oct, 2007 21:44

• To all my friends and colleagues I wish to extend a most sincere Sputnik day greeting.

  Fifty years ago a rocket raised from the Kazakhstan steppes to change history, policy and the very fabric of human culture.  Those of us who have made space an avocation owe a debt of respect and gratitude to those whose efforts made our present (and future) possible, and today's celebration gives us a chance to express this gratitude.

  Antonio Elias

• #328 by CFE on 05 Oct, 2007 04:24
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  Propforce - 4/10/2007  11:28 AM
  
  

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  antonioe - 4/10/2007 7:54 AM

  Oh, nonononono.... the wing is there for one thing and one thing only; LIFT.  As a matter of fact, as I mentioned earlier, the original design did NOT have one; it earned its way into the design!  That lift does two inordinately important things for Pegasus:

  It rotates the flight path angle up (or, conversely, it prevents it from going further negative after drop).  Otherwise, you would have to use rocket thrust to do that, and that would both consume a lot of precious ?V ("turning loss") and would require a scary (over 45 degrees at significant dynamic pressure) angle of attack to accomplish it

  Antonio,

  Thanks for a detailed response.  It's much appreciated.

  How would your design (the Pegasus) be different if you have a supersonic carrier aircraft available?

  Also, why the L1011?  That looks like an awfully big carrier for Pegasus.  Can you do it with a smaller carrier aircraft?

  
  From what I've read, I'm operating under the belief that the L-1011 was chosen by virtue of its tall landing gear, minimizing ground clearance issues.
  
  In regards to Skyrocket's post about the Boeing AirLaunch (winged Athena-2) proposal, how do you feel about launching a Pegasus-type rocket from the upper surface of the mothership?  My impression is that most engineers are highly leery of this approach.  Even if there's merit to it, I think the crash of the M-21 Blackbird during the D-21 launch is enough to scare them away.
• #329 by Jim on 05 Oct, 2007 12:42
• L-1011 because it was available and cheap
• #330 by Skyrocket on 05 Oct, 2007 12:50
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  Jim - 5/10/2007  2:42 PM
  
  L-1011 because it was available and cheap

  
  and because it has no centerline main undercarriage like the DC-10, MD-10, MD-11, B-747
  
  
• #331 by Jim on 05 Oct, 2007 12:51
• Not all DC-10's had a centerline main undercarriage
• #332 by Skyrocket on 05 Oct, 2007 12:52
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  CFE - 5/10/2007  6:24 AM
  In regards to Skyrocket's post about the Boeing AirLaunch (winged Athena-2) proposal, how do you feel about launching a Pegasus-type rocket from the upper surface of the mothership?  My impression is that most engineers are highly leery of this approach.  Even if there's merit to it, I think the crash of the M-21 Blackbird during the D-21 launch is enough to scare them away.

  
  The Shuttle glide tests showed, that a deployment from the upperside is possible (although it is riskier than from the underside)
  
  
• #333 by Jim on 05 Oct, 2007 12:53
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  CFE - 5/10/2007  12:24 AM
   Even if there's merit to it, I think the crash of the M-21 Blackbird during the D-21 launch is enough to scare them away.

  
  Not really, remember orbiter-SCA?   M-21/D-21 was due to shock interaction, which is not present on the other concepts
• #334 by Skyrocket on 05 Oct, 2007 12:57
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  Jim - 5/10/2007  2:51 PM
  
  Not all DC-10's had a centerline main undercarriage

  
  You're right. DC-10 without centerline main undercarriage would have been possible like this conversion to fight wildfires shows:
  http://www.air-and-space.com/20050617%20Victorville/DSC_1842%20DC-10-10%20N450AX%20Tanker%20Air%20Carrier%20910%20left%20side%20l.jpg
  
• #335 by antonioe on 05 Oct, 2007 13:38

• You all had very good insights as to the selection of the L-1011.  By the way, our preferred approach was to continue to use DFRC's B-52, but the Center's management (at the time) made the decision that while they were glad to support "early development and use" of Pegasus they did not feel comfortable supporting "commercial operations" (a decision they would regret a short number of years later when they were struggling to justify replacement of the old B model with an H).

  When we realized we would have to aquire a carrier aircraft (or the services thereof) we let the word out to see if anybody would independently acquire such an aircraft, modify it, and lease or rent it to us, preferably "by the glass".

  We received a very small number of responses; one organization (a wealthy individual, mostly) proposed to buy a B-747SP and install a wing pylon; gave us a detailed tech presento, and it seemed feasible.  One catch though: the business deal, when translated into english, was equivalent to our funding the entire aircraft acquisition and mod through guaranteed usage, leaving him with absolutely zero risk.  What a deal (for him)!

  Dan Raymer, of AIAA design book fame, had just left (or was in the process of leaving) Lockheed Commercial and steered us towards the L-1011, performing the preliminary feasibility studies which centered on aircraft performance.  What was attractive about the L-1011 was that while still being a very capable aircraft, it was being retired in droves, which made its price very attractive.  We ended up buying an ex-Air Canada, ship number 1067, if I remember correctly (Lockheed began the L-1011 serial numbers at 1001, so our ship was the 67th off the line).  We paid $10.5M for it.  I signed the purchase contract for Orbital (the story of the purchase of the L-1011, the fun test flight with the Air Canada crew, my meeting in Murana, AZ our crew, including the legendary Bill Weaver and Johnny Lear - Bill Lear's equally famous son - will soon be an exciting episode of the "Saga of Pegasus", whenever I decide to get off my fat @$$ and resume writing it).

  It was only after we selected the L-1011 that we discovered a large number of coincidental design features that actually made the mod, at least from a structural standpoint, a lot easier than it could have been.  Amongst them was the fact that the L-1011 fuselage did not have a single KEEL LONGERON, the way the DC-10, B-747, etc. have, but TWIN "KEELSONS", that is, two longitudinal members at each side of the centerline, all along the length of the fuselage.

  And, as luck had it, the separation between these keelsons was perfect to hold the four-hook carriage assembly that held Pegasus through four rolling pins, whose separation had been designed years before the L-1011 entered the picture!!!  And I can go on...

  As I always said: the be successful, you need to be smart, work hard, and have luck.  Two out of three are insufficient.

• #336 by antonioe on 05 Oct, 2007 13:53

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  Skyrocket - 4/10/2007 10:23 AM The wing discussion reminds me on Boeings "AirLaunch System" concept, which surfaced some years ago.

  The graphic you show was definitely NOT produced by Boeing engineering - it looks more like a Photoshop marketing trick, where they took essentially a Pegasus and a B-747 and put one on top of each other.  Let me prove it:

  1. Pegasus' wing is mounted on the top of the rocket body to share the mechanical attachments to the carrier aircraft pylon.  On a top-mounted version, you would expect the wing to be mounted on the BOTTOM of the rocket body for precisely the same reasons.
  2. The Pegasus wing tips were clipped to fit the 22-ft wingspan limitation caused by the B-52's inboard engine nacelle (what a coincidence!  the X-15 wingspan was... 22 ft!).  On top of the 747 there is no need for that feature, yet the picture shows Pegasus-style clipped wingtips.
  3. One problem with a top mount (not the only one, alas) is that the rocket wing has to generate a LOT more lift that on a drop; in the latter case you can get by with about 0.4 g's of lift; on a top-mount you need at least 0.8 g's, and that assumes that the carrier aircraft performa a rather severe pitch-down simultaneously.  Therefore the wing of a top-mount air-launched rocket would be about twice the size, relatively speaking, of a bottom-drop one.
  4. Similarly, on a top mount, you need the rocket to have a definitely better L/D than the carrier aircraft, even if the latter deploys its gear and put on full flaps and spoilers to "dirty up" its L/D.  That would drive you to choose a much more aerodynamically efficient wing than Pegasus, with a much higher aspect ratio.  The picture's wing looks just too much like a bottom-dropped Pegasus.

  From this I deduce that the picture in question was untouched by Boeing engineering, who knows better (specifically, Dick Cervisi, whom I know and respect and who is no fool.)

  I think I also answered the question "why not mount on the top, where you don't have to mess around with ground clearance, etc".  Oh, there is also the slight complication of attaching, and then working on, a rocket many tens of feet off the ground and around some large obstacls such as wings.  You end up with a structure the size, complexity and cost of the Shuttle's Mate/Demate facility :frown:

• #337 by meiza on 05 Oct, 2007 14:07
• "to be successful, you need to be smart, work hard, and have luck.  Two out of three are insufficient." Is this original? Can I steal that?
• #338 by Skyrocket on 05 Oct, 2007 14:09
• Yes, the image looks indeed like it was produced by someone, who had a idea how a winged booster (i.e. the Pegasus) looks like and what are the dimensions of a Castor-120/Castor-120 core and applied this to some 3d-Software. BTW, a Pegasus-like supersonic-capable wing would not have been making sense for this vehicle, as the Wing in this concept would have been dropped before going supersonic. A high lift subsonic wing would have been the wing of choice, if this project had been put into reality.
  
  Aditionally the X-37 payload might have produced some additional aerodynamic trouble.
• #339 by antonioe on 05 Oct, 2007 14:53

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  meiza - 5/10/2007 9:07 AM "to be successful, you need to be smart, work hard, and have luck. Two out of three are insufficient." Is this original? Can I steal that?

  I think I made it up - at least I haven't heard it from anyone - but I'm willing to bet a chocolate milkshake somebody else said it before me.

  The fee for usage is 0.2 cents/repetition (plus the risk that somebody throws a bookend at you) - see my agent.