Q&A: Pegasus Designer Dr. Antonio Elias

• #420 by Skyrocket on 06 Dec, 2007 15:27
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  A_M_Swallow - 6/12/2007  5:19 PM
  
  The grey pipes on the side of the nozzles made it look like there was a second set behind.
  

  
  No, these are just the exhaust tubes from the turbo-pumps of the NK-33 engines
• #421 by Big Al on 06 Dec, 2007 17:07
• Another question here, I had read somewhere that because Pegasus is carried in an airplane with people in it ,that it has to be man rated. Any truth to this?
• #422 by edkyle99 on 06 Dec, 2007 17:25
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  CFE - 5/12/2007  11:57 PM
  
  I would guess that it's a custom-designed second stage.  I would even guess that ATK was the manufacturer (after all, who else is left to tackle a solid rocket this big?)
  
  As a wild guess, ATK may even be using a single segment from the shuttle's SRB as the basis for the Taurus-II (still hoping to make "Cygnus" official) second stage.

  
  I think the SRB segments are much too big for this application.  Taurus II appears to need a second stage motor that weighs somewhere in the 25-35 tonne range.  A single SRB segment contains nearly 125 tonnes of propellant.  
  
  A Minuteman first stage M55 motor is about the right mass at 23 tonnes, but it is a bit long (7.5 meters almost), probably doesn't have the right thrust profile - and probably isn't available for commercial use at any rate.  
  
  But what about solid motor clusters?  Four SR19 motors (Minuteman second stage) arranged to fire in parallel would be about right, but again probably wouldn't be available.  An SR19 is 4.12 meters long, 1.33 meters diameter, and weighs about 7 tonnes loaded.  I don't know if any ATK Orion motors could be used this way.  Orion 50XL is a bit too light to work in a set of four.  Orion 50SXL is too heavy to work in a cluster, too light to work by itself, and too long.  
  
  ATK has been working on modified Orion motors for the missile defense project, but I can't find any details on them.  Maybe something is spinning off from that massive program.
  
   - Ed Kyle
• #423 by crab nebula2 on 06 Dec, 2007 18:06
• On the Ares I threads there is much discussion of resonat combustion vibration problems with solid propellant rocket motors.  Did Orbital have to deal with this problem in developing Pegasus and Taurus?
• #424 by yinzer on 06 Dec, 2007 19:04
• I know the Taurus had to use devices to isolate the payload from the launch vehicle vibrations.
  
  As for the second stage, I'd think that the length/diameter ratio of most solid rocket motors designed for first stage applications would result in T/W ratios that were much too high.  What with the earlier talk about much cheaper and more effective thrust vector control systems and the relatively large size of the required Taurus II second stage, I'd suspect it's an all-new motor.
  
  On the other hand, I remember reading that ATK could vary the length of the Castor 120 by a fair bit while keeping the diameter fixed, and that could pretty easily give you a motor of about the right size and shape.
• #425 by antonioe on 07 Dec, 2007 00:25

• Quote

  crab nebula2 - 6/12/2007 1:06 PM On the Ares I threads there is much discussion of resonat combustion vibration problems with solid propellant rocket motors. Did Orbital have to deal with this problem in developing Pegasus and Taurus?

  Pegasus: no.   Taurus: yes (aero313: do you care to comment?)

   

• #426 by edkyle99 on 07 Dec, 2007 20:15
• Here is my guess-sketch of Taurus II showing approximately how a 45% shortened Castor 120 might fit into the 4.5 meter long upper stage section.  A 45% Castor 120 would be 4.5 meters long, including about 0.92 meters of nozzle length, and 2.36 meters in diameter.  That leaves 1.54 meters of width inside the second stage shroud, which might accommodate third stage bipropellant tanks and thrusters, etc..  In other words, the trim stage could be built around the Castor motor in the same way that a satellite propulsion bus is built around a kick stage motor.
  
  A 212 tonne first stage and a 23.7 tonne second stage could put 5.5 tonnes into low earth orbit assuming that a bipropellant trim third stage weighed about 0.5 tonnes at most.  Replacing most of the trim stage mass with a Star 48B provides enough delta-v to put at least 1.84 tonnes (the Delta 7925 GTO rating) into GTO, and possibly as much as 2 tonnes.
  
  My only concern about this guess is that the Castor motor would provide a real kick in the pants.  Initial thrust to weight would be 2.3-2.6, but this would increase to a fairly high number just prior to motor burnout - unless late-burn thrust can be tailored down sufficiently.
  
  Note:  Dimensions are my approximations based on scaling from the published Orbital Sciences drawing and on comments in this thread.  
  
   - Ed Kyle

  ---

  
• #427 by deiter on 08 Dec, 2007 15:09
• The Orbital site shows additional strap on boosters in the artistic concept drawing on the T2 page (http://www.orbital.com/AdvancedSpace/AdvancedLaunchSystems/TaurusII/). Does anyone know if this is planned for T2?
• #428 by antonioe on 08 Dec, 2007 16:08

• The initial design (over a year old by now) had a 3m diameter core, a single AJ-26 and 2 to 4 strapons.  The current design has a 3.9m core, two AJ-26's and no strap-ons (like the original Thor... but you know what eventually happened to that one...)

  That picture is - alas - over a year old... it's there to confuse the casual observer, not eagle-eyed NSF forum participants... :laugh:

• #429 by antonioe on 08 Dec, 2007 16:16

• Quote

  edkyle99 - 7/12/2007 3:15 PM Here is my guess-sketch of Taurus II...

  You are a meter or two short, but otherwise it's an excellent reverse-engineering job.  Your second stage is a bit heavy and, you are right, unless the design stresses low combustion rate (therefore lower thrust) it could impart too high an acceleration to the upper stack.  But it's amazing what solid motor designers can do to the thrust profile and burn rates!

  By the way, we only recently baselined the actual fairing profile...

  There is a story that when Grumman designed the Gulfstream II (the first jet version of the turboprop Gulfstream) engineering designed the entire aircraft EXCEPT the vertical tail.  They gave the VP marketing a "kit" consisting of the vertical tail area, approximate desired aspect ratio, and approximate desired location of the center of pressure.  Other than that, they said "have at it", in other words, it's purely a MARKETING shape... well, we did something of the same for the fairing... we prepared a "kit" of four fairing shapes that had essentially the same useable volume inside, similar aerodynamic and mass properties, and were about the same cost.  Which one to use is a "marketing" decision...

  We voted.  One individual, of course, had 50.1% of the vote... :laugh:

   

• #430 by aero313 on 08 Dec, 2007 18:26
• Quote

  antonioe - 6/12/2007  8:25 PM
  
  

  Quote

  crab nebula2 - 6/12/2007 1:06 PM On the Ares I threads there is much discussion of resonat combustion vibration problems with solid propellant rocket motors. Did Orbital have to deal with this problem in developing Pegasus and Taurus?

  Pegasus: no.   Taurus: yes (aero313: do you care to comment?)

  
  I already did in the Ares 1 thread.
• #431 by aero313 on 08 Dec, 2007 18:30
• Quote

  yinzer - 6/12/2007  3:04 PMOn the other hand, I remember reading that ATK could vary the length of the Castor 120 by a fair bit while keeping the diameter fixed, and that could pretty easily give you a motor of about the right size and shape.

  
  Don't confuse marketing with reality.  Even the relatively small stretch to the Pegasus motors to go from the standard to the XL version resulted in brand new motors, new tooling, new fabrication engineering, and another qual program.  ANYTHING you do to a solid rocket motor results in a brand new motor.
• #432 by CFE on 08 Dec, 2007 21:11
• Will the propellant tanks on the first stage be fabricated in-house, or will they be subcontracted?
• #433 by gospacex on 09 Dec, 2007 05:42
• Pegasus seems to ignite ~5 seconds after drop. By this time it accumulates ~50 m/s of speed in free fall. Why 5 seconds and not, say, 3?
• #434 by sitharus on 09 Dec, 2007 08:05
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  gospacex - 9/12/2007  6:42 PM
  
  Pegasus seems to ignite ~5 seconds after drop. By this time it accumulates ~50 m/s of speed in free fall. Why 5 seconds and not, say, 3?

  
  I've always assumed this was to give aircraft that dropped it time to clear the area. If something goes wrong and the rocket explodes you don't want debris hitting the launcher, especially as it has people on board.
• #435 by antonioe on 09 Dec, 2007 20:34

• Quote

  gospacex - 9/12/2007 12:42 AM Pegasus seems to ignite ~5 seconds after drop. By this time it accumulates ~50 m/s of speed in free fall. Why 5 seconds and not, say, 3?

  You are right about the 5 seconds, but not about the 50 m/s.  Right off the hooks, the wing produces something like 0.15 g's of lift (it has essentially the same angle of attack as the Carrier Aircraft), and a pitch-up maneuver starts immediately after release; by the time stage 1 ignites, the wing is producing over half a g of lift at about 9-10 degrees angle of attack.  Lift increases rapidly after ignition, so that angle of attack has to be reduced (else the trajectory turn upwards excessively).

  Actually, *I* did the safety analysis to compute the safe distance for ignition - it was my ticket to the Launch Panel Operator seat on flight 1!!!    If you simulate the trajectories of debris from a potential failure at start-up, it is easy to observe that the significant forward equivalent airspeed (read: dynamic pressure) distorts the particle paths (relatively to the moving airplane-rocket reference frame).  This effect significantly reduces the probability of a large enough particle hitting the carrier aircraft, so that 100 to 150 m was sufficient to reduce the probability of aircraft-downing damage below my chances of dying of natural causes during the flight.  At zero airspeed, or in vacuum, the safe distance for the B-52 "target size" would have been much larger. Also, part of the separation is caused by the carrier aircraft experiening a slight excess vertical acceleration after drop. The L-1011 is smaller than the B-52, and the dynamic pressure at release higher, but the XL stage 1 motor is larger than the original motor, so we ended up keeping the same distance.

  Actually, the IMU in Pegasus is monitoring the acceleration at separation and the integrated separation distance is one of the "yes" votes that must happen for ignition (the others begin related to attitude, electrical bus health, etc.)

  Without the wing lift, it would indeed fall ballistically (or nearly so); 3 seconds would have resulted in insufficient separation, more than 5 excessive energy loss.  To engineer is to compromise, and "optima" seldom exist.

• #436 by simonbp on 10 Dec, 2007 23:18
• So, here's the real question: Have you built your own paper model Pegasus and Stargazer?
  
  http://jleslie48.com/0304sg/Stargazer18.jpg">
  
  http://jleslie48.com/0304sg/pegasus04.jpg">
  
  1:96 scale: http://jleslie48.com/0304sg/
  
  1:48 scale (just the rocket): http://jleslie48.com/peg_picturemanual.zip
  
  1:48 scale (with Hyper-X): http://jleslie48.com/pegasus_hyper_x_2.pdf
  
  There's also a Minotaur available: http://www.ericksmodels.com/paper/models/enhminotaur/enhminotaur.html
  
  Simon
• #437 by gospacex on 11 Dec, 2007 00:01
• Is Stargazer at "max throttle" at Pegasus launch? (If not, why?) What is the Mach number? Do you dream of having a supersonic carrier aircraft? - for one, it can fly higher, reducing drag losses even further... and a bit of additional speed never hurts!
  
  Probably stupid idea, but - is it feasible to install some rocket thrusters on the (supersonic) carrier and boost speed (and/or pitch nose up) prior to Pegasus release?
• #438 by vt_hokie on 11 Dec, 2007 00:43
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  gospacex - 10/12/2007  8:01 PM
  
  Do you dream of having a supersonic carrier aircraft? - for one, it can fly higher, reducing drag losses even further... and a bit of additional speed never hurts!

  
  Just gotta avoid !
• #439 by kevin-rf on 11 Dec, 2007 11:56
• Actually, somewhere in the thread Antonio discussed why they did not go with a supersonic carrier aircraft... Have fun rereading the thread :-)