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#300 by CFE on 29 Sep, 2007 00:26
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Do you put any credence in the studies which show cost savings from using the same propellants in all stages of your launch vehicle? I tend to think that a LOX-Kero first stage with solid upper stages would achieve most of the operational cost benefits of an all-LOX/Kero vehicle, because the upper stages won't need to be fueled at the launch site.
I assume that a solid upper stage would be a new development, assuming a 4-meter constant diameter. -
#301 by antonioe on 29 Sep, 2007 04:34
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CFE - 28/9/2007 7:26 PM Do you put any credence in the studies which show cost savings from using the same propellants in all stages of your launch vehicle?
Sure! Ceteris paribus, indeed it is (you have all the necessary infrastructure there already, empty LOX/Kerosene stages are relatively light, performance is not too shabby, etc. etc.). Problem is, we could not find a suitable engine (using a rather extensive set of criteria for "suitable", some of which I am not at liberty to express until after PDR, but which will eventually become obvious).
That is the main problem with "theoretical" dictums: they are O.K. as long as all the other variables are kept constant. In the real world, muda happens and you end up having to do "suboptimal" things. But I love that - that's why I'm an engineer and not a scientist.
QuoteI tend to think that a LOX-Kero first stage with solid upper stages would achieve most of the operational cost benefits of an all-LOX/Kero vehicle, because the upper stages won't need to be fueled at the launch site.
Yes, but on the other hand, they are heavier to handle, have a lower Isp (but reasonable mass fractions and oodles of thrust!) and the recurring cost can be surprisingly steep. There is another, perhaps more esthetical, reason to use the same technology on all stages: the design is "balanced" in terms of performance... now, there are many sucessful counter-examples, but I believe in Von Karman's dictum that if it looks right, it probably IS right (he was thinking about aircraft aerodynamics when he said that, but I think it applies to almost anything.)
Anyway, here I am with a LOX/Kerosene S1, begging for a Cryo US, and most likely ending up having to have a solid... so much for theoretical considerations... RATS!
QuoteI assume that a solid upper stage would be a new development, assuming a 4-meter constant diameter.
As in the Pegasus Stage 3, it could have a smaller outer diameter than the airframe without necessarily requiring a neckdown (it costs a few Kg, though). A 4 m diameter solid upper stage with the right amount of propellant mass would be too flat (as in "pancake") to be practical, anyway (not impossible, just impractical). A more important issue is "is there a solid out there with the right amount of propellant"?
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#302 by Skyrocket on 01 Oct, 2007 15:46
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As i just got the latest Orbital press release on the successful flight of an OBV vehicle some questions arose:
Is the OBV supposed to get a "real" name sometimes or will it just retain the three letter acronym? All other of your major vehicles have nice mythological.
By the way: how did the first OBV prototype mission ("Taurus-Lite") differ from the operational OBV vehicle. The images i have seen on the MDA site look like a OBV with Pegasus payload shroud.
Was "Taurus-Lite" the launch vehicle name or was it a OBV, which flew a mission called "Taurus-Lite"?
( http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=10010&posts=1&start=1 - James Lowe)
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#303 by JIS on 01 Oct, 2007 16:09
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Regarding current talk about Ares 1 1stg thrust oscillations I'm wondering how serious this effect is on Orbital launchers. Is a ride on the single solid motor much more “rough” than on the liquid stage? Also what are the requirements for the roll control? Are the roll torques the same every flight or do they differ significantly? I've heard that the roll control doesn't scale with motor length but rather with diameter. Also bigger motors tend to be more stable in roll. Could you provide sime insight into this problem dear Dr. Elias?
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#304 by antonioe on 02 Oct, 2007 00:38
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Skyrocket - 1/10/2007 10:46 AM As i just got the latest Orbital press release on the successful flight of an OBV vehicle some questions arose: Is the OBV supposed to get a "real" name sometimes or will it just retain the three letter acronym?
Yes, it is an acronym. "In the beginning" (as the HHGTTG would say) GMD (Ground-based Missile Defense) Prime Contractor Boeing created (with a little prodding from Orbital's friends in high places) a competition for "Alternative Boost Vehicle" just in case (ahem!) the baseline Boeing-built Boost Vehicle (the "BV") ran into... well... "additional trouble".
Two companies bid the ABV competition: Lockheed and Orbital. And Boeing giveth them names, and the Lockheed offering he nameth the "Lockheed Boost Vehicle" (LBV) and the Orbital he nameth... well... the "Orbital Boost Vehicle" (OBV)... Duh!
And Boeign evaluateth the Orbital proposal, and saw that it was good. And Boeing then sayeth upon Lockheed: "Oh good and faithful bidder: although ye have lost the Alternative Boost Vehicle competition, Lo! I shall completely forgo the manufacturing of any hardware element of the GMD system, and I shall entrust to you the task of finishing the development of the baseline BV, which thou shall improve in performance and reliability. Take the sacred paper bag of parts, be fruitful and add, and it shall be called the "BV Plus".
But the OBV kept outperforming the BV, even in its "plus" version, and Boeing decideth that the OBV shall be the PRIME Boost Vehicle, and BV Plus the BACKUP. Then, after a number of test flights, it became apparent that since the OBV was delivering as promised, there was little point in keeping two boost vehicle programs running, and Boeing terminated BV Plus (Gunther: you still carry the OBV as the "alternate" in your site... well, the "primary" does not exists any more!)
I doubt the OBV will ever be recognized as an individual vehicle: it's part of the GMD interceptor (add the kill vehicle); maybe the entire combo (OBV + KV) will one day have a nickname... "Falcon"? (everything seems to be called "Falcon" these days... F-16, DARPA's ORS rocket effort, SpaceX's rockets, other things...)
Quotehow did the first OBV prototype mission ("Taurus-Lite") differ from the operational OBV vehicle. The images i have seen on the MDA site look like a OBV with Pegasus payload shroud
OBV was to be the penultimate "Pegasus without wings". But, in addition to removing the wings, OBV had to differ from Pegasus is many other respects, such as ground- (and eventually, silo-) launch. So we proposed to quickly modify a Pegasus to a ground-launch configuration and launch it from a surface position to demonstrate its performance, etc. before embarking on additional mods; and frankly, also to prove to Boeing and MDS that little Orbital could make all these changes in a short time - indeed, "Taurus-lite" flew something like 13 months after contract award. And, believe me, going from an air-launch to a ground launch is not a trivial change! (environments, sea-level ignition of Stage 1, aero and mechanical loads, ground support equipment, software, etc. etc!....)
Since the prototype OBV used the TVC-ed version of the pegasus S1 motor that Taurus used, it was dubbed "Taurus-lite", even though it used a conventional Pegasus fairing. Only one was built.
Trivial pursuit question #17: Why does the Pegasus fairing have a rounded shape, when all other Orbital LV's (and most other LV's, by the way) seem to have a bi-conic or poly-conic shape? (Note: answering "because it's air launched" is not enough... you have to explain why).
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#305 by antonioe on 02 Oct, 2007 00:51
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JIS - 1/10/2007 11:09 AM Regarding current talk about Ares 1 1stg thrust oscillations I'm wondering how serious this effect is on Orbital launchers.
I'm not an expert on this subject but I have not heard anybody loose any sleep about it.
QuoteIs a ride on the single solid motor much more “rough” than on the liquid stage?
Yes, esp. ground acoustics. Before we launched our first Taurus, nobody had ever ground-launched a Peackeeper Stage 1, and that worried everybody. The larger solids (e.g. the Shuttle SRB's) use some form of pad acoustics suppression system (e.g., water deluge). Taurus does not, but has other methods of tackling vibration that I cannot disclose in a public site.
QuoteAlso what are the requirements for the roll control? Are the roll torques the same every flight or do they differ significantly? I've heard that the roll control doesn't scale with motor length but rather with diameter. Also bigger motors tend to be more stable in roll. Could you provide sime insight into this problem dear Dr. Elias?
It stands to reason from f=ma that larger diameter rockets
- Have a much larger roll moment of inertia (goes as the SQUARE of the radius, therefore of the diameter), thus react more slowly to roll disturbances, and
- A roll control RCS has a larger moment arm, therefore more effectiveness.
Yes, this is textbook physics; I can't go any further without asking our ITAR people to check it, and I'm too lazy to do so, so please don't dig too deep into technical details, please. I'll be glad to exchange e-mails on unclassified detailed technical subjects with anyone I can prove is a US citizen.
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#306 by yinzer on 02 Oct, 2007 02:22
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antonioe - 1/10/2007 5:38 PM
Trivial pursuit question #17: Why does the Pegasus fairing have a rounded shape, when all other Orbital LV's (and most other LV's, by the way) seem to have a bi-conic or poly-conic shape? (Note: answering "because it's air launched" is not enough... you have to explain why).
Mach and alpha at max-q? Estimating based on what's in the mission planner's guides, the Atlas V 552 sees 900 psf at about Mach 1.3, while the Pegasus sees 1500 psf at nearly Mach 4. Atlas also flies a load relief trajectory while Pegasus is pulling up.
So I'd guess some combination of better heating characteristics due to the detached shock, reduced lift forces at non-zero angles of attack, maybe better acoustics. I would say sacrificing transonic drag for supersonic drag, but don't know enough to back that up. I'm almost certain it's due to the difference in conditions at max-q though. -
#307 by antonioe on 02 Oct, 2007 02:28
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warm... warm...
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#308 by yinzer on 02 Oct, 2007 02:41
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The only other things I can think of are small size and the various square-cube laws that go with it such as the tradeoff between skin friction drag and wave drag, and maybe the shape of the dynamic pressure vs. time curve (longer and smoother rather than the big peak and sharp drop common to ground launched vehicles).
But at this point, I'll let other people guess. -
#309 by pippin on 02 Oct, 2007 08:13
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Less drag and less disturbancees at low speeds (around ignition) to keep it controllable before, at and shortly after ignition?
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#310 by JIS on 02 Oct, 2007 10:36
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antonioe - 2/10/2007 1:38 AM
What about interaction with mother aircraft?
#17: Why does the Pegasus fairing have a rounded shape, when all other Orbital LV's (and most other LV's, by the way) seem to have a bi-conic or poly-conic shape? (Note: answering "because it's air launched" is not enough... you have to explain why). -
#311 by JIS on 02 Oct, 2007 11:38
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antonioe - 2/10/2007 1:51 AM
p>Yes, this is textbook physics; I can't go any further without asking our ITAR people to check it, and I'm too lazy to do so, so please don't dig too deep into technical details, please. I'll be glad to exchange e-mails on unclassified detailed technical subjects with anyone I can prove is a US citizen.
Thanks for answer. I'm happy with general discussion. -
#312 by simonbp on 02 Oct, 2007 22:45
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Higher dynamic pressure -> needs a structurally stronger nose -> a hemisphere is stronger than a biconic per unit mass?
Simon
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#313 by antonioe on 04 Oct, 2007 03:03
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Thanks for playing "Pegasus trivial pursuit". Jack came closest to the original design intent, although he did use a little bit of a "shotgun technique" at the end:
Quoteyinzer - 1/10/2007 9:22 PM Mach and alpha at max-q?
YES! (at least the alpha part). When I started flying Pegasus trajectories, first on my modest 3DOF sim written in Microsoft C, then on PC-POST (1987!!!) I started seeing q-alphas (product of dynamic pressure times angle of attach) of 20,000 psf-degrees or so. Holy Reynolds Number, Batman! That means that even if the vehicle is aerodynamically stable, small perturbations in the airflow can cause significant aerodynamic moments, even if they are eventually counterbalanced by, say, the moment from the tail that tends to stabilize the motion.
Now, a biconic shape has much lower drag at zero alpha, and is inmensely easier to construct. But my fear was that at the very high q-α's, a small flow detachment would cause a major transient in the vehicle's attitude. Also, when attached to the airplane, the rocket would NOT be at zero alpha, and the slope of the alpha/lift curve for a biconic nose shape is higher than a spherical nose, and so is its drag (that's why subsonic airliner noses are round, not pointy) so a rounded-nose Pegasus would twist the B-52's rather flexible wing less than a biconic-nose one. And Pegasus, with is low Aspect Ratio wing, pretty lousy drag charateristics anyway (the reduced gravity losses from the wing's lift more than offsets that, though...)
In practice, we never flew Pegasus at such ridiculous q-α's, so it would probably have been OK if it had a biconic nose. However, the round nose works, and you know the cardinal rule in rocketry: if it works, leave it alone!
As it turns out, the round nose also reduces significantly the aerodynamically induce noise, esp, at high q-α's. That, and the air launch, makes Pegasus one of the acoustically quietest, if not THE most quiet ride to space (well, I have not compared user's guides... now somebody is going to do just that and call me a liar...) Also, it's quite volumetrically efficient...
Now, for the shotgun part:
QuoteSo I'd guess some combination of better heating characteristics due to the detached shock
Oh, come on! the benefits of detached shocks start becoming significant at high Mach numbers, say, 10 and above! Remember Chapman's equation... v cubed!!!
Quotereduced lift forces at non-zero angles of attack,
Yes...Quotemaybe better acoustics.
Yes, another hit...QuoteI would say sacrificing transonic drag for supersonic drag
Hmmm... yes, but mostly under the aircraft - not in free flight - and due to the non-zero alpha condition there...You missed "it looks cool"...
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#314 by Propforce on 04 Oct, 2007 05:07
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Antonio,
A curious question on the Pegasus wing. I appologize if you've covered this in this thread but I have not gone through the entire 20 pages of posts.
Why the need for delta-wing design? Is it needed to stabilize the rocket after separation from the L1011 and before the SRM light? I've actually hear rumor that the wing was needed for NATO treaty reason but not for technical reason.
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#315 by yinzer on 04 Oct, 2007 08:01
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Hah! A couple of hits from the shotgun technique isn't so bad. Had no idea what chapman's equation was, but thanks to your mention the next time the shotgun blast will be even more accurate.
I think I've said it before, but I think it's great that you come here and answer questions and talk about this stuff. I learn a lot and I'm sure others do too. I hope you keep coming back and wish you the best success in all your endeavors, especially the Taurus II or whatever it ends up being called. LDCM is paying $124M - you gotta be able to undercut that. -
#316 by Jim on 04 Oct, 2007 11:35
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Propforce - 4/10/2007 1:07 AM
Antonio,
A curious question on the Pegasus wing. I appologize if you've covered this in this thread but I have not gone through the entire 20 pages of posts.
Why the need for delta-wing design? Is it needed to stabilize the rocket after separation from the L1011 and before the SRM light? I've actually hear rumor that the wing was needed for NATO treaty reason but not for technical reason.
it is for the pullup -
#317 by antonioe on 04 Oct, 2007 14:54
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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. 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.
Even a simple 3DOF trajectory sim will convince you that even a wing with pitiful L/D as Pegasus', if properly sized (about 0.4 g's of lift at separation conditions and reasonable alpha) more than pays for its weight and drag costs.
I know of three alternatives to the wing (there may be others):
- 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.
- 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.
- Fly at scary flight path angles. Lossy and risky.
The original (pre-wing) design for Pegasus did (3)
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#318 by antonioe on 04 Oct, 2007 14:57
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yinzer - 4/10/2007 3:01 AM LDCM is paying $124M
For an EELV? Actually, I think that's a bargain!!!
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#319 by Skyrocket on 04 Oct, 2007 15:23
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The wing discussion reminds me on Boeings "AirLaunch System" concept, which surfaced some years ago. (A winged Castor-120/Castor-120/Star-92 stack)
In this case, the wings served mainly to lift the vehicle from the back of the B-747 carrier aircraft. The whole Wing-and-Fin assembly was to be jettisoned about five seconds after ignition.
See https://research.maxwell.af.mil/papers/ay2004/afit/AFIT-GAE-ENY-04-M04.pdf
