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Thread: Why Pegasus?

  1. #1
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    Why Pegasus?

    I've seen several threads over the years discussing air-launched rockets, and was under the (possibly mistaken) impression that there wasn't a compelling reason for such a system.
    And yet, here is Pegasus.

    According to the article, this mission will be launched from 39,000 feet, near the equator.
    What is the big upside here? Is it the ability to launch from virtually any latitude?
    I may have many faults, but being wrong ain't one of them. - Jimmy Hoffa

  2. #2
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    Yes and two other points : The launch occurred high above any bad weather so your window of opportunity is wider. And you can plan to have the first stage go back in a part of the sea where there is no risk of hitting a fishing boat or else.

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    And it's the only LV currently on NASA's approved list for primary payloads that small.

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    And it's a small payload on a fast launcher that can be launched from a mobile military aircraft platform from anywhere in the world into orbital or suborbital trajectories to anywhere in the world... you do the math.
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    Actuall Stargazer isn't a 'mobile military aircraft'.

    It's a converted Tristar that used to fly as a commercial passenger aircraft for Air Canada. It did launch from B52's early on, but more than 90% of its flights have been from the L-1011 converted Tristar.

    The reason for Pegasus existing isn't military.

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    Quote Originally Posted by djellison View Post
    Actuall Stargazer isn't a 'mobile military aircraft'.

    It's a converted Tristar that used to fly as a commercial passenger aircraft for Air Canada. It did launch from B52's early on, but more than 90% of its flights have been from the L-1011 converted Tristar.
    Ah, my mistake. I thought it could still be launched from B-52 aircraft

    The reason for Pegasus existing isn't military.
    And squirrel guns can only be used by squirrels. Got it.
    Et tu BAUT? Quantum mutatus ab illo.

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    Quote Originally Posted by Extravoice View Post
    I've seen several threads over the years discussing air-launched rockets, and was under the (possibly mistaken) impression that there wasn't a compelling reason for such a system.
    I don't know how you are applying "compelling" to the reasoning. I remember a lot of discussion about the sizes of payloads and how feasable some of the applications are.

    I think the limitation is purely the payloads. Pegasus is great for these small payloads (980lbs) but I think it would be very difficult to scale up.
    Even if we build a system to be carried by the AN-225's upper limit, we are up to about 5 tons to LEO. I don't know how useful that would be compared to conventional systems once the cost of the larger booster is considered.

    I don't know how much flexibility is really needed for launch locations or windows. Certainly an air launched system increases the launch windows and locations to almost anything. Although launch windows are important, I would think most payloads could wait for optimal weather and windows.

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    Quote Originally Posted by NEOWatcher View Post
    but I think it would be very difficult to scale up..
    http://www.stratolaunch.com/

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    I think a thread in this forum about stratolaunch was where I got the impression that air-launched rocketry was not considered practical.
    I may have many faults, but being wrong ain't one of them. - Jimmy Hoffa

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    Well clearly it IS practical as it's being done regularly by Pegasus, and investors are backing it for launches an order of magnitude larger.

    Now - does it make financial sense as the sizes that Stratolaunch are considering? Personally, I don't think so.

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    5 tons to LEO might work for a small aerospace-taxi intended for a short duration flight to a space station. Is that what Stratolaunch is looking at, cause I didn't find details on their site. (Wikpedia says 6100 kg, even better.)
    Et tu BAUT? Quantum mutatus ab illo.

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    Quote Originally Posted by djellison View Post
    Yes; I know about that. I just didn't have any numbers to use, and AN225 is something that people have actually seen. I said "difficult" not "can't".

    Which matches up precisely with your statement:
    Quote Originally Posted by djellison View Post
    Now - does it make financial sense as the sizes that Stratolaunch are considering? Personally, I don't think so.

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    You can also get some extra horizontal velocity component from the carrier aircraft on top of the Earth's rotational contribution.

    And Pegasus just delivered NuSTAR into orbit.


    Disclosure:..., but I've never worked on either Pegasus or NuSTAR. :-)

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    Quote Originally Posted by sts60 View Post
    You can also get some extra horizontal velocity component from the carrier aircraft on top of the Earth's rotational contribution.

    And Pegasus just delivered NuSTAR into orbit.


    Disclosure:..., but I've never worked on either Pegasus or NuSTAR. :-)
    Too bad it's so hard to get a really fast carrier aircraft and separation so that the speed increase from standstill was more than negligible.
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    The Pegasus user's guide specifies a standard launch profile of 39,000 ft and Mach 0.82, which converts to about 540 mph. Since NuStar was launched from the equator, it gets about 1,000 mph from the Earth's rotation, so there is roughly a 50% additional gain from the aircraft given its low inclination (6 degrees). That's nothing to sneeze at, even though it is not a primary reason for using the launcher and is only a few percent of orbital speed; every little bit helps.

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    I wonder if the old XB-70 could be used or modified as a carrier aircraft. It was designed to be a bomber of sorts and is, at least by aircraft standards, very, very fast.

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    Cheaper than just buying a Falcon 9 off the shelf? I doubt it.

    XB70'd payload is about 22.5 tons. Pegasus is 18 - 23tons (depending on config)

    Once you add the drag of it being onboard and handling hardware etc - I think it would be pretty marginal

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    Yeah, you are probably right. On the other hand, the fact we can say 'buying a Falcon 9 off the shelf' is pretty exciting, no?

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    Heck yeah

    Actually, a Falcon 1e could loft more than a Pegasus for something like $10-11M.

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    Quote Originally Posted by sts60 View Post
    The Pegasus user's guide specifies a standard launch profile of 39,000 ft and Mach 0.82, which converts to about 540 mph. Since NuStar was launched from the equator, it gets about 1,000 mph from the Earth's rotation, so there is roughly a 50% additional gain from the aircraft given its low inclination (6 degrees). That's nothing to sneeze at, even though it is not a primary reason for using the launcher and is only a few percent of orbital speed; every little bit helps.
    Right, my calculations come in at roughly 3% of LEO speed, but I neglected to add in the additional speed from an equatorial launch versus a launch at higher latitude.
    Et tu BAUT? Quantum mutatus ab illo.

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    Showing my ignorance of orbital mechanics, I'll ask: Is there a potentially substantial fuel savings due to the ability to launch from virtually any latitude (and therefore orbital inclination), compared to launching from a fixed site and adjusting the orbit through brute force?
    I may have many faults, but being wrong ain't one of them. - Jimmy Hoffa

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    Quote Originally Posted by Extravoice View Post
    Showing my ignorance of orbital mechanics, I'll ask: Is there a potentially substantial fuel savings due to the ability to launch from virtually any latitude (and therefore orbital inclination), compared to launching from a fixed site and adjusting the orbit through brute force?
    Probably more of a savings in fuel on the satellite itself than in launch fuel. Although if it allows you to reduce the launch mass of the satellite, that would translate into additional savings. Launching from the equator was one of the main attributes of Boeing's Sea Launch program, which turned out to be a commercial failure.
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    Quote Originally Posted by ravens_cry View Post
    I wonder if the old XB-70 could be used or modified as a carrier aircraft. It was designed to be a bomber of sorts and is, at least by aircraft standards, very, very fast.
    The soviets were working on something along those lines: the MiG-105
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    Quote Originally Posted by ravens_cry View Post
    I wonder if the old XB-70 could be used or modified as a carrier aircraft. It was designed to be a bomber of sorts and is, at least by aircraft standards, very, very fast.
    Quote Originally Posted by djellison View Post
    XB70'd payload is about 22.5 tons. Pegasus is 18 - 23tons (depending on config)

    Once you add the drag of it being onboard and handling hardware etc - I think it would be pretty marginal
    Best thing is that the XB-70 was the first waverider design, so that drag might not be that much of a problem.

    And launching at Mach 3 would shave off a bit more weight from the Pegasus.

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    Would the heat from friction at those speeds have a negative effect on the Pegasus's fuel?

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    I don't think so. Considering that the Pegasus uses solid fuel that burn much hotter.

    There's also that the XB-70 may not heat up that much, since its a waverider design. I expect that the payload, in the case the Pegasus, will be kept out of the high pressure air that heats up the plane.

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    Quote Originally Posted by Warringer View Post
    I don't think so. Considering that the Pegasus uses solid fuel that burn much hotter.

    There's also that the XB-70 may not heat up that much, since its a waverider design. I expect that the payload, in the case the Pegasus, will be kept out of the high pressure air that heats up the plane.
    Piggy-back instead of belly-slung? As long as the vehicles can survive separation at such a high speed. I wonder if giving the mothership an extra kick in speed and altitude with rockets would be worth it.
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    Quote Originally Posted by NEOWatcher View Post
    I don't know how you are applying "compelling" to the reasoning. I remember a lot of discussion about the sizes of payloads and how feasable some of the applications are.

    I think the limitation is purely the payloads. Pegasus is great for these small payloads (980lbs) but I think it would be very difficult to scale up.
    Even if we build a system to be carried by the AN-225's upper limit, we are up to about 5 tons to LEO. I don't know how useful that would be compared to conventional systems once the cost of the larger booster is considered.

    I don't know how much flexibility is really needed for launch locations or windows. Certainly an air launched system increases the launch windows and locations to almost anything. Although launch windows are important, I would think most payloads could wait for optimal weather and windows.



    I am not sure if it should be noted but the Pegasus rocket was an "initial DARPA" project. --And as with a sizable number of their projects---I have "heard" it described that DARPA lost its control of the project and it was handed over to the Air Force or NASA--for that matter.

    But the document that I am quoting from states that since its initial launch--the "pegasus rockets" have put some 38 satellites into orbit (up until 2007).


    Here is the image (?) of its maiden flight---> Courtesy DARPA

    I have been unable to load the image I will try in a subsequent post.

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    Quote Originally Posted by John Jaksich View Post
    I am not sure if it should be noted but the Pegasus rocket was an "initial DARPA" project. --And as with a sizable number of their projects---I have "heard" it described that DARPA lost its control of the project and it was handed over to the Air Force or NASA--for that matter.

    But the document that I am quoting from states that since its initial launch--the "pegasus rockets" have put some 38 satellites into orbit (up until 2007).


    Here is the image (?) of its maiden flight---> Courtesy DARPA

    I have been unable to load the image I will try in a subsequent post.
    As stated I haven't been able to manipulate the jpeg files so----> here is a link to CalTech-> and the NuStar launch vehicle using a pegasus rocket.


    http://www.nustar.caltech.edu/news/5...etail-template

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