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Thread: Weather balloons into space.

  1. #61
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    Hop_David, I realize you are frustrated, but you can still be polite while expressing your frustration. Hornblower has apologized for his error; let's end this with your accepting it.
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  2. #62
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    I think part of confusion in regards to whether getting above the atmosphere would offer significant savings is that trips to the Moon which was in the original question are being conflated into just getting to LEO.
    Getting to the Moon requires such a large propellant load that the savings of high altitude launch are relatively small.
    However, if what you want is just to get to LEO then the savings are significant if you could find a way to loft large rockets to high altitude without using propellant.


    Bob Clark

  3. #63
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    Quote Originally Posted by Hornblower View Post
    In the process of evaluating jraitchi's post which was a direct response to the OP,
    In addition to responding to the OP he made numerous comments on what he thought was being said. His perception that I was advocating balloon launch is wrong.

    Further, he stated altitude confers insignificant benefit. He mentioned 200 km altitude. This is also wrong. According to my model, launching the Saturn V from 200 km would've conferred an extra 1.1 km/sec which would result in a mass savings of 30 to 50%.

    Quote Originally Posted by Hornblower View Post
    I lost track of who said what elsewhere in the thread. My bad, my apology.
    This thread has a low signal to noise ratio so it's understandable that you were confused. Apology accepted.

  4. #64
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    Quote Originally Posted by RGClark View Post
    I think part of confusion in regards to whether getting above the atmosphere would offer significant savings is that trips to the Moon which was in the original question are being conflated into just getting to LEO.
    Getting to the Moon requires such a large propellant load that the savings of high altitude launch are relatively small.
    However, if what you want is just to get to LEO then the savings are significant if you could find a way to loft large rockets to high altitude without using propellant.


    Bob Clark
    It's a common misperception that adding 1 km/sec to a large delta V budget would make little difference. Some would expect the difference between a 19 and 20 km/sec delta V budget to be 5%. This is not the case.

    Here's a spreadsheet showing what adding 1.1 km/sec does to various delta V budgets. The right delta V budgets take the budgets to the left and adds 1.1.


    You will see that adding 1.1 to the larger delta V budgets also increases the mass ratio by approximately 30 to 50 percent.

  5. #65
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    Quote Originally Posted by Hop_David View Post
    It's a common misperception that adding 1 km/sec to a large delta V budget would make little difference. Some would expect the difference between a 19 and 20 km/sec delta V budget to be 5%. This is not the case.
    ...
    You will see that adding 1.1 to the larger delta V budgets also increases the mass ratio by approximately 30 to 50 percent.
    Hmm. You may be right. The difference is even worse when you take into account the high altitude allows you to get close to full vacuum Isp.
    A LH2/LOX engine might get 455 s vacuum Isp. But because of the reduction of the Isp at sea level the average Isp over the flight might be 420 s.
    Then a 20,100 s delta-V requirement at a 420 s average Isp would give a mass ratio of e^((20,100/420*9.8)) = 132. But if you subtract off 1,100 m/s and use the full vacuum Isp, then you get a mass ratio of e^((19,000/455*9.8)) = 71.

    Bob Clark

  6. #66
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    Quote Originally Posted by RGClark View Post
    Hmm. You may be right. The difference is even worse when you take into account the high altitude allows you to get close to full vacuum Isp.
    For the Saturn V lower stage, vacuum ISP differs from sea level by 17%. This was accounted for in my model Saturn V spreadsheet. The accelerations in my model are pretty close to the actual accelerations with the exception of the first part of the flight. My model starts with 17% more acceleration. But this advantage tapered off after a few minutes.

    Rocket acceleration a is a vector quantity. I'll denote the magnitude of this vector as |a|.

    The angles in my spreadsheet are the ship's angle from horizontal at different times in the horizontal burn. The horizontal component of a has magnitude |a|cos(angle). The vertical component has magnitude |a|sin(angle).

    Throughout the first part of the 11 minute burn I kept the vertical component a a little bit more than 9.8 meters/sec^2 so the ship could maintain altitude. I show a total gravity loss of .6 km/sec. Actual Saturn V gravity loss was 1.5 km/sec. This is a savings of .9 km/sec. Overall my final horizontal velocity was 1.1 km/sec greater than what the actual Saturn V achieved after 11 minutes.

    So my model takes into account vacuum ISP as well as shorter vertical ascent.

    Moreover, on an airless world acceleration along horizontal tracks to achieve orbital velocity is possible. On earth's surface, the troposphere forbids this scheme. Accelerating along horizontal tracks would cut the 1.5 km/sec gravity loss to virtually zero. On horizontal tracks, the acceleration could be provided by an external power source rather than relying on reaction mass that must be carried aboard the ship. This changes the game dramatically. On airless worlds, SSTO RLVs are far more plausible than here on earth. On the moon, reusable launch vehicles that move between the moon's surface and EML1 are plausible even without railguns.

    I will say it again: Earth's atmosphere is a very significant factor in missions to LEO and beyond.

  7. #67
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    I have seen enough well-informed "rocket science" in this thread to conclude that lifting any given rocket to a very high altitude with a balloon before lighting it off will increase the payload it can deliver to escape velocity.

    Philippe's friend insists he can build something in his back yard that could reach the Moon if is aim is accurate enough. Surely he is not contemplating a manned mission. Such a vehicle would be too big to escape detection by authorities who would lawfully stop it before it is anywhere near ready for launch. However, a vehicle weighing a ton could be lifted to 100,000 feet with existing balloons which technically could be launched from a one-acre lot. That would require no more audacity than demonstrated by that family who escaped from East Germany in a hot air balloon under cover of darkness.

    Can we prove that such a stunt is impossible? Only if we are sure of his limitations. With an unlimited budget, sufficient technical and machine shop knowhow, and access to the necessary propellants, it would in theory be possible. As a practical matter I would guess next to impossible for a solo operator.

  8. #68
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    Quote Originally Posted by Hornblower View Post
    I have seen enough well-informed "rocket science" in this thread to conclude that lifting any given rocket to a very high altitude with a balloon before lighting it off will increase the payload it can deliver to escape velocity.
    No, that's not right.

    For a balloon to lift a rocket to a given altitude, it has to be buoyant at that altitude. That is, the overall density must be even less than the tenuous air at that altitude.

    At 35 kilometers, air density is around 1/100 of sea level. Very difficult to imagine a balloon plus a sizable payload having density less than this. I would imagine 35 km is about the ceiling. But the air at 35 km is still too dense to move orbital velocity through, you'd burn up. It would be better if you achieved orbital velocity higher up, more like 100 km. So your vertical ascent is only slightly smaller and you get a limited gravity loss benefit. My guess is that 35 km would cut .4 km/sec off the vertical ascent penalty. Depending on how fast your rocket gains altitude, you'd still need 1 to 1.5 km/sec to climb the rest of the vertical ascent.

    And you need to achieve orbital velocity. At 100 kilometers altitude, orbital velocity is 7.84 km/sec. Adding the ascent delta V to 7.84, I believe the rocket would have a 8.8 to 9.3 km/sec delta V budget.

    For this delta V budget, the propellent mass would need to be about 8 or 9 times the dry mass of the rocket. This isn't "any given rocket" but a rather exceptional rocket. Unless the rocket's dry mass was exceptionally tiny, Phillipe's friend couldn't get it in his back yard. Given a 1 ton rocket as you mention, the rocket's dry mass would be 125 to 110 kilograms. This would of course include rocket engines, communication and navigation electronics, the tanks to hold propellent, power source for refrigeration and electronics, fairing, radiation shielding, etc. Perhaps the payload is an ant.

    The ballistic coefficient is a factor in how much air drag reduces velocity. This includes the ratio of rocket's cross sectional area to its mass. As a shape gets larger, it's surface area to mass ratio goes down. So a tiny vehicle suffers more loss from air friction than a large vehicle like the shuttle or a Saturn V.

    And as I've mentioned, balloons are vulnerable to wind. It would be hard to control the position and direction during ascent.

    As I've said more than a dozen times, balloon launch is a bad idea.

    But...

    That is not to say getting above earth's atmosphere is insignificant. The earth's atmosphere is a very significant factor when planning missions.

  9. #69
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    According to this graphic from the NASA Columbia Science Balloon Facility, you could get 4 tons to ~40km with the ULDB (Ultra Long Duration Balloon). I'm not sure how much higher you might get with a lower payload. Unless the max altitude for such balloons is more dependent on burst height than bouyancy.

    I still think that using balloons for something useful wouldn't be practical. However, I wonder if there are other ways to loft/lift a rocket to that altitude before firing its main engines, such as tethers or an inflatable-tower or a combination of them. If we use something smaller to get to LEO for rendezvous instead an all-in-one-moonshot, the masses might be manageable.

    On the other hand, I had been toying with the idea of giant airships as motherships for smaller ascent vehicles.
    Et tu BAUT? Quantum mutatus ab illo.

  10. #70
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    Quote Originally Posted by Ara Pacis View Post
    According to this graphic from the NASA Columbia Science Balloon Facility, you could get 4 tons to ~40km with the ULDB (Ultra Long Duration Balloon). I'm not sure how much higher you might get with a lower payload. Unless the max altitude for such balloons is more dependent on burst height than bouyancy.
    4 tons to 40 km? That's better than what I thought possible. But still not much benefit for achieving orbit and not worth the trouble, in my opinion.

    Quote Originally Posted by Ara Pacis View Post
    I still think that using balloons for something useful wouldn't be practical. However, I wonder if there are other ways to loft/lift a rocket to that altitude before firing its main engines, such as tethers or an inflatable-tower or a combination of them. If we use something smaller to get to LEO for rendezvous instead an all-in-one-moonshot, the masses might be manageable.
    I remain agnostic if SSTO RLV is doable. Although at this time I'm giving Skylon better than even odds.

    You mention tethers. I think momentum exchange tethers could help achieve SSTO RLV.

    If the end of the tether at the capture/release point is moving 1 km/sec slower than orbital velocity, that changes the picture a lot. It would shave 25 to 30% off your needed propellent mass.

    Besides propellent to dry mass ratio, re-entry is another major obstacle for SSTO RLVs. Atmospheric abuse scales roughly with velocity cubed. A ship re-entering earth's atmosphere at 7 km/sec rather than 8 km/sec would suffer 67% of the abuse.

    If the tether end is moving 2 km/sec less than orbital velocity, that makes even more dramatic cuts to minimum requirements. A 40 to 50% cut in propellent mass and about half the normal re-entry abuse.

  11. #71
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    Quote Originally Posted by Hop_David View Post
    I remain agnostic if SSTO RLV is doable. Although at this time I'm giving Skylon better than even odds.

    You mention tethers. I think momentum exchange tethers could help achieve SSTO RLV.

    If the end of the tether at the capture/release point is moving 1 km/sec slower than orbital velocity, that changes the picture a lot. It would shave 25 to 30% off your needed propellent mass.

    Besides propellent to dry mass ratio, re-entry is another major obstacle for SSTO RLVs. Atmospheric abuse scales roughly with velocity cubed. A ship re-entering earth's atmosphere at 7 km/sec rather than 8 km/sec would suffer 67% of the abuse.

    If the tether end is moving 2 km/sec less than orbital velocity, that makes even more dramatic cuts to minimum requirements. A 40 to 50% cut in propellent mass and about half the normal re-entry abuse.
    Ah, I'm not suggesting an SSTO RLV. I was thinking of a 3 stage system as part of an integrated space infrastructure. An airship that uses aerodynamic lift in addition to bouyancy rockets up to a high altitude and high speed release point. The small ascent vehicle is dropped and uses a moderate 2nd stage to take it to orbit (or possibly suborbital rendezvous with a rotovator tether) to a large space station hangar bay. The small vehicle might be used for descent using large retro-rockets (and perhaps a tether release or a retrograde electromagnetic catapult launch) to reduce entry speeds significantly and then using parachutes and/or hang-glider style wings for flight control once out of the supersonic flight regime.
    Et tu BAUT? Quantum mutatus ab illo.

  12. #72
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    I am remaining focused on how to respond to Philippe's friend.

    I stand by my conclusion that any given rocket can achieve more delta V if launched from high altitude rather than at sea level. This could be from a mountaintop, a hypothetical tall tower, or a balloon. We gain delta V for the following reasons, which I have ascertained from what looks like good exercises in "rocket science" elsewhere in this thread:

    1. Atmospheric drag during the burn is reduced at all times.

    2. The performance of the thruster is improved at low atmospheric pressure.

    3. We have the option of pitching forward more and sooner, reducing the gravity loss and getting a forward assist from Earth's rotation.

    I am fully aware that atmospheric drag will have more adverse effect on a small vehicle than on a large one. I do not expect to achieve anywhere near the payload to liftoff mass ratio that a Saturn V or a Titan III could give us. I would be pleased if a one-ton stack with lots of upper stages could deliver a .22 bullet to escape velocity.

    If our friend insists he could do it from his back yard, I would ask him the following questions:

    4. How much delta V do you think you need, and why?

    5. What do you use for fuel?

    6. How do you obtain the fuel?

    7. If cryogenic stuff such as liquid oxygen is needed, how do you manage the refrigeration?

    8. How big a vehicle are you contemplating?

    9. What is the dry mass of each stage?

    10. What is the effective exhaust velocity of the thruster in each stage?

    11. How much thrust does each stage produce?

    12. How long is the burn time of each stage?

    13. How much is atmospheric drag going to slow you down during the early portion of the ascent?

    14. How large a payload can this stack deliver?

    If he can answer these questions, then we can ask him about such things as the guidance system and the method of lighting off the successive stages at the proper times.

  13. #73
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    Quote Originally Posted by Garrison View Post
    Are you sure your friend isn't talking about this:

    Team ARCA

    I remembered that during the Ansari X-prize competition one of the contenders proposed using a balloon as part of the launch system and when I went looking for some confirmation I found the link above. Now whether it will remotely work as shown in the animation in practice I don't know, but you can at least tell your neighbour that someone beat them to the punch, and they need way more hardware than just your average weather balloon.
    on the other hand enthusiasts have done remarkable things with balloons and a cheap camera:

    Pictures from the edge of space with a 30 camera

    Just saw this today:

    Moon Balloon Has Mostly Successful Test Flight
    by NANCY ATKINSON on OCTOBER 5, 2010
    http://www.universetoday.com/74983/m...l-test-flight/

    The "rockoon" idea has been done before to launch suborbital rockets. The ARCA team wants to use it launch a rocket to the Moon. They were only able to achieve 40 km altitude with this test flight, not yet to the 100 km required for suborbital.
    An unusual aspect of the design is that they have the stages connected by tethers, at least according to the animation. I couldn't tell if this is how this test flight was carried out. I would think the tethers would cause the lower stages to gyrate wildly hanging many meters below.
    Anyone know if this has any kind of weight advantage doing it this way?


    Bob Clark

  14. #74
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    Quote Originally Posted by RGClark View Post
    Just saw this today:

    Moon Balloon Has Mostly Successful Test Flight
    by NANCY ATKINSON on OCTOBER 5, 2010
    http://www.universetoday.com/74983/m...l-test-flight/

    The "rockoon" idea has been done before to launch suborbital rockets. The ARCA team wants to use it launch a rocket to the Moon. They were only able to achieve 40 km altitude with this test flight, not yet to the 100 km required for suborbital.
    An unusual aspect of the design is that they have the stages connected by tethers, at least according to the animation. I couldn't tell if this is how this test flight was carried out. I would think the tethers would cause the lower stages to gyrate wildly hanging many meters below.
    Anyone know if this has any kind of weight advantage doing it this way?
    The ARCA team is using a theory that connecting the stages with tethers rather than having them solidly connected provides stability without using fins or gimbaled engines:

    ARCASPACE.
    5.3 Popescu-Diaconu Stabilization Method
    http://en.wikipedia.org/wiki/ARCASPACE


    Bob Clark

  15. #75
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    Quote Originally Posted by RGClark View Post
    The ARCA team is using a theory that connecting the stages with tethers rather than having them solidly connected provides stability without using fins or gimbaled engines:

    ARCASPACE.
    5.3 Popescu-Diaconu Stabilization Method
    http://en.wikipedia.org/wiki/ARCASPACE


    Bob Clark
    Which is physically not possible.

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