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B Missile reentry

  1. May 15, 2017 #1
    A couple of days ago, there was this beautiful ballistic missile launched from North Korea.
    While reading about this momentous event, I was surprised to learn that said missile had climbed over 2,000 km before crashing into the sea some 700 km from its launchpad.

    Since this missile flew into space, way beyond most satellites, it had to reenter Earth atmosphere before landing, or rather, crashing.

    Yet, considering how close from its launchpad it crashed, it appears that it reentered the atmosphere at a very steep angle, almost vertically.

    I thought that it was not possible for an objet to enter Earth atmosphere "head on" without either being sent back into space, or disintegrating upon impact.

    The space shuttle, for example, had to reenter the atmosphere at a very precise angle to avoid such a fate.

    Could someone shed some light on how missiles do reenter the atmosphere, apparently with much less trouble than other objects such as a shuttle?
     
  2. jcsd
  3. May 16, 2017 #2

    marcusl

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    The space shuttle was basically an aeroplane that came in at a low angle so it was in position to fly, maneuver, and land at lower altitudes. It also had windows for the pilots which needed to be shielded from the direct heat of reentry, accounting for the high angle of attack. A missile nose cone has no windows, and can be coated with thick insulation for reentry. Since a soft landing is not required or desired, a steep angle is used. As you point out, it disintegrated upon impact.

    BTW, I don't find a nuclear-capable IRBM to be "beautiful" so much as "horrible."
     
  4. May 16, 2017 #3
    Marcusl, thanks for the explanation.
    This leads me to another question:
    - is there any reason as to why a missile should reach such a high altitude, is that customary for ballistic missiles in general? After all the higher a missile climbs, the more time it wastes and the more the effect of surprise is diminished...

    As far as "beautiful" is concerned, this is just a word borrowed from this famous guy who uses it for all kinds of purposes, generally the wrong kind: the beautiful border wall and so on...
     
  5. May 16, 2017 #4

    Bandersnatch

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    The reason for it to climb almost vertically like that is that it lands somewhere relatively close - either because you don't want to aggravate anyone by landing your test rockets on their soil or anywhere near their coast, or because you want to keep it on your radar throughout the whole flight.
    Remember that it was just a test. If your rocket can reach 2000km altitude, then it's capable of accelerating to approx 5km/s. Once you know that, it's just a matter of plotting its flight path at a shallower angle for it to reach a faraway target.
     
  6. May 16, 2017 #5
    This specific missile was shot to such a high altitude to test its range, they change its angle of attack and calculate how far it can reach a target based on the missile's trajectory at its apex.
     
  7. May 16, 2017 #6
    Got it!
    Yet, sending a missile 2,000 km up and then let it fall another 2,000 km back into the ocean is not the same as having it flying much more horizontally over 4,000 km to reach some predetermined target.
    I wonder what one can conclude from such a test with regards to the potential reach of the missile in case of use for a strike.
     
  8. May 16, 2017 #7

    Bandersnatch

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    But it is. It's a ballistic missile, you see. It burns its engines for a relatively short while, to clear the atmosphere and get some speed, and then it just follows a ballistic trajectory. It will always fall unpowered (and raise unpowered for part of its journey).
     
  9. May 16, 2017 #8
    Understood, so this was quite a technical achievement...better not be a neighbor of Mr Kim...one never knows what might fall in one's backyard...
     
  10. May 16, 2017 #9
    You also have to factor in rotation of the earth, and how long it can relax in sub orbit and how much distance it can gain from this. The test reveals a lot of information when you map it all out.
     
  11. May 18, 2017 #10
    Why are such weapons ballistic and not just like a powered always controllable flight vehicle eg a jet plane.

    Is a missile just another name for a rocket?
     
  12. May 18, 2017 #11

    Drakkith

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    Speed through the atmosphere is limited by the air and all the different effects it brings. It also introduces vulnerabilities since an opponent can shoot down your missile. Getting the missile up and out of the atmosphere lets you reach absurdly high speeds, allowing for quick deliveries of the payload (warhead) and renders the missile immune to nearly all defenses and countermeasures.

    A rocket is usually a term referring to an air or space vehicle powered by a rocket engine. A missile is a guided, self-propelled munition, whereas a rocket, in the context of a munition, is an unguided, self-propelled munition. Missiles can be propelled by either jet engines or rocket engines while rockets are powered by rocket engines.

    Note that I deliberately put the term 'munition' in there as it is an important distinction. For example, Project Mercury, the first American program aimed at putting men into space, used a modified Atlas-D as as a launch vehicle. The Atlas-D was an operational ICBM at the time and the weapon system would be considered a missile. However, the modified Atlas-D's of Project Mercury were rockets, not missiles, since their payload wasn't an explosive warhead.

    Note that the distinction between missile and rocket isn't always this clear cut, as the U.S. has guided weapons referred to as rockets.
     
  13. May 18, 2017 #12
    So loosely speaking if it has blowy up bit stuck on the nose its a missile .

    Self propelled means on board thrust, in contrast to say a ballistic arrow. is a bullet self propelled because it originated from the propellent cartridge it was part of hence a bullet in your definition is a missile? Or does the bullet need to carry an explosive charge eg an artillery shell is a missile?

    Also missiles have guidance so how can they be ballistic?

    A rocket propelled grenade is a missile, fits your definition?
     
  14. May 18, 2017 #13

    Drakkith

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    If it's guided, yes. If it's unguided, then 99% of the time it's not a missile, but a rocket. Rocket munitions usually use solid-fueled rocket engines because they are MUCH cheaper than jet engines and require next to no maintenance. When combined with the cost-savings associated with a complete lack of the sophisticated electronics used in guidance and targeting, we can often afford to launch dozens of rockets to take out a single target and it may still cost less than using one missile.

    A bullet is neither self-propelled nor guided. Note that technically a bullet is the projectile part of a cartridge when referring to most modern firearms.
     
  15. May 18, 2017 #14
    Sorry I don't get guided, rockets are maneuverable hence guided??

    how can something guided be called ballistic ie if its guided it is not ballistic by definition???

    What am I missing?

    Also missiles use jet engines???
     
  16. May 18, 2017 #15

    Drakkith

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    That's because the military uses 'rocket' to mean something different than the usual definition.

    Basically:
    • If it's a Munition:
      • And Guided - Missile
      • And Unguided - Rocket
    • If it's not a Munition:
      • Uses a Rocket Engine - Rocket
      • Doesn't Use a Rocket Engine - Not a Rocket
     
  17. May 18, 2017 #16
    Thanks I think I get it now.
     
  18. May 18, 2017 #17

    Drakkith

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    Once out of the atmosphere and up to speed, the payload is essentially in free fall. An object in free fall is following a "ballistic" trajectory. It's the same kind of path that an unpowered projectile follows. So even though ICBM's are guided during ascent, a large portion of the warhead's flight is unpowered with little ability to guide itself except to make small adjustments. Hence they are "ballistic" missiles.
     
  19. May 18, 2017 #18
    OK, these definitions seem a bit rubbery but appreciate you taking the time.

    Is it linguistically redundant to say " guided missile", by definition a missile is guided.

    Should just be called missile or alternatively - guided munition equipped rocket.
     
  20. May 18, 2017 #19

    Drakkith

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    You're under the assumption that these definitions are:
    1. Static
    2. Developed recently

    No definition is entirely static and many words have changed drastically over the years.
    In addition, the word "missile" is actually much older than just 60 years or so. Dictionary.com has the following as its definitions of 'missile' and shows its origin as being from around 1600:

    As with almost everything, context is important.
     
  21. May 18, 2017 #20

    berkeman

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    Cruise missiles certainly do...
    As Drak points out, Ballistic Missiles are guided at launch and during the lift phase, then largely unguided for most of the rest of the flight. Some ICBM re-entry platforms do have some limited guidance capability near the end of the flight for evading countermeasures and for improved accuracy to the target.

    https://en.wikipedia.org/wiki/Intercontinental_ballistic_missile
     
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