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Space ship design idea, how to approach with out air resistance

  1. Jun 16, 2011 #1
    ok so here is what i was thinking

    when a space ship enters the atmosphere the reason for the high impact velocity is that the air is spinning at 66000mh AND that to keep an orbit you need certain speed with is different from the earths rotation... now if you slow down/speed up to match earths rotational velocity you start to "fall" faster then you could match the required speed

    now here is my idea and let me know guys if you see something wrong with this theory

    lets assume you have enough fuel to compensate for the gravitational pull , in other words you use your engines to "hover" till you reach the required speed to sink in to the atmosphere.. would this work ?

    secondly the earth poles the atmosphere is not sniping obviously because those are the axles, so could you not just fly the thing there and enter the atmosphere there with out any major air resistance ? (again of course compensate for gravity on approach with engines)

    then once you enter the atmosphere slowly you just use normal jet engines to fly where ever you want to land

    as you dont have the high velocity entry, the design do not have to withstand the heat and pressure of current landings, the spaceship could be designed like a conventional airplane with jet engines

    again dual engines, jet engine for air, rocket engines for space

    did i miss anything ? is this possible ? it seems way cheaper and safer to design such craft.. no ?
     
  2. jcsd
  3. Jun 16, 2011 #2

    cjl

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    The problem isn't the atmosphere's spin - the problem is the orbital speed of the spacecraft. In order to stay up, the spacecraft is orbiting the earth at around 7 km/s, which is quite a lot faster than the speed of the earth's spin (which is only around 1000 mph at the equator, or 400m/s approximately). Yes, you could avoid heating by slowing down before reentry, but you would need to remove an enormous amount of speed first. You would need so much fuel that the entire vehicle would be absolutely huge, since you would need to slow down just as much as you had to speed up during ascent. As a result, it turns out that it is far lighter, cheaper, and more efficient to simply use the atmosphere to slow you down on the way in. Yes, heat shielding is heavy and expensive, but it's far lighter and cheaper than the alternative.
     
  4. Jun 17, 2011 #3
    thank you for the answer, this brings me how ever to a new idea and question

    so to have a stable orbit you need to 7km/s (25000km/h)
    BUT what if you just go to the moon and back ? do you need to archive stable orbit or can you not just descend right away at arrival ?

    why im asking this ?! the moon is 384403 kilometers away so if you go 25000kmh it would only take 16 hours to get there, now how about only going 5000kmh then the trip would take 80 hours ? there for would need a hell of a lot less fuel to slow down to the required slow decent.

    a jumbo jet size (and similarly shaped) space ship could carry a lot of fuel, plus you would not need the initial rocket fuel to get to the higher atmosphere as the jet engines get you there

    of course i understand that such craft could never archive stable orbit as it does not have the required amount of fuel to speed up and slow down.. but again is stable orbit necessary for shuttling to the moon ?

    did i got something wrong this time ?
     
    Last edited: Jun 17, 2011
  5. Jun 17, 2011 #4

    Drakkith

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    Staff: Mentor

    The amount of fuel it takes to launch something into space is enormous. The weight of the heat shield is probably much less than the combined weight of both the engines AND the fuel needed for the jet engines.

    Also, you cannot avoid major acceleration and deceleration to the moon and back. A spaceship MUST reach a minimum velocity away from the Earth or gravity will simply pull it back. And then you must go beyond this minimum, otherwise it will take you longer and longer to reach the moon, as the Earth is slowing you down as you go along. This is bad because people need food, water, and oxygen. The longer the trip takes the more of these items you will need, which increases weight and fuel. No matter how you look at it, if you wish to reach the moon in a short period of time you MUST use a high velocity.
     
  6. Jun 17, 2011 #5

    russ_watters

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    Siix, you're ignoring the issue of gravity entirely. You use much less fuel going into earth orbit, then lunar orbit, then landing on the moon than you would going straight up to the moon because of gravity.
     
  7. Jun 17, 2011 #6
    ...and siiix, your whole concept of how to get to the moon and generally navigating through space is flawed. I'm not surprised because I don't fully understand it either, and I doubt many people do.

    Either way, I originally realized something was up when I read that Apollo, going 25000mph, takes 3 days to get to the moon. The moon is not THAT far away.

    So I tried wrapping my head around it. Here's what I concluded: first of all, you need to go 25000mph to be able to even escape earth's gravity and actually attempt to get to the moon. The thing to understand is that the 25000mph is not a straight earth-to-moon line. The spaceship starts out orbiting earth, and then speeds up to go into an "orbit" that moves it away from the earth. Still, the largest vector of that 25000mph is moving across the earth's surface, not straight to the moon.

    THEN, as you're getting closer to the moon, you're moving against earth's gravity. Essentially you're going uphill to get to the moon, and as such you are constantly slowing down. By the time you actually do get to the moon, that 25000mph is only a few thousand mph, and slowing down to go into orbit around it is no big deal.

    But either way, you do a quick burn to get to 25000mph and then you coast for three days all the way to the moon. It IS possible to go straight up at 5000mph, but you'll constantly need to be burning the engines not only to balance out gravity, but to actually make progress against it and reach the moon. Considering how a spaceship weighs MILLIONS of pounds (and kilos, whichever you prefer), your engines will need to produce at least that for the thing to just get off the ground...no to mention to actually climb the 200000? miles to the moon.

    In order to produce millions of pounds of thrust you'll probably burn a jumbo jet's worth of fuel in a few minutes. And then what...how many more hours do you have to go? You're going to need to bring more fuel, which will increase the mass of the spaceship, which will require more fuel to get it off the ground AND climb, which will increase it's mass more, which means you will need more fuel, which means more mass, which means more fuel....and on and on and on until you end up with a 5,000,000lb spaceship of which 4,500,000lb is fuel. This is why a jumbo jet is not going to cut it.

    As for using jet engines...if you can find any powerful (and damn heavy) enough to supply the millions of pound of thrust then that's great and all for the first, what, 30 kilometers? I doubt they'll do anything other than weigh you down after that...and you still have 384403km to go.

    Someone please correct me if I'm wrong here. I kind of thought up most of this myself and nobody has ever confirmed it...
     
    Last edited: Jun 17, 2011
  8. Jun 17, 2011 #7
    ok so speculations aside, because obviously im not capable of making the precise calculations of fuel needed

    this is what i was noticing , and this is what made me thinking:

    note the size difference shuttle to rocket
    images?q=tbn:ANd9GcSFo98c0H538o3u5Kskz9V7BRmk6OPQiOwNIDihYDlyNY2sBUJ25A.jpg
    and now the size difference regular size jet to shuttle
    USRV01P07_02.jpg

    and now imagine the the same thing with a super jumbo jet:
    800px-Buran_on_An-225_%28Le_Bourget_1989%29_1.JPE

    from what i can tell the plane is bigger then the rocket, you add to this that the fuel you save using the jet engines for initial acceleration about 1000kmh+1000kmh of earths rotation and the 1st 15km attitude

    in addition if its unavoidable you could have extra boost rockets for the initial acceleration that you lose, like the smaller rockets of the shuttle

    according to wiki you do not need to "escape velocity" to escape earths gravity IF you have engines to compensate, you can do that at any speed


    BUT here is the important part , it should be possible to land a craft with out sitting in orbit ?!? because in this case i think thats the real fuel waster, you would need to get to the minimum speed needed to stay there

    now remember you have fuel to maneuverer around, so you would need to burn your engines longer BUT just to compensate for gravity when needed, you save your self the fuel to getting up/down to/from 25000kmh

    on the moon i guess its tricky, maybe with very precise calculations just keep drooping in an angle with rocket engines on so the speed gets to 0 compared to the surface, such craft would anyhow need thrusters that can compensate for gravity on both earth and moon other wise it would just fall down on the moon and outside the earth atmosphere

    craft
    {====| < main push

    ^
    hovering push, to compensate for gravity outside the atmosphere

    this is the flight patch they normally use, so they do establish orbit before take off and landing
    5105040212_346b792ee5_m.jpg

    but WHY ? why not just keep going ? and then at the landing again, why not just land strait up with out orbiting around, thats would save huge amounts of fuel

    (now obviously this only works for the moon as you cant go 5000kmh to mars, it would take to long)
     
  9. Jun 17, 2011 #8

    cjl

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    What you're missing is that at liftoff, the shuttle weighs 4-5 times more than any airplane, and the large majority of that weight is fuel. Spacecraft need a LOT more fuel than airplanes do, and that's only going to be a larger problem if you did what you are proposing.
     
    Last edited: Jun 17, 2011
  10. Jun 17, 2011 #9
    I agree that absolutely it is possible to simply walk out of the gravity well instead of shooting yourself out of it.

    The problem is that, currently, you HAVE TO use a rocket to do it. Even if we start at 100 kilometers above the earth, if the rocket is just hovering there it has to constantly thrust its engines at least as much as it weighs, simply to keep from falling back down.

    A jumbo-jet-rocket which weighs 400,000kg would need 400,000kg equivalent thrust just to keep from falling.

    Now, I did some math basic math and checked how much thrust and for how long a space shuttle booster works. Basically...it works out to where it has enough thrust/ burn time to support a 747 for ~6 minutes. If we strapped 5 of them onto a 747, we could keep it from crashing back down to the ground for about half an hour. This is just to keep it motionless at a point above the ground.

    What you are proposing is to not only do that, but to accelerate it to 5000kph, and to keep it there for 80 hours! That's vastly more thrust and for much, much longer than what we can even dream of.

    As if that's not enough, all along I ignored a very important issue, the one that is the limiting factor of all rockets: the fact that 5 space shuttle boosters weigh ~3,000,000 kg! So you can't just slap on more and more to keep the thing up in the air, because you're going to have to increase the thrust more and more to compensate for their own weight. You'll get to the point where you have to add millions of kilos of weight for just an extra minute of hover time.

    This is why, yes, you can "walk" your way up to space, but you're not going to do it with a rocket. You're going to need something more exotic, like a space elevator, in order to dream about these things.
     
  11. Jun 17, 2011 #10

    cjl

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    You're making a large conceptual mistake here. In space, distance isn't what determines the amount of fuel you use. What uses fuel is changes in velocity. The path they took to the moon was almost the best possible route in terms of minimizing velocity change. Your approach (slow all the way there, straight in) would actually require a much larger velocity change than the route they took, even though it would be shorter.
     
  12. Jun 17, 2011 #11

    Drakkith

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    Staff: Mentor

    There's no possible way to make a jet aircraft both light enough to get into space AND with big enough engines and wings to support several times the aircrafts weight in fuel at a minimum. Yes, you would still need that much to get into space.

    Which adds more weight meaning you need bigger engines and wings and structure to support the dead weight of the boosters. Which increases weight even more...see where I'm going?

    That is correct, however the key to understand is that the longer you take to get away from the earth, the more fuel you burn. Accelerating to tens of thousands of MPH is actually more efficient than sitting at 5,000 mph and constantly burning the engines fighting against gravity. You spend a longer amount of time near the earth where the gravity is greater than when you are further away.

    Did you know that the original plan for the Apollo moon landers was for the ENTIRE spacecraft to land? However it was quickly realized that to do that would require massive amounts of fuel to support all that weight. That is one of the main reasons they went to a seperate moon lander. It minimizes the weight that you have to land and then takeoff.

    Incorrect. It is MUCH more efficient to do a hard quick burn and get up to speed than it is to spend longer in a gravity well. Also, if you only have part of the spacecraft land, like the moon lander, you don't have to deccelerate the rest of the spacecraft. You can leave it in orbit.

    The main engines are the thrusters.

    It would not as I've explained above.
     
  13. Apr 21, 2012 #12
    ok so i did some more research, i see i did a couple of mistakes in my idea, but in general it seems i was on to something;;
    as it came a stumbled on a design that is somewhat similar to my idea, and already in phase3 development (during Phase 3 (2011-2013) pre-orders for SKYLON vehicles will be received, and the vehicle manufacturing consortium formed.) ... now of course there are major differences as well

    the similarities are that its a plane that slowly exits and enters the atmosphere, using jet engines in the atmosphere and for orbit it using hydrogen powered boosters (no huge rockets, no heat shield)

    http://www.reactionengines.co.uk/index.html [Broken]

    now what would be interesting would a design like this (with necessary changes) be able to fly further then earth orbit ? how about orbiting the moon to pick up or drop of cargo ?

    mars is probably to far for this design.. i think
     
    Last edited by a moderator: May 5, 2017
  14. Apr 21, 2012 #13

    mfb

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    2016 Award

    Staff: Mentor

    As described on the website, the SKYLON vehicle would fly similar to conventional rockets. The main difference is that it can launch from a runway and is designed to be air-breathing up to Mach 5.5, which saves a bit of oxygen fuel.

    It would fly to high altitude and speed like a jet. At that point, it has Mach 5.5, is somewhere at ~30km height (guess) and nearly horizontal. You can find similar parameters in conventional rockets, and the remaining part is really similar: SKYLON and rockets use their fuel to accelerate up to orbital velocity, slowly gaining additional height.
    In order to land, the deceleration is done with the help of the atmosphere, both with conventional designs and SKYLON.

    Going to orbit first is a good idea for every mission. You don't waste too much energy during the ascent, and you can use the full orbital energy to go to any other places.

    Right, especially as it is not designed for missions longer than ~1 week. But you could launch several parts of a rocket, assemble it in space, and reach mars with this rocket.
     
  15. Apr 21, 2012 #14

    rcgldr

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    But that takes much more energy than to accelerate to escape velocity by applying thrust perpendicular to the force of gravity (which is done as much as possible once a spacecraft gets past the outer edge of the atmosphere).

    Earth orbit is established before take off (trans lunar injection) to moon, because the thrust needs to be applied at a specific point in the orbit, which is different than when the orbit is first acheived. I'm not sure why they wait for 2 or 3 orbits to do this. Orbit is also established before moon landing, since only a part of the craft orbiting the moon actually descends, with only a part of the descending part returning back to the lunar orbiting craft. Re-entry into the earths atmosphere does not involve an orbit, just a precise angle and position to avoid generating too much heat or not slowing down enough and ending up in a very long elliptical orbital path.

    Wiki has more info:

    http://en.wikipedia.org/wiki/Trans_Lunar_Injection

    http://en.wikipedia.org/wiki/Hohmann_transfer_orbit

    http://en.wikipedia.org/wiki/Trans_Earth_Injection

    http://en.wikipedia.org/wiki/Appolo_11

    http://en.wikipedia.org/wiki/Apollo_program
     
    Last edited: Apr 21, 2012
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