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Why do you aerospace engineers have such a hard time making spacecraft?

  1. Jun 20, 2007 #1
    isnt possible to make cost efficient spacecraft by putting multiple types of rockets on a ship which can then alternate rocket engine at different levels of the atmosphere and eventually escape into space?
     
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  3. Jun 20, 2007 #2

    russ_watters

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    Lots of engines? Sounds expensive...
     
  4. Jun 20, 2007 #3

    FredGarvin

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    Let's start with the easiest question for you....Are you referring to manned or non-manned space flight?

    Every engine that gets put on an aircraft has to get certified (expensive). Every engine has to have its associated components (weight). Every engine costs money to make (again, expense). The more engines you have the more complicated the control system is (cost)...There are a whole lot of tradeoffs to consider other than an engine's Isp in a vehicle design.
     
    Last edited: Jun 20, 2007
  5. Jun 20, 2007 #4

    Janus

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    It also sounds inefficient, as the rocket engines not in use at a given time just become extra dead weight that the engine in use has to lift
     
  6. Jun 20, 2007 #5

    D H

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    This is exactly what we do right now. We do not use a single-stage-to-orbit. The systems that carry vehicles from the Earth's surface to space comprise a number of rockets. Multiple stages are needed because a single-stage-to-orbit vehicle would need to carry its entire empty structure all the way to orbit. With present fuel technologies, doing so be very expensive.
     
  7. Jun 20, 2007 #6

    Danger

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    There is a compromise approach, but it's still not as good as dropping dead stages. Variable-geometry nozzles such as the (still experimental?) linear aerospike can operate at peak efficiency throughout the climb. That doesn't resolve the issue of all the dead weight, but jettisonable fuel tanks would help.
     
  8. Jun 20, 2007 #7
    The most efficient I'm thinking right now at the moment is the strategy made by Scaled Composites for the X-Prize competition. Have an aircraft carry the spacecraft from high altitudes and launch it from there. I think the Antonov An-225 could do just that for heavier manned orbital flights.

    But maybe in the future, an aircraft could be made just for that and could cruise above 70,000 ft and still carry very heavy payloads, that makes a good launching platform. Instinctively, this craft would be BWB or possibly a high aspect ratio flying wing with extended booms for elevator pitch control, that makes for a high L/D craft with high altitude capability with excellent structural integrity for carrying heavy payloads.
     
  9. Jun 20, 2007 #8

    FredGarvin

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    There is no aerospike that has flown to my knowledge. There are engines that have movable skirts to help compensate for altitude, but not to the extent of the spike.
     
  10. Jun 20, 2007 #9

    D H

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    Launching a vehicle from an aircraft worked for SpaceShipOne precisely because it was only a suborbital vehicle. SpaceShipOne had a maximum specific energy of about 1 megajoule. Compare this to a vehicle in low Earth orbit, which has a specific energy of 32 megajoules or more. Launching a vehicle from an aircraft doesn't buy much capability in terms of getting a spacecraft into orbit. The first stage would still be needed. Much better is to make the various stages completely reusable (Rocketplane-Kistler K1, Space-X Falcon-9).
     
  11. Jun 20, 2007 #10
    what im trying to say is that, why doesn't NASA develop manned vehicles with multiple types of engines, for example, the ship uses lets say engine A to get to one point of the atmosphere,

    then engine A is shut off but not broken off the ship, then you activate engine B, C, and eventually escape into space

    that way the engines are saved so NASA doesnt have to waste money to make new ones
     
  12. Jun 20, 2007 #11
    Oomair, many reasons why it's not done like that were already given in the previous answers you got...
     
  13. Jun 20, 2007 #12

    D H

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    The crucial phrase here is "but not broken off the ship". If the vehicle doesn't discard spent stages it has to haul all that dead weight into orbit. The rocket equation says this is cannot be done. It would be better to have a single-stage-to-orbit vehicle (which we can't do yet) than a multi-stage-to-orbit that doesn't discard spent stages. Moreover, returning this dead weight to Earth will be a considerable challenge. The return vehicle will have to be a lot heftier to be able to keep that awkward weight under control when it hits the atmosphere at Mach 25. That's even more dead weight that has to be hauled into orbit.
     
  14. Jun 20, 2007 #13

    Danger

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    Oomair, you might be getting misled by some ideas that have been kicking around for atmospheric machines, such as taking off with turbojets and switching to scramjets when supersonic. Those things are not intended to go orbital, and they don't have to carry an oxydizer.
     
  15. Jun 20, 2007 #14
    Oomair may have a point, but not literally with multiple engines. I've read about some 'single stage' concepts put forth by reputable aircraft companies. One of them actually envisioned a turbojet that 'morphed' and doubled as rocket engine. The turbojet seemed to close it's inlet, compressor and combustion area, and used the nozzle section as the rocket engine.

    I was thinking that turbojets are rather complex and heavy. So I might propose a ramjet instead that morphs to scramjet, and finally morphing to rocket engine to reach orbit. Scramjet are indeed not intended to go orbital and that's when the rocket engine or 'rocket mode' takes over.

    The idea understandably, makes the vehicle carry much less oxidizer needed than would a pure-rocket design, that enormous fuel weight saved could directly translate to greater payloads carried and of course, much improved economy of space flights. The challenge now is designing a morphing engine that is efficient in wide range of flight regimes as well as light in weight.

    There has been real plans to use the the scramjet for use with single stage orbital vehicles, but scramjet technology and hypersonic flight is still relatively early as of now. There's not even one manned flight conducted to date.

    @D H, the energy comparison, is very enlightening to know, thanks, so that would make aerial launches very much insignificant gain in energy
     
  16. Jun 20, 2007 #15
    Hypersonic research has been going on since the 60's gaming_addict. The problem has been finding materials that will survive the high viscous heating.
     
  17. Jun 21, 2007 #16
    ok one thing i want to get straight in my head here is that does a space shuttle that is returning to orbit just dive into orbit at a high speed and makes a landing like a plane or does it stay in a suborbital area for a few hours by slowly coming down each atmospheric level so that the heat doesn't dissolve it?
     
  18. Jun 21, 2007 #17
    It takes a little over an hour to start from deceleration burn to landing. And yes, you are right with your 2nd statement.

    -It starts by deceleration burn
    -Skips over the upper atmosphere, much like a skipping stone thrown over the surface of a lake
    -By maintaining high angle of attack, you create a 'lot of drag' and the glide path steepens <-do that if you think you might overshoot your target landing area, conversely, you reduce angle of attack if you feel like coming short.
    -You could also do a series of 'S-Turns' or steep break turns left and right to bleed more speed if you think you might overshoot
    ^^All of the above maneuvers are done above 200,000 ft traveling at hypersonic velocities.

    When you have gone below hypersonic at lower altitudes, the glide bath then becomes more steep.

    So to summarize, glide path during high altitude hypersonic phase is relatively shallow, done at high angle of attack(to expose the more heat resistant bottom tiles) and to experience the least heating possible(least aerodynamic pressures) but at a longer period of time. Once the craft have gone supersonic, lower altitude, and nearing the landing area, that's when the glide path becomes very steep, to have good speed for landing, and of course to make up for the space shuttle's very poor aerodynamic efficiency :)
     
  19. Jun 21, 2007 #18

    FredGarvin

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    You really should be careful with the words you choose. I have been on a couple of programs that looked into hybridization between air breathing and rocket propulsion systems. That's exactly what they are, hybrids. The whole "morphing" thing, other than sounding cool, is a pipe dream brought about, mostly, by people looking for government research funding. I can imagine my Chevy Blazer morphing into a Ferrari when I want to go into "race car" mode. You're saying the same thing.

    I would challenge you to look at the weight of a turbojet versus the combined weight of a thruster package and the associated turbo pumps and delivery systems.


    Oh yeah...those pesky details we have to think about I guess. Morphing....light....efficient over a wide operating range....That's not a challenge. That's unrealistic dreaming.

    Considering that the first real scramjet model research craft testing was in 2002 and then 2004, it may be a bit of a stretch to say "we haven't even had a manned flight yet." There's at least a couple of decades of research and development before we see anything close to a manned flight.
     
  20. Jun 21, 2007 #19
    Yep, that 'hybrid' term didn't come to mind so I used 'morph' instead. When I meant the 'morphing' thing, here's what I meant:

    http://en.wikipedia.org/wiki/Pratt_&_Whitney_J58

    That's a real example of an engine that 'morphed' into another, from a turbojet into ramjet. So 'hybrid' it is! Sorry I'm not a native speaker so I could get lost with words :)

    I know one of them will be heavier(the turbojet will be the lighter in weight, I assume), and I presume you'd say it won't matter if I used ramjets or turbojets. But I prefer using the ramjet because, it will be used from high subsonic to mach 4 in which, a ramjet will by far outperform a turbojet in thrust levels. Turbojets can only match a ramjet in reheat but it becomes so inefficient

    But you're right, these things, ramjet to scramjet, then rocket, is still decades away.

    My best bet at present would still be a BWB because it can be stiffened so easily and made very strong. Thus a very light body and yet strong could be made, making single stage flight possible and reentries safer.
     
  21. Jun 21, 2007 #20

    D H

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    This is all wrong. The Shuttle has only done a skip reentry in simulations. It has never done one in reality. Skip reentry has a very low Technology Readiness Level (TRL 3, to be precise). It is one of the many technologies that need to be beefed up considerably before we even consider sending people to Mars.
     
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