Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > sci.physics > #531366

Re: It's not rocket science--we need a better way to get to space

Show all headers | View raw

On 9/11/2015 7:41 PM, nuny@bid.nes wrote:
> On Sunday, November 8, 2015 at 9:39:38 PM UTC-8, Sylvia Else wrote:
>> On 8/11/2015 5:27 PM, nuny@bid.nes wrote:
>>> On Saturday, November 7, 2015 at 6:38:47 AM UTC-8, Sam Wormley
>>> wrote:
>>>> It's not rocket science--we need a better way to get to space
>>>>> http://phys.org/news/2015-11-rocket-sciencewe-space.html
>>>>
>>>> Rockets are a terrible way of getting to space.
>>>
>>> (nothing-new article snippet snipped)
>>>
>>> To paraphrase Churchill:
>>>
>>> "It has been said that rocketry is the worst way to get to space
>>> except all the others that have been tried."
>
> (snip)
>
>>> Multifunctional engines (turbojets that switch to various
>>> supersonic modes and or to pure rocket) are old hat in SF but are
>>> very hard to implement in real life, and are not likely to
>>> provide the benefits usually claimed- not because of
>>> unreliability, but because no multipurpose tool can be as
>>> efficient in any one of its functions as a purpose-specific tool.
>>> In other words, the Skylon system can not be as efficient as a
>>> standard airbreather in that phase of its operation as an
>>> ordinary, optimized turbojet, nor can it in its other modes
>>> (ignoring for the moment that hypersonic jets are not exactly
>>> mature enough tech to determine their efficiency or reliability).
>>> There's also that pesky issue of having to accelerate and
>>> decelerate empty fuel tanks for much of a given flight.
>>
>> It's worth noting that the Sabre engine does not function as a
>> turbo jet (or ramjet) at any stage of its operation. Propulsion is
>> achieved by means of rockets throughout. During the air breathing
>> phase, the rockets receive hydrogen from the tanks, and oxygen (and
>> nitrogen) from the air. In non air-breathing mode the rockets
>> receives all their propellants from the tanks.
>
> That's sort of correct but handwaves away some subtleties. The
> precooler is the first of a series of tradeoffs in the Sabre engine-
> it has to bypass some (very hot) air which incurs a drag penalty.
> Another is that the precooler needs lots more LH2 (to accept heat
> from the airstream via the helium loop) per second than the
> combustion chamber needs, meaning some LH2 has to be dumped overboard
> which wastes its fuel value. Instead, the dumped no-longer-L H2 is
> cleverly burned with the bypass air in ramjets that ring the rocket
> nozzle, producing thrust intended to offset most of the initial drag
> penalty. This is a really neat way to combine tradeoffs productively,
> but it's a stack of delicate balances.
>
> [Claiming the Sabre is a rocket in all modes of operation seems to me
> to be disingenuous (yes, I know, the claim didn't originate with
> you). Just because the incoming air compressor is not driven by a
> power turbine in the exhaust stream doesn't, to me, disqualify the
> Sabre as a form of "turbojet". And, doesn't "air breathing rocket"
> strike anyone else as an oxymoron? The Sabre straddles two
> conventional pigeonholes in airbreathing mode but still has a foot
> firmly in each. I'm more comfortable calling it a hybrid but perhaps
> that's just old-fogey-ism on my part.]

The essence of a jet - be it turbojet, ramjet, or scramjet, is that the 
fuel is burnt in a hot and moving air stream. Yet that is nothing like 
the way the Sabre engine works. The captured air enters the combustion 
chamber while it is very dense and very cold. It's not actually liquid, 
but not far from it. It is also moving slowly, or at least, at a speed 
comparable to that of the hydrogen. The design of the combustion chamber 
and nozzle have every similarity to that of a rocket engine, and nothing 
at all resembling a jet.

Of course, we needn't get into an argument about the meaning of words, 
but the reasoning about its inevitable inefficiency due to its hybrid 
nature just doesn't work. That's not to say that the conclusion is 
necessarily wrong, but if it is, it must be reached by a different line 
of argument.

>
>> As for having to accelerate the tanks, that reduces the payload
>> fraction, but you get the tanks back intact. So you need a larger
>> vehicle for a given payload, but you don't have to keep rebuilding
>> it. One needs to do the sums before one can form a view about
>> whether that's worthwhile.
>
> That comes under "specific impulse", aka thrust-to-weight ratio. The
> larger that is, the more non-reactant mass you can afford to carry
> around. The Sabre's is *designed* to be in double digits, and if
> testing confirms the design goals it will be worthwhile, as you put
> it, but the pudding isn't done yet.
>
> I'm not against new ideas that advance the art, or to multimode
> machines in general (I specifically dislike single-purpose tools even
> if they're the best at their task). The (IMNSHO) most beautiful
> manmade thing ever to fly, the SR-71, flew on engines that (mostly)
> smoothly transitioned from pure turbojet to pure ramjet modes, a
> technological miracle for its time, giving it a still-unequaled
> flight envelope. Even so, for its entire operational lifetime it was
> actively juggling a series of tradeoffs giving it a reputation for
> temperament, "unstarting" in ramjet mode at inconvenient moments for
> no apparent reason. That's bad enough in level flight at Mach 3 or so
> at 80,000 ft with no payload on the line- boosting through that
> altitude at Mach 5 with passengers aboard... let's just say I want
> the pudding to consistently earn five stars before I'd get aboard.
>
> The SR-71 reminds me of another subtlety influencing engine, and
> overall aircraft, efficiency, namely how the airframe shape affects
> airflow into and past the engine(s). Most of the renderings for
> concept planes using the Sabre look, well, less than optimal to me.
> That's a somewhat different area of physics though.

Not sure what your objections are there. The engines have to be mounted 
where they are to maintain the centre of gravity as the propellants are 
consumed. They also have to far enough from the nose, and sufficiently 
inboard that the nose shock wave doesn't intersect the inlets during the 
air-breathing phase.

During the air-breathing phase, the engine inlets are inside the nose 
shock-cone, but are otherwise unaffected by the body, as far as I can 
see. This should make their design and testing so much more straight 
forward. Further, after the double-shock inlet, the flow into the 
pre-cooler is subsonic, which makes life a lot easier when it comes to 
developing that.

Sylvia.

Back to sci.physics | Previous | Next — Previous in thread | Find similar | Unroll thread

Thread

It's not rocket science—we need a better way to get to space Sam Wormley <swormley1@gmail.com> - 2015-11-07 08:38 -0600
  Re: It's not rocket science—we need a better way to get to space Poutnik <poutnik4nntp@gmail.com> - 2015-11-07 16:01 +0100
  Re: It's not rocket science—we need a better way to get to space gilber34 <fafa@invalid.com> - 2015-11-07 09:22 -0600
    Re: It's not rocket science—we need a better way to get to space Wally W. <ww84wa@aim.com> - 2015-11-07 10:39 -0500
      Re: It's not rocket science—we need a better way to get to space Edgardo Alcantara <edgardoalcantara@carosseryrepair.org> - 2015-11-07 15:44 +0000
        Re: It's not rocket science—we need a better way to get to space Wally W. <ww84wa@aim.com> - 2015-11-07 11:00 -0500
        Re: It's not rocket science—we need a better way to get to space Fabian Russell <root@localhost.localdomain> - 2015-11-07 21:47 +0000
          Re: It's not rocket science—we need a better way to get to space Edgardo Alcantara <edgardoalcantara@carosseryrepair.org> - 2015-11-08 00:49 +0000
            Re: It's not rocket science—we need a better way to get to space Fabian Russell <root@localhost.localdomain> - 2015-11-08 01:40 +0000
              Re: It's not rocket science?we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-08 03:27 +0000
                Re: It's not rocket science?we need a better way to get to space Fabian Russell <root@localhost.localdomain> - 2015-11-08 03:43 +0000
                Re: It's not rocket science?we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-08 04:10 +0000
                Re: It's not rocket science?we need a better way to get to space Fabian Russell <root@localhost.localdomain> - 2015-11-08 04:42 +0000
                Re: It's not rocket science?we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-08 06:23 +0000
                Re: It's not rocket science?we need a better way to get to space Edgardo Alcantara <edgardoalcantara@carosseryrepair.org> - 2015-11-08 16:01 +0000
                Re: It's not rocket science?we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-08 17:33 +0000
                Re: It's not rocket science?we need a better way to get to space Ludwig Böhm <ludwb@widespectrum.com> - 2015-11-11 23:56 +0000
                Re: It's not rocket science?we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-12 00:29 +0000
                Re: It's not rocket science?we need a better way to get to space Ludwig Böhm <ludwb@widespectrum.com> - 2015-11-12 00:38 +0000
                Re: It's not rocket science?we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-12 01:11 +0000
                Re: It's not rocket science?we need a better way to get to space Ludwig Böhm <ludwb@widespectrum.com> - 2015-11-12 16:07 +0000
                Re: It's not rocket science?we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-12 18:47 +0000
                Re: It's not rocket science?we need a better way to get to space kefischer <emoneyjoe@iglou.com> - 2015-11-12 16:06 -0500
                Re: It's not rocket science?we need a better way to get to space Ludwig Böhm <ludwb@widespectrum.com> - 2015-11-12 21:22 +0000
                Re: It's not rocket science?we need a better way to get to space Ludwig Böhm <ludwb@widespectrum.com> - 2015-11-12 16:08 +0000
                Re: It's not rocket science?we need a better way to get to space kefischer <emoneyjoe@iglou.com> - 2015-11-11 20:55 -0500
                Re: It's not rocket science?we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-12 02:09 +0000
                Re: It's not rocket science?we need a better way to get to space kefischer <emoneyjoe@iglou.com> - 2015-11-11 23:08 -0500
                Re: It's not rocket science?we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-12 04:45 +0000
                Re: It's not rocket science?we need a better way to get to space kefischer <emoneyjoe@iglou.com> - 2015-11-12 03:28 -0500
                Re: It's not rocket science?we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-12 18:58 +0000
                Re: It's not rocket science?we need a better way to get to space kefischer <emoneyjoe@iglou.com> - 2015-11-12 16:11 -0500
                Re: It's not rocket science?we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-12 21:43 +0000
              Re: It's not rocket science—we need a better way to get to space kefischer <emoneyjoe@iglou.com> - 2015-11-07 22:53 -0500
                Re: It's not rocket science—we need a better way to get to space Fabian Russell <root@localhost.localdomain> - 2015-11-08 04:14 +0000
                Re: It's not rocket science—we need a better way to get to space kefischer <emoneyjoe@iglou.com> - 2015-11-07 23:43 -0500
                Re: It's not rocket science—we need a better way to get to space Edgardo Alcantara <edgardoalcantara@carosseryrepair.org> - 2015-11-08 16:25 +0000
                Re: It's not rocket science?we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-08 06:24 +0000
          Re: It's not rocket science?we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-08 03:29 +0000
            Re: It's not rocket science?we need a better way to get to space Fabian Russell <root@localhost.localdomain> - 2015-11-08 03:40 +0000
              Re: It's not rocket science?we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-08 04:11 +0000
      Re: It's not rocket science—we need a better way to get to space Sam Wormley <swormley1@gmail.com> - 2015-11-07 09:58 -0600
        Re: It's not rocket science—we need a better way to get to space gilber34 <fafa@invalid.com> - 2015-11-07 10:01 -0600
        Re: It's not rocket science—we need a better way to get to space Wally W. <ww84wa@aim.com> - 2015-11-07 11:21 -0500
          Re: It's not rocket science—we need a better way to get to space Sam Wormley <swormley1@gmail.com> - 2015-11-07 10:23 -0600
            Re: It's not rocket science—we need a better way to get to space Wally W. <ww84wa@aim.com> - 2015-11-07 11:27 -0500
              Re: It's not rocket science—we need a better way to get to space Sam Wormley <swormley1@gmail.com> - 2015-11-07 10:29 -0600
                Re: It's not rocket science—we need a better way to get to space gilber34 <fafa@invalid.com> - 2015-11-07 10:35 -0600
                Re: It's not rocket science—we need a better way to get to space Sylvia Else <sylvia@not.at.this.address> - 2015-11-08 14:01 +1100
        Re: It's not rocket science—we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-07 18:53 +0000
          Re: It's not rocket science—we need a better way to get to space Sam Wormley <swormley1@gmail.com> - 2015-11-07 14:09 -0600
            Re: It's not rocket science—we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-07 20:22 +0000
              Re: It's not rocket science—we need a better way to get to space Sam Wormley <swormley1@gmail.com> - 2015-11-07 14:41 -0600
                Re: It's not rocket science—we need a better way to get to space jimp@specsol.spam.sux.com - 2015-11-07 21:19 +0000
            Re: It's not rocket science—we need a better way to get to space Wally W. <ww84wa@aim.com> - 2015-11-07 18:58 -0500
  Re: It's not rocket science--we need a better way to get to space "nuny@bid.nes" <Alien8752@gmail.com> - 2015-11-07 22:27 -0800
    Re: It's not rocket science--we need a better way to get to space Sylvia Else <sylvia@not.at.this.address> - 2015-11-09 16:39 +1100
      Re: It's not rocket science--we need a better way to get to space "nuny@bid.nes" <Alien8752@gmail.com> - 2015-11-09 00:41 -0800
        Re: It's not rocket science--we need a better way to get to space Sylvia Else <sylvia@not.at.this.address> - 2015-11-09 23:16 +1100

csiph-web