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Multiple Engines???



 
 
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  #21  
Old November 24th 03, 11:55 PM
Henry Spencer
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Default Multiple Engines???

In article ,
David Shannon wrote:
Alas, SSTO fuel fraction is prohibitive.

Not necessarily. When people have been pushed hard to try to build
expendable stages with that sort of fuel fraction, they have generally
succeeded. And with 1960s technology, too, in some cases.


Yup, true. I meant (thinking context evident) "RLV" SSTO. Apologies.
Wings, heatshield, deorbit propellant - all dip hard into payload.


Leaving off the wings helps considerably with that. :-) Deorbit fuel is
not a big deal, but heatshield and landing systems are certainly an issue.
On the other hand, we *do* have better technology now than the guys who
built (e.g.) the Titan II first stage.

To me, it seems challenging, but far from hopeless, especially if you are
willing to innovate rather than just believing parametric models -- based
on orthodox past practice! -- for everything. (How much does a horizontal
lander's landing gear weigh? Orthodox parametric guesswork is 4%. NASA
RLV parametric guesswork is 3%. The B-58 landing gear, in 1957, was
1.5%... and the Voyager gear was 0.9%.)

Assume the VentureStar was built and worked as advertised.
257 klb inert, 50 klb payload, 2313 klb LHOx, 8 Aerospikes, fuel
fraction .883
2 smaller editions, 3 Aerospikes on Booster, and 1 on Orbiter, have
*together* 198 klb inert, 50 klb payload, 927 klb LHOx, 4 Aerospikes,
fuel fraction .824
The only added complexity is crossfeed, already proven on the STS.


No, the added complexity is that now you have to develop three different
configurations -- two different vehicles plus the stack. That has a
tendency to cost 2-3x as much as a single vehicle. It also adds a bunch
more failure modes.

The SRB separation may look simple but it isn't; NASA spent a lot of time
and money making sure it would work.


But work it does, yes?


So far, yes. :-) That doesn't mean it's a good idea, especially for a
new design that wants reliability and low development cost.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |
  #25  
Old November 26th 03, 08:55 AM
David Shannon
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Default Multiple Engines???

Leaving off the wings helps considerably with that.

D'accord. I would be happy with a steerable parachute on a capsule -
as long as I can land back at the takeoff point ( R! L! V!, R! L! V!)

2 smaller editions, 3 Aerospikes on Booster, and 1 on Orbiter, have
*together* 198 klb inert, 50 klb payload, 927 klb LHOx, 4 Aerospikes,
fuel fraction .824
The only added complexity is crossfeed, already proven on the STS.


No, the added complexity ... three different configurations ...
It also adds a bunch more failure modes.


Yes indeed.
So, don't use differing configurations in the stages.

Consider the design path where the stages are externaly identical
(eg General Dynamics "Triamese"
http://www.abo.fi/~mlindroo/SpaceLVs/Slides/sld020.htm )

This approach has theoretical limitations that are outweighed by
practical advantages. You only have to spend skull-sweat on the orbiter -
the boosters are simplified variants (no OMS, less TPS, etc, etc)
I can live with the risks in stage separation.
  #26  
Old November 26th 03, 08:55 AM
David Shannon
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Posts: n/a
Default Multiple Engines???

Leaving off the wings helps considerably with that.

D'accord. I would be happy with a steerable parachute on a capsule -
as long as I can land back at the takeoff point ( R! L! V!, R! L! V!)

2 smaller editions, 3 Aerospikes on Booster, and 1 on Orbiter, have
*together* 198 klb inert, 50 klb payload, 927 klb LHOx, 4 Aerospikes,
fuel fraction .824
The only added complexity is crossfeed, already proven on the STS.


No, the added complexity ... three different configurations ...
It also adds a bunch more failure modes.


Yes indeed.
So, don't use differing configurations in the stages.

Consider the design path where the stages are externaly identical
(eg General Dynamics "Triamese"
http://www.abo.fi/~mlindroo/SpaceLVs/Slides/sld020.htm )

This approach has theoretical limitations that are outweighed by
practical advantages. You only have to spend skull-sweat on the orbiter -
the boosters are simplified variants (no OMS, less TPS, etc, etc)
I can live with the risks in stage separation.
  #27  
Old November 26th 03, 09:11 AM
David Shannon
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Default Multiple Engines???

But how do you get the first stage back?

A 1993 NASA study (TP-3335) considered the 2STO case
where 47% of the ascent propellant was carried by the booster,
which had a fuel fraction of .874.

It staged at T+105 seconds at 83,175' and 2,900 ft/sec.
Even so, it was then only 10.5 nautical miles laterally from the pad.
A glide return was perfectly feasible.
  #28  
Old November 26th 03, 09:11 AM
David Shannon
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Posts: n/a
Default Multiple Engines???

But how do you get the first stage back?

A 1993 NASA study (TP-3335) considered the 2STO case
where 47% of the ascent propellant was carried by the booster,
which had a fuel fraction of .874.

It staged at T+105 seconds at 83,175' and 2,900 ft/sec.
Even so, it was then only 10.5 nautical miles laterally from the pad.
A glide return was perfectly feasible.
  #29  
Old November 26th 03, 05:11 PM
Henry Spencer
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Default Multiple Engines???

In article ,
David Shannon wrote:
Consider the design path where the stages are externaly identical
(eg General Dynamics "Triamese"...
This approach has theoretical limitations that are outweighed by
practical advantages. You only have to spend skull-sweat on the orbiter -
the boosters are simplified variants (no OMS, less TPS, etc, etc)


The practical problem with biamese and triamese is that almost anything
you do to simplify the boosters starts you off down the slippery slope of
building two different vehicles. It's very hard to stop that.

Just leaving systems out looks easy, but often it means a lot of extra
engineering to assess what *happens* when you leave those systems out,
and what drives development cost is not materials but engineering effort.
Later on, when weight is excessive or there's a bit of a performance
shortfall, well, we're already building two different configurations, so
we'll just make them a little *more* different...

Biamese or triamese is a win only if the boosters are the *same* as the
orbiter. Same TPS; if it doesn't get as hot, that's nice. Same OMS;
okay, we can leave its tanks empty on the boosters. Same systems, all of
them. Maybe we fill the boosters' cargo bays with tanks, but if so, any
permanent fittings we need to add go in the orbiters too. It takes very
strong engineering leadership to make this work.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |
  #30  
Old November 26th 03, 05:11 PM
Henry Spencer
external usenet poster
 
Posts: n/a
Default Multiple Engines???

In article ,
David Shannon wrote:
Consider the design path where the stages are externaly identical
(eg General Dynamics "Triamese"...
This approach has theoretical limitations that are outweighed by
practical advantages. You only have to spend skull-sweat on the orbiter -
the boosters are simplified variants (no OMS, less TPS, etc, etc)


The practical problem with biamese and triamese is that almost anything
you do to simplify the boosters starts you off down the slippery slope of
building two different vehicles. It's very hard to stop that.

Just leaving systems out looks easy, but often it means a lot of extra
engineering to assess what *happens* when you leave those systems out,
and what drives development cost is not materials but engineering effort.
Later on, when weight is excessive or there's a bit of a performance
shortfall, well, we're already building two different configurations, so
we'll just make them a little *more* different...

Biamese or triamese is a win only if the boosters are the *same* as the
orbiter. Same TPS; if it doesn't get as hot, that's nice. Same OMS;
okay, we can leave its tanks empty on the boosters. Same systems, all of
them. Maybe we fill the boosters' cargo bays with tanks, but if so, any
permanent fittings we need to add go in the orbiters too. It takes very
strong engineering leadership to make this work.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |
 




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