Why is a LOX/Kero SSTO not rather easy?

Which has nothing to do with his assertion that simply scaling up and
throwing testing out would make it cheaper.

--
"Yea, all israel have transgressed thy law, even by departing, that they
might not obey thy voice; therefore the curse is poured upon us, and the
oath that is written in the law of Moses the servant of God, because we have
sinned against him." Daniel 9-11

Am Sun, 14 Sep 2003 15:28:08 -0700 schrieb "Larry Gales":

[...]
Anywho, the problem with adding a small number of engines is
that because of that same limited throttle range you have to
rely on all of them. With three times as many engines and
all of them needed you've just upped the chance of *a* failure
involving an engine by a factor of 3. In order to keep the
failure rate the same you need to have enough redundancy so
that you have the same probability for equivalent
configurations. For example, if you have a 1 engine rocket
that uses a 1% failure rate engine then you have a 1% engine
related failure rate. But if you up that to 3 engines you
end up with a 3% failure rate vehicle. To get back below
1% you only need to add an extra engine though, you'll have a
4% failure rate for one engine out but only a .06% rate (I
think) of falling below the minimum number of engines needed
for flight. Though, of course, that brings symmetry issues
into play since you usually have to shutdown engine pairs
unless it's the center engine.

You will achieve a stable flight, as long as the thrust vectors of all
engines will go through te center of gravity (CG) of the rocket
complex. So, if you provide gimballing enough to move the thrust
vectors according to the moving CG with emptying of the tanks, any
engine can cut out without losing stability. I know, that this is
problematic while in atmospheric flight because of aerodynamic loads,
but in principle this method works.

And this "trick" everytimes works then, when there are engines enough
to acieve the goal of not losing the payload, even when one engine
fails. And you don't NOT need to switch off engines symmetrically. The
disadvantage of this is, that you lose a bit performance, if the
thrust vector does not direct exactly to the acceleration vector. So
that is used (normally) only, if there is no possibility to switch off
the engine - e.g. solid boosters like found on many launchers.

Well known examples are the Delta-II/III, or the "old" Ariane-4, where
the booster engines have thrust vectors going through CG and not
parallel to acceleration vector. And there are even asymmetrically
mounted booster arrangements, like in Delta-74xx models...

In other cases, when all or main thrust is generated by engines with
throttling/switch off capability (usually liquids) it is mostly easier
to invest in gimballing capacity of (part of) the engines to reduce
the risk of necessity of off-switching or throttling of healthy
engines - as long as there IS enoungh thrust anymore. And that depends
on the total number of engines and the thrust reserves of them.


cu, ZiLi aka HKZL (Heinrich Zinndorf-Linker)
--
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http://zili.de X No HTML in
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Am Sun, 14 Sep 2003 15:28:08 -0700 schrieb "Larry Gales":

[...]
Anywho, the problem with adding a small number of engines is
that because of that same limited throttle range you have to
rely on all of them. With three times as many engines and
all of them needed you've just upped the chance of *a* failure
involving an engine by a factor of 3. In order to keep the
failure rate the same you need to have enough redundancy so
that you have the same probability for equivalent
configurations. For example, if you have a 1 engine rocket
that uses a 1% failure rate engine then you have a 1% engine
related failure rate. But if you up that to 3 engines you
end up with a 3% failure rate vehicle. To get back below
1% you only need to add an extra engine though, you'll have a
4% failure rate for one engine out but only a .06% rate (I
think) of falling below the minimum number of engines needed
for flight. Though, of course, that brings symmetry issues
into play since you usually have to shutdown engine pairs
unless it's the center engine.

You will achieve a stable flight, as long as the thrust vectors of all
engines will go through te center of gravity (CG) of the rocket
complex. So, if you provide gimballing enough to move the thrust
vectors according to the moving CG with emptying of the tanks, any
engine can cut out without losing stability. I know, that this is
problematic while in atmospheric flight because of aerodynamic loads,
but in principle this method works.

And this "trick" everytimes works then, when there are engines enough
to acieve the goal of not losing the payload, even when one engine
fails. And you don't NOT need to switch off engines symmetrically. The
disadvantage of this is, that you lose a bit performance, if the
thrust vector does not direct exactly to the acceleration vector. So
that is used (normally) only, if there is no possibility to switch off
the engine - e.g. solid boosters like found on many launchers.

Well known examples are the Delta-II/III, or the "old" Ariane-4, where
the booster engines have thrust vectors going through CG and not
parallel to acceleration vector. And there are even asymmetrically
mounted booster arrangements, like in Delta-74xx models...

In other cases, when all or main thrust is generated by engines with
throttling/switch off capability (usually liquids) it is mostly easier
to invest in gimballing capacity of (part of) the engines to reduce
the risk of necessity of off-switching or throttling of healthy
engines - as long as there IS enoungh thrust anymore. And that depends
on the total number of engines and the thrust reserves of them.


cu, ZiLi aka HKZL (Heinrich Zinndorf-Linker)
--
/"\ ASCII Ribbon Campaign
\ /
http://zili.de X No HTML in
/ \ email & news