Multiple Engines???

It seems that for rockets of multiple stages with only one fuel combination,
there is an interesting engineering decision.

Consider a two stage rocket where both stages burn the same fuel
combination.

You could use 1 engine for the upper stage, and 4-ish engines of the same
design for the lower stage. The advantage is that you need only design one
engine type. The disadvantage is that with 4-ish engines on the lower stage
you probably cannot tolerate an engine failure, and clearly not a catastrophic
failure. Therefore you might lose a bit of reliability (which you might get back by spending the saved money on reliability).

Alternatively, you might use two different engine designs, a large
and a small. This reduces the total part count while increasing the
total unique part count. It probably increases cost and reliability.

Does anyone have any numbers that might help convince which is the
better path? For example, is motor design cost a large part of the
overall vehicle cost? Are most failures due to motor failures? Is a
single large motor likely to weigh less and/or have a higher ISP than
a few smaller (but still large) motors?

Basically, anyone have any good arguments for either choice?

-Thanks
-Talleyrand

P.S. Is it reasonably easy to tailor an engine to
atmosphere or vacuum operation with changes to the engine bell;
things like turbopump and cooling systems can remain the same?

"Charles Talleyrand" wrote in message
...
It seems that for rockets of multiple stages with only one fuel
combination,
there is an interesting engineering decision.

Consider a two stage rocket where both stages burn the same fuel
combination.

You could use 1 engine for the upper stage, and 4-ish engines of the same
design for the lower stage. The advantage is that you need only design
one
engine type. The disadvantage is that with 4-ish engines on the lower
stage
you probably cannot tolerate an engine failure, and clearly not a
catastrophic
failure.

Use five, one in the center, and design so that 3.5 or 4 at rated thrust
would be sufficient. Then, run 5 at 80-90%; if the center engine dies, the
others can be revved up. If an outer one dies, the opposite one can be cut
back for balance while the others rev up.
--
If you have had problems with Illinois Student Assistance Commission (ISAC),
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in the works.

The important point is that propellant is cheap, cheap, cheap, and loss of a launcher is as expensive as all hell.

Best reliability calls for a completely reuseable single stage launcher, with engines of such a size and number that, if at any
time one of them must be shut down, the others throttle up to compensate, and you just keep going. Use of two parallel stages is
a distant second because separating two vehicles at high speed in the atmosphere is not simple. (A two parallel stage launcher
can be designed in which separation occurs outside the atmosphere; this does make the return of the first stage a bit more
difficult.) Series staging is terrible because you take off without the second stage engines running, and thus without knowing
whether they will start and run correctly. In any case there's little reason to use multiple different engine designs in one
vehicle.

The "reusable" part means that you can get all of the design and manufacturing errors out of every flight article before it goes
into service. This also means that, in service, the chances of a catastrophic engine failure are negligible.

Best reliability calls for a completely reuseable single stage launcher

Alas, SSTO fuel fraction is prohibitive. 2STO typically uses 1/3 the
propellant for a given payload, although vehicle empty weights are higher.

separating two vehicles at high speed in the atmosphere is not simple.

Shuttle does it every mission - SRBs from ET, ET from Orbiter.

Series staging is terrible because you take off without the second stage engines running, and thus without knowing whether they will start and run correctly.

Agreed

In any case there's little reason to use multiple different engine designs in one vehicle.

Servicing a single engine type is cheaper, if all other things are equal.
4 on the booster and 1 on the Orbiter would support a single engine core,
with differing bell arrangements.

In article ,
David Shannon wrote:
Best reliability calls for a completely reuseable single stage launcher

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.

Reusability is the uncertain part, but that's true of TSTO systems too.

2STO typically uses 1/3 the
propellant for a given payload, although vehicle empty weights are higher.

However, since propellant costs are negligible, and empty mass and
complexity are the expensive parts...

separating two vehicles at high speed in the atmosphere is not simple.

Shuttle does it every mission - SRBs from ET, ET from Orbiter.

Note the words "in the atmosphere". The ET separation occurs in vacuum.

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

Servicing a single engine type is cheaper, if all other things are equal.
4 on the booster and 1 on the Orbiter would support a single engine core,
with differing bell arrangements.

In fact, NASA planned roughly that for the original two-reusable-stage
shuttle.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

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.

2STO typically uses 1/3 the
propellant for a given payload, although vehicle empty weights are higher.

However, since propellant costs are negligible, and empty mass and
complexity are the expensive parts...

Hmmm. I misspoke. Try these numbers....

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.

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?

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. |

(David Shannon) wrote in message
2STO typically uses 1/3 the propellant for a given payload, although vehicle empty weights are higher.

I don't have a problem with 2STO for a RLV. But how do you get the
first stage back? If you have good mass ratio between the 1st and 2nd
stage, its likley to be going fast at the seperation point making it
quite hard to do.

Greg

"Richard Schumacher" wrote in message ...

The important point is that propellant is cheap, cheap, cheap, and loss of a launcher is as expensive as all hell.

Best reliability calls for a completely reuseable single stage launcher, with engines of such a size and number that, if at any
time one of them must be shut down, the others throttle up to compensate, and you just keep going.

This is not obvious.

In a world where engines explode upon failure, having exactly one engine per stage is best.

In a world where there are finite development dollars, and those dollars can buy reliability, having
one TYPE of engine per vehicle is best.

It has been suggested to me in private email that the correct answer to this delima is to have one engine
FAMILY, but with multiple engine sizes per family.


The "reusable" part means that you can get all of the design and manufacturing errors out of every flight article before it goes
into service. This also means that, in service, the chances of a catastrophic engine failure are negligible.

I dunno. Seems to me that airplanes occasionaly suffer failure despite their resability, and it's not clear why
repeated testing makes catastrophic failure preferencially less likely than any other type.

By the way, 80 or less column posts are usually appreciated.
Format, format, format!

Charles Talleyrand wrote:
"Richard Schumacher" wrote:
The important point is that propellant is cheap,
cheap, cheap, and loss of a launcher is as expensive as all hell.

Best reliability calls for a completely reuseable single stage
launcher, with engines of such a size and number that, if at any
time one of them must be shut down, the others throttle up to
compensate, and you just keep going.

This is not obvious.

In a world where engines explode upon failure, having exactly
one engine per stage is best.

Engines actually rarely explode on failure; going back
through the history of flight failures shows almost exclusively
systems failure followed by shutdown, or accidental shutdown,
without any uncontained failure. It's not unknown but is a lot
rarer than 'graceful' shutdowns.

Cost and complexity constraints as well as reliability
analysis do argue for single engines per stage on expendables,
and five or more on reusables with abort-to-orbit (fewer if
abort-to-ground is ok).

In a world where there are finite development dollars,
and those dollars can buy reliability, having
one TYPE of engine per vehicle is best.

That does have its limits. It's great on SSTO and Stage
and a Half, and some TSTO concepts. It sucks on three or
more stage vehicles where the GLOW of the first stage may be
a hundred or more times the GLOW of the last stage...

It has been suggested to me in private email that the correct
answer to this delima is to have one engine
FAMILY, but with multiple engine sizes per family.

Hard to do that; engines don't scale very well in terms of keeping
similar design and construction. Similar operating concept and
specifications? Sure. But the parts won't be descended or
related very closely.

One reasonable exception is truncated versus long nozzles...
*that* isn't such a big change. You can keep the exact same
pumps, combustion chamber, injector, etc.

The answers to some of these questions vary significantly when
you look at serious RLV operability and seriously far out BDB
designs, as optimizations start to pull in unexpected ways.


-george william herbert