Multiple Engines???

In article ,
Charles Talleyrand wrote:
In a world where engines explode upon failure, having exactly one
engine per stage is best.

Fortunately, this is not such a world. Explosions of fully-developed
liquid-fuel engines are very rare. (Solids are another matter.) They
may refuse to work but they seldom do anything messy.

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.

As has already been noted, this doesn't work, because many aspects of
rocket engines don't scale straightforwardly.

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...

Yes, but note that this is *extremely* rare by rocket standards, and also
that they very seldom crash as a result. The two facts are closely
related: what is needed is reusability *and* fault tolerance. Given that
combination, you can debug each flight article -- things will go wrong,
but you survive them, and fix them before the next flight. The result is
a much lower failure rate in operational service, and the fault-tolerance
means that you usually survive even the failures which do still happen.

(A comparison of the X-15 and the Atlas A -- similar-sized vehicles with
broadly similar performance specs, built at around the same time with
about the same technology level -- is instructive. The X-15 was reusable
and fault-tolerant, the Atlas A was neither. X-15 operational reliability
was far higher, *and* its development program was much cheaper.)
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

Heinrich Zinndorf-Linker wrote:
Am 20 Nov 2003 20:38:05 -0800 schrieb "George William Herbert":
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.

Ok, then search for "hard start" instead - that's the euphemism used
for engine explosion when it occurs on ignition. You will maybe
surprised to find a significant number of them...

No, "hard start" is not a euphemism for an engine
explosion on ignition. A hard start is a hard start;
some buildup of propellant prior to complete ignition,
followed by rapid high pressure combustion.

A hard start that leads to system or mechanical failure
or engine dissassembly is no longer called a hard start.

I just skimmed through the launch failure reports in
3rd edition Iaskowiwitz (Space Launch Systems).
There are a few engine failures with not enough
details to tell if they were hard starts or another
failure, but all the attributed engine failures were
from other causes, unless I missed one.

Could you please point to actual examples of
vehicles lost due to engine hard starts?
Not development accidents; production vehicle
losses. Hmm. Well, the X-15 might count,
but even that is stretching it somewhat.


-george william herbert

In article ,
George William Herbert wrote:
Could you please point to actual examples of
vehicles lost due to engine hard starts?
Not development accidents; production vehicle
losses. Hmm. Well, the X-15 might count,
but even that is stretching it somewhat.

? The only X-15 actually lost was a mid-air breakup due to control-system
misbehavior (complicated by poor display design, pilot vertigo, and a
hypersonic spin), long after engine cutoff.

One of the X-15s was badly damaged during a ground test, but that was a
tank burst due to overpressurization after an engine shutdown.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

On 22 Nov 2003 19:21:55 -0800, (George William
Herbert) wrote:

No, "hard start" is not a euphemism for an engine
explosion on ignition. A hard start is a hard start;
some buildup of propellant prior to complete ignition,
followed by rapid high pressure combustion.

You also get a hard start (or hard light) when the fuel being pumped
into the afterburner doesn't light immediately and pools in the burner
can. Then when it does light, it goes "whoomp", blows out a flame as
long as the airplane, and give the crew a kick in the rump.

Could you please point to actual examples of
vehicles lost due to engine hard starts?
Not development accidents; production vehicle
losses. Hmm. Well, the X-15 might count,
but even that is stretching it somewhat.

A number of fighters, but since they're not rockets, they don't count.
The F-106 only had hard starts because of the way it was designed, but
it flew for decades that way.

Mary

--
Mary Shafer Retired aerospace research engineer

Henry Spencer wrote:

(A comparison of the X-15 and the Atlas A -- similar-sized
vehicles with broadly similar performance specs, built at around
the same time with about the same technology level -- is
instructive. The X-15 was reusable and fault-tolerant, the
Atlas A was neither. X-15 operational reliability was far
higher, *and* its development program was much cheaper.)

But on the other hand the Atlas A was hands down the better
investment.

Jim Davis

Am 22 Nov 2003 19:21:55 -0800 schrieb "George William Herbert":

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.

Ok, then search for "hard start" instead - that's the euphemism used
for engine explosion when it occurs on ignition. You will maybe
surprised to find a significant number of them...

No, "hard start" is not a euphemism for an engine
explosion on ignition. A hard start is a hard start;
some buildup of propellant prior to complete ignition,
followed by rapid high pressure combustion.

I know of occurences, where the term 'hard start' WAS used as an
euphemism to explain, why a mission goal could not be achieved.

One example I can pick out is the Ariane-5 mission (L#142) that should
have launched Artemis and BSAT-2B to GTO - BSAT was lost completely,
and Artemis lost much mission time by that so called hard start...

So what?

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

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?

On 23 Nov 2003 07:41:11 GMT, Jim Davis
wrote:

Henry Spencer wrote:

(A comparison of the X-15 and the Atlas A -- similar-sized
vehicles with broadly similar performance specs, built at around
the same time with about the same technology level -- is
instructive. The X-15 was reusable and fault-tolerant, the
Atlas A was neither. X-15 operational reliability was far
higher, *and* its development program was much cheaper.)

But on the other hand the Atlas A was hands down the better
investment.

Really? This means you put a cash value on the increased knowledge of
hypersonic flight the X-15 produced. What number did you arrive at
and how? What were the respective ROIs?

Mary

--
Mary Shafer Retired aerospace research engineer

Heinrich Zinndorf-Linker wrote:
Am 22 Nov 2003 19:21:55 -0800 schrieb "George William Herbert":
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.

Ok, then search for "hard start" instead - that's the euphemism used
for engine explosion when it occurs on ignition. You will maybe
surprised to find a significant number of them...

No, "hard start" is not a euphemism for an engine
explosion on ignition. A hard start is a hard start;
some buildup of propellant prior to complete ignition,
followed by rapid high pressure combustion.

I know of occurences, where the term 'hard start' WAS used as an
euphemism to explain, why a mission goal could not be achieved.

One example I can pick out is the Ariane-5 mission (L#142) that should
have launched Artemis and BSAT-2B to GTO - BSAT was lost completely,
and Artemis lost much mission time by that so called hard start...

So what?

So, find a second such incident.

Just about everything in the world has happened once
to a space launch. The things that happen over and over
again are where we need to focus attention, for the most part.

There are clear patterns such as leaving rags / pipe covers /
other foreign objects in pipes and tanks; guidance systems losing
their minds and not having a backup; solid rocket motors going
boom; etc. Hard starts are not a clear pattern.


-george william herbert

Mary Shafer wrote:

But on the other hand the Atlas A was hands down the better
investment.

Really?

Really.

This means you put a cash value on the increased
knowledge of hypersonic flight the X-15 produced.

Not a cash value, but a value.

What number
did you arrive at and how?

The Atlas has had a far greater impact on the world than has the
X-15. Indeed, the development of the ICBM ensured that no
operational vehicles would follow from the X-15 research program.
It became a technological dead end.

What were the respective ROIs?

Hard to quantify exactly, but there is certainly a lot more money
in ballistic/orbital flight than in hypersonic flight.

Jim Davis