How much mass was saved by using a two-stage LM in Apollo?

Couldn't be too much - extra engine; propellant and pressurant tanks; extra
insulation on the underside of the Ascent Stage etc, as well as the
separation mechanism and other systems.

On Mar 24, 6:03*am, "Alan Erskine" wrote:
Couldn't be too much - extra engine; propellant and pressurant tanks; extra
insulation on the underside of the Ascent Stage etc, as well as the
separation mechanism and other systems.

I thought it was more about reducing the mass the ascent stage engine
had to lift back off the Moon.

"Matt" wrote in message
...
On Mar 24, 6:03 am, "Alan Erskine" wrote:
Couldn't be too much - extra engine; propellant and pressurant tanks;
extra
insulation on the underside of the Ascent Stage etc, as well as the
separation mechanism and other systems.

I thought it was more about reducing the mass the ascent stage engine
had to lift back off the Moon.

Yeah, but if you had an empty 'Descent stage' (minus all the bits-and-pieces
I mentioned above), it wouldn't be _that_ much heavier. I might have to do
some heavy-duty research to find out - check out system weights etc and
deduct them from the DS structure.

Remember, it would also mean the descent stage would be lighter and
therefore use less propellant during descent; making more propellant
available for the combined ascent.

A lot. A single-stage LM would have had to drag all of the hardware
used for descent back into lunar orbit. I don't know exactly how heavy
that equipment would have been, but for a first approximation we can
assume it would have been like lugging the descent stage into orbit.
This is a bit on the pessimistic side, since a single-stage LM would
have needed only one main engine, one set of main propellant tanks and
so on. But it's probably not far off: he ascent engine, for example,
was much lighter than, descent engine, since it was simpler (no
throttling needed) and of much lower thrust.

According to Apollo by the Numbers, Apollo 11's LM had a mass of
16,200 lbm at touch-down. The mass of the ascent stage alone at lunar
lift-off was 10,800 lbm, which gives us a dry weight for the descent
stage of about 5400 lbm.

The mass of Apollo 11's ascent stage as it rendezvoused with the CSM
was about 5040 lbm. Thus, keeping the descent stage would have doubled
the mass to be returned to lunar orbit. Since all of that mass first
had to be landed on the moon, the fully-loaded weight of the LM at the
beginning of descent to the lunar surface would have been much
greater.

Even if I've overestimated the mass by fifty percent, we're still
talking about a single-stage LM that is much heavier than the actual
two-stage version.

The two stage version has the safety advantage that there's a back-up
engine available should the descent engine fail during landing. In
addition, the ascent engine can be very simple, and hence reliable,
since it requires no throttle.

A lot. A single-stage LM would have had to drag all of the hardware
used for descent back into lunar orbit. I don't know exactly how heavy
that equipment would have been, but for a first approximation we can
assume it would have been like lugging the descent stage into orbit.
This is a bit on the pessimistic side, since a single-stage LM would
have needed only one main engine, one set of main propellant tanks and
so on. But it's probably not far off: he ascent engine, for example,
was much lighter than, descent engine, since it was simpler (no
throttling needed) and of much lower thrust.

According to Apollo by the Numbers, Apollo 11's LM had a mass of
16,200 lbm at touch-down. The mass of the ascent stage alone at lunar
lift-off was 10,800 lbm, which gives us a dry weight for the descent
stage of about 5400 lbm.

The mass of Apollo 11's ascent stage as it rendezvoused with the CSM
was about 5040 lbm. Thus, keeping the descent stage would have doubled
the mass to be returned to lunar orbit. Since all of that mass first
had to be landed on the moon, the fully-loaded weight of the LM at the
beginning of descent to the lunar surface would have been much
greater.

Even if I've overestimated the mass by fifty percent, we're still
talking about a single-stage LM that is much heavier than the actual
two-stage version.

The two stage version has the safety advantage that there's a back-up
engine available should the descent engine fail during landing. In
addition, the ascent engine can be very simple, and hence reliable,
since it requires no throttle.

"Alan Erskine" wrote in message
...

"Matt" wrote in message
...
On Mar 24, 6:03 am, "Alan Erskine" wrote:
Couldn't be too much - extra engine; propellant and pressurant tanks;
extra
insulation on the underside of the Ascent Stage etc, as well as the
separation mechanism and other systems.

I thought it was more about reducing the mass the ascent stage engine
had to lift back off the Moon.

Yeah, but if you had an empty 'Descent stage' (minus all the
bits-and-pieces
I mentioned above), it wouldn't be _that_ much heavier. I might have to
do
some heavy-duty research to find out - check out system weights etc and
deduct them from the DS structure.

Remember, it would also mean the descent stage would be lighter and
therefore use less propellant during descent; making more propellant
available for the combined ascent.


Consider you need tanks and engine. The mass of the tank compared to the
mass of the fuel probably isn't that much.

So extra tankage probably doesn't hurt as much there as one might think.

The mass of the engine I'm guessing is a more significant issue.

The descent stage was about 2/3rds of the total mass of the entire LM stack.
Of that, the fuel as about 80% of the descent stage mass.

(Descent stage: approximately 10,000 kg, fuel 8,000 kg, ascent stage 4,600
kg, fuel 2,300 kg)


So the total mass of the fuel alone is about the same as total mass of the
descent stage. Figure if you did it as a SSTO, your fuel would have to go
way up. (Someone else can plug all this into equations.)

So first pass, fairly significant I'd say.

"Greg D. Moore (Strider)" wrote in message
...
Consider you need tanks and engine. The mass of the tank compared to the
mass of the fuel probably isn't that much.

The DS tanks could have been extended - not by much either - they were
considerably greater diameter than the AS tanks. Also, the AS tanks were
spherical - basically, two domes welded together; so a small (cylindrical)
extension in the length of the DS tanks would have been much lighter than
that. Sure, as a % of propellant capacity, the saving might not have been
that great, but if all the systems _exclusive_ to the AS that were
essentially duplicates of the DS systems; the difference might be
substantial.


So extra tankage probably doesn't hurt as much there as one might think.

And then there's all the insulation panels under the AS. Think of the
increase in useable volume inside the AS. And there's all the panelling on
top of the DS - designed to prevent the thrust of the ASE from penetrating
into the DS. Wouldn't need that either.


The mass of the engine I'm guessing is a more significant issue.

According to EA (http://www.astronautix.com/engines/tr201.htm), the AS
Engine weighed 113kg. Plus all the pressurising tanks; plumbing (and the
pyrotechnics that go with stage sep) Does anyone know how much the landing
gear weighed? hmmmm....

I'm sure there's not going to be that much of a difference in total mass -
and then there's the residual prop in the DS - AS11 had 20 seconds, but the
other landings had over 60 seconds each - that adds up to quite a bit of
mass too.

Alan Erskine wrote:
Couldn't be too much - extra engine; propellant and pressurant tanks; extra
insulation on the underside of the Ascent Stage etc, as well as the
separation mechanism and other systems.



The Russian LK used the same engine for landing and ascent, after
braking out of lunar orbit via a "crasher" stage that had no landing
gear on it.
We had a lot more propellant tankage devoted to braking the LM out of
lunar orbit, so it certainly behooved us to try and get rid of its
weight, as well as that of the landing gear, before ascent.
One of the main things that drove the two-stage decision on the LM was
the desire for having a pristine ascent engine for lift-off, rather than
one that had been fired during descent.
The Soviets got around this on the LK by having two separate engines
combined into one that had a single main engine combustion chamber with
a pair of secondary combustion chambers flanking it. On ignition, both
sets of combustion chambers would ignite, with the outer pair shutting
down if the central engine was working properly.
The landing gear was to remain on the Moon, serving as a launching pad
for the main body of the LK.

Pat

"Pat Flannery" wrote in message
...


Alan Erskine wrote:
Couldn't be too much - extra engine; propellant and pressurant tanks;
extra
insulation on the underside of the Ascent Stage etc, as well as the
separation mechanism and other systems.



The Russian LK used the same engine for landing and ascent, after braking
out of lunar orbit via a "crasher" stage that had no landing gear on it.
We had a lot more propellant tankage devoted to braking the LM out of
lunar orbit, so it certainly behooved us to try and get rid of its weight,
as well as that of the landing gear, before ascent.

True. In addition to the mass of the landing gear is the mass of the
structure in the descent stage which was there to support and distribute the
landing loads, which was a pretty big unknown before the first landing. No
one really knew the physical properties of the lunar surface. That and no
one knew how well the astronauts would do landing the LEM with the
possibility of a lot of lunar dust being kicked up and obscuring the view.
It was a real possibility that the landing could be pretty hard if the
descent engine blew away enough dust to reveal a hard, rocky surface and the
blowing dust made the view so bad the landing was hard to begin with.

One of the main things that drove the two-stage decision on the LM was the
desire for having a pristine ascent engine for lift-off, rather than one
that had been fired during descent.

I think this was partly another byproduct of the unknown dust issue. That
and there was always the possibility that the descent engine would get bent
by hitting a rock or something else on the surface.

The Soviets got around this on the LK by having two separate engines
combined into one that had a single main engine combustion chamber with a
pair of secondary combustion chambers flanking it. On ignition, both sets
of combustion chambers would ignite, with the outer pair shutting down if
the central engine was working properly.
The landing gear was to remain on the Moon, serving as a launching pad for
the main body of the LK.

If it were to serve as a launching pad, that would be landing gear and some
supporting structure, right?

Jeff
--
A clever person solves a problem.
A wise person avoids it. -- Einstein

Jeff Findley wrote:
If it were to serve as a launching pad, that would be landing gear and some
supporting structure, right?


Yup, basically a framework ring the main part of the spacecraft rested on.
This also held surface experiment gear, landing radar, batteries, and
some of the communication antennas.
There's a video of the ascent he http://www.russianspaceweb.com/lk.html
Although he doesn't show the outer two engine nozzles shutting down.
When on the surface, the engine bells were covered by clamshell doors.

Pat