Lifting Starships

> From: Timothy Little [email removed]
> Subject: Streamline, airframe, and rockets.

Guide:
> >Me:
> Tim Little

> Ports without an atmosphere are also likely to have low gravity -
> hence landing under thrust should be easier than on Earth.

Ya'll better check the world generation system in Traveller again.
A planet is likely to have any kind of atmosphere.

> If the thrust is less than the surface gravity of the landing site,
> it will have *serious* problems.

Agreed.  That was my point.  Aerodynamic lift will NOT make up for a
lack of downward thrust.

> > There MUST be downward pointing rockets on ships in order for them
> > to land in such circumstances.
>
> Maybe they are not designed to land under such circumstances?  The
> incidence of high-G airless planets should be rather low, shouldn't it?

I haven't tried TNE world generation yet, but in Trav & MegaTrav they
were Not unknown.  Also, my reading of TNE says that an airframe ship can
land on any planet regardless of atmosphere.

> Besides which, why would any ship equipped with rockets *not* call
> the direction of thrust 'down'?  In space, it won't matter much (it
> will probably simplify artificial gravity or inertial compensation).
>
> If you need thrust in order to land, it makes sense to get the most
> effective use of it by directing it (largely) straight down.  Beside
> which, the ship will be designed to take a large force along the axis
> of the main drive - if you wanted to land the ship on it's side, there
> would need to be extra support for that, too.

Here's where my Trav background does me a disservice.  You are quite
correct in saying that any sensibly designed ship would land on its tail.
It would be less costly structurally, and would require less artificial
gravity and inertial compensation than having your a "Down" side (perhaps
"Ventral" is a better word in these circumstances) being along the side
of the ship.  Unfortunately, 90% of all published Traveller, MegaTrav
(and the TNE illustrations) do precisely this.  The only sensible designs
I can think of off the top of my head are the Mercenary Cruiser, the
Azhanti High Lightning, and the Lab Ship (though that's a different matter)
However, since we are Stuck with the problem that our beloved Scoutship,
Free Trader, Far Trader, Fat Trader, Kinunir, etc. etc. etc. have this
"Ventral" surface and orientation, that is the problem we must address.
The example gave one of the worst offenders the Donosev class scout
surveyor.  Take a look at the deck plans.  No one thought for more than
2 seconds before setting pen to paper how she would be laid out.  The
deck SHOULD be perpendicular to the drive.  It SHOULD land on it's tail.
It doesn't.  (sigh)

> > As an unstreamlined ship will not
> > be able to use lift at ANY time, this will be the only normal way
> > the ship could land at ALL.
>
> How about a high-tech version of a parachute?  Of course, launching
> would be extremely difficult.  Perhaps it would need to make use of a
> separate launching vehicle?

Nearly all unstreamlined ships in Trav DO carry auxilluries.
But many of these ships still persist in attempting to land.
Also the example we're trying to discuss is how one Might land
if it had to.

> > Barring specially fitted rockets, in order to maneuver properly, a
> > ship's rocket engines must be able to be gymbaled.  (You can't have
> > them pointing in one direction only all the time.)
>
> Why not?  I imagine a fixed main thrust engine, with a number of smaller
> maneuver thrusters.  It's certainly a lot easier than engineering a
> gimbal that will work reliably under the high-stress (probably high-
> temperature and vibratory) conditions of a main rocket.

Take a look at every rocket launched since the V-2.  All of them use
gymbaled thrust.  From the Sapwood to the Saturn V to the Space Shuttle.
Why?  Well, if you did have rockets pointing in every orientation you
might use, you would need to build them of comperable power to the main
engine (I'm not referring to orientation thrusters here, but only main
engines)  If you needed to correct a thrust from your main engine, you
need something comparable.  That means you would need engines of comparable
size and weight to the fixed main engine.  So, when these things aren't
correcting thrust, they are just dead weight.  VERY inefficient.  Gymballing
your engines is a minor consideration by comparison.

> >  It is true that
> > the amount that the engines must be deflected is limited, but with
> > contra-gravitic drives, you won't need much deflection at all.
> > >From my reading of the paragraph outtake from FF&S, the contragrav
> > takes care of 99% of the weight.  So, you need a 1/100th of a G
> > vector from your engines in order to get a full G upward to equal
> > your weight and a bit more to take off.  Well, if you have 1 G
> > engines, the necessary deflection is Less than one degree.
>
> If you're using a 1G drive to counter 0.01G of weight, what about the
> 0.99995G that's propelling you *forward*?  Surely you want the smallest
> possible speed when you land?

Who says you keep the engine pointing in the same direction all the time?
You could (and probably would under the circumstances) land backward,
using the engines as breaking power.  (BTW, cosine of 1 degrees is only
0.998476952.  :-)

> > A 1 degree deflection gives you a net thrust of .0075 G's upward.
> > Just barely enough.  Fear not, a deflection of 6.3 degrees will
> > give you a full net thrust of 0.1 G in an upward direction.  This
> > is a negigable defection and plenty of thrust for landing.
>
> Too much thrust!  There's still 0.995G thrust pushing you forawrd.
> (After 1 minute on an airless planet, this is 2100 km/h)

No, cosine of 6.3 degrees is only 0.99396.  :-) And as I said, you
need not be keeping your engine pointing in one direction all the
time allowing your vector to build up.  Landing backward would
probably be best.  Also, here I was trying to cover what I would consider
to be the absolute minimums needed for a gymballing engines.  If you
put less limit on the amount the engines may be traversed, your
"Forward" thrust will be substantially reduced.

> > As we have seen in the case of the streamlined ship, we can not
> > exceed the 1000kph limit on the structure.
>
> None of the early manned space capsules were airframe - and they re-
> entered at *much* higher speeds than this.  Heat was a large problem,
> of course.

(Sigh) actually the space capsules certainly were "airframe" structures.
"Streamlined" as defined by Traveller, means something who's structure
is capable of travelling at transsonic speeds.  "Airframe" structures
are ones which can withstand supersonic stresses.  I also recall reading
that the Apollo command module was actually designed to use it's heat
shield for lift in a very, very limited capacity.  It was supposed to
be able to "fly" though I think they were more describing a superior
control of plummeting.  :-)

> What does a ship have to look like to qualify as `unstreamlined'?

Say, something like the Apollo Lunar Lander, Skylab,
Mir, Station Freedom, Salyut, The Donosev Scout ship, The Space:1999
Eagle, and virtually any design seen on the new Star Trek.  (There,
that ought to be enough examples!)

> For instance, is a pure sphere considered `streamlined'?

Yes, (I haven't seen BL, but Trav & MegaTrav defined a sphere as
"streamlined", but not "airframe".

> > [300 kph] This is a VERY low speed
> > for you to try to enter the atmosphere at.  One would have to come
> > to almost zero ground speed for a ship to enter the upper
> > atmosphere.  You would have to have some pretty good meteorological
> > data to avoid winds of 300kph on re-entry.
>
> I assume the 300kph limit is for `standard' atmospheres (100 kPa).
> At higher altitudes, the limit would (should?) be higher due to lower
> density.  Also, if there are no high-shear regions, the problem will be
> self-correcting.

Not necessarily, 300kph is a low speed for re-entry no matter what
the pressure differences may do.  Say we double that for allowable
speeds in the upper atmosphere.  That's still Abominably low airspeed.
Say you're up in orbit.  You can bring yourself to near zero ground speed
using your engines, and then begin your descent through the atmosphere.
To then attempt to avoid building up a few hundred kph vector, and avoid
pockets of windshear, gusts and jet streams is on the difficult side.
A VNE (Velocity Never Exceed) of 300kph is pretty dang low, unless you're
flying a Cessna.  :-)

> (In a long-distance flight, the difference in air velocity between start
> and finish points can be around 3000 km/h.  The difference is gradually
> accomodated during the flight)

Most of the flights you describe do not require re-entry through the
winds of the upper atmosphere, and are not undertaken by planes with
a VNE of 300kph.

> > Doing so, even with
> > good data, and a ground side weather station at the starport should
> > be pretty dang close to impossible.
>
> Compared to the task of instellar navigation?  Or even getting into an
> orbit that is guaranteed not to hit anything else up there?
> Are you kidding?

Without precise atmospheric data on wind speed and VNE of 300kph?  No
I am not kidding.  Stars are relatively easy to detect.  How does one
remotely detect wind speed?  Unless it's raining, your doppler weather
radar won't show you a thing.  Best way to go would be to have an
"airframe" ship out in front of you guiding you down to avoid turbulance
and gusts and giving you constant windspeed updates.  Without that
asisstance, I would say that if the starport is below TL 8 it would be
impossible or close to.
- --
Scott 2G Kellogg