How are/were rockets stabilised

With the source of thrust so far behind the centre of mass, large
rockets would seem to be unstable in flight. So how is this overcome?

I could imagine a sensitive gyro in the rockets nose, which enables a
microprocessor to calculate the deviation from the planned line of
flight. This then instructs actuators to repoint the engine to
compensate (or a vane to redirect the thrust). Is this broadly how its
done?

If so, how were the early rockets kept in line? A suspect digital
control might have been available for Apollo, but what about the
earlier rockets?

The problem is referred to as a vertical pendulum. Basically like
balancing a
broom in the palm of your hand. You move your hand back and forth to
keep
the broom vertical. The weight being at the top actually reduces the
difficulty.

As to what was before digital, the control was analog and in fact for
simple
systems controlled with a very low parts count and is most often has a
faster response than digital systems. The negatives that have moved
such
systems into disfavor: That your mathematical constants must be know
and
hard-wired into the design, components have to be precision with very
little
drift, and late design changes means tearing open the box to modify the
circuit. To some degree analog components are always in the loop but
mostly to provide the muscle for the digital controls.

Even the German V2 rocket of the 1940s used a gyroscope. The thrust was
redirected by graphite vanes in the engine exhaust. I'm not sure, but
the control system might have been completely mechanical in that era.

Dave

wrote:

With the source of thrust so far behind the centre of mass, large
rockets would seem to be unstable in flight. So how is this overcome?

Rocket stability does not depend on where the nozzle is placed. If you look
at Robert Goddards worlds-first liquid fueled rocket:
http://www.epower-propulsion.com/epo...d%20Rocket.jpg

you'll see that he placed the nozzle on top, because he believed this to be
crucial for stability. As it turned out, it is irrelevant, where the exhaust
nozzle is placed. Which is why it's always placed on the bottom.

I could imagine a sensitive gyro in the rockets nose, which enables a
microprocessor to calculate the deviation from the planned line of
flight. This then instructs actuators to repoint the engine to
compensate (or a vane to redirect the thrust). Is this broadly how its
done?

Sounds good to me :-) Something along those lines, at least.

If so, how were the early rockets kept in line? A suspect digital
control might have been available for Apollo, but what about the
earlier rockets?

Now, basic rocket stability is provided by the airflow over the rocket body
and fins. That's actually the only reason to put fins on a rocket.
Basically, if a rocket's center of gravity is located at least one body
diameter (rule of thumb) in front of the rocket's center of pressure, the
rocket is positively stable, and will direct itself back to its original
path, if diverted. See

Stability of a Model Rocket
http://www.grc.nasa.gov/WWW/K-12/airplane/rktstab.html

Model Rocket Stability
http://www.apogeerockets.com/educati..._stability.asp

Fins for Rocket Stability
http://members.aol.com/ricnakk/fins.html

/steen

"Steen" wrote in message
. ..
wrote:

With the source of thrust so far behind the centre of mass, large
rockets would seem to be unstable in flight. So how is this overcome?

Rocket stability does not depend on where the nozzle is placed. If you
look
at Robert Goddards worlds-first liquid fueled rocket:
http://www.epower-propulsion.com/epo...d%20Rocket.jpg

you'll see that he placed the nozzle on top, because he believed this to
be
crucial for stability. As it turned out, it is irrelevant, where the
exhaust
nozzle is placed. Which is why it's always placed on the bottom.

For flight in the atmosphere, it's definately not irrelevant. In fact, for
atmospheric flight, sticking the engine at the top, like Goddard did,
results in a design that's very easy to make stable. When you stick the
engine at the bottom, you can often get away without much of a control
system, by adding some fins to the back, which insures that the center of
pressure is below the center of gravity.

However, note that stability does not mean guided. If you want the rocket
to follow a certain course, or hit a certain target, you'll need an active
control system no matter what. With an aerodynamically unstable design, the
control system will need to react very quickly. An aerodynamically stable
design relaxes this considerably.

I could imagine a sensitive gyro in the rockets nose, which enables a
microprocessor to calculate the deviation from the planned line of
flight. This then instructs actuators to repoint the engine to
compensate (or a vane to redirect the thrust). Is this broadly how its
done?

Sounds good to me :-) Something along those lines, at least.

The V2 did essentially the same thing, without the benefit of a
microprocessor. ;-)

If so, how were the early rockets kept in line? A suspect digital
control might have been available for Apollo, but what about the
earlier rockets?

Now, basic rocket stability is provided by the airflow over the rocket
body
and fins. That's actually the only reason to put fins on a rocket.
Basically, if a rocket's center of gravity is located at least one body
diameter (rule of thumb) in front of the rocket's center of pressure, the
rocket is positively stable, and will direct itself back to its original
path, if diverted. See

Stability of a Model Rocket
http://www.grc.nasa.gov/WWW/K-12/airplane/rktstab.html

Model Rocket Stability
http://www.apogeerockets.com/educati..._stability.asp

Fins for Rocket Stability
http://members.aol.com/ricnakk/fins.html

But this still isn't the same as active guidance. A good stiff crosswind
will send an aerodynamically stable rocket off course. If I remember my
model rocket terminology, this is called shuttlecocking. I believe the
larger the fins, the larger the shuttlecocking in a crosswind.

Jeff
--
Remove icky phrase from email address to get a valid address.

writes:

With the source of thrust so far behind the centre of mass, large
rockets would seem to be unstable in flight. So how is this overcome?

I could imagine a sensitive gyro in the rockets nose, which enables a
microprocessor to calculate the deviation from the planned line of
flight. This then instructs actuators to repoint the engine to
compensate (or a vane to redirect the thrust). Is this broadly how its
done?

If so, how were the early rockets kept in line? A suspect digital
control might have been available for Apollo, but what about the
earlier rockets?

From http://www.astronautix.com/lvs/v2.htm:

The development of the V-2 guidance system evolved as follows:

* The A3 system

* The Sg 52 platform and control system developed for the A5 in
1938-39 This was a stable control system based on rate gyros,
but was found not to be an ideal solution

* The Sg 64 improved system developed for the A5 and tested in
launches

* The Sg 66 developed by Kreiselgeraete in 1940-42 for the
A4. This electromechanical control system used unbalanced gyro
accelerometers and a 'Mischgeraet' (mixing device) to combine
inputs, thereby eliminating the need for rate gyros. However
Kreiselgeraete was only experienced in naval equipment, and
applying the principle to rocket guidance was completely foreign
to them. Neverthelss, since Kreiselgeraete was a small company,
they could respond with flexibility and speed to the rocket
team's requests.

Siemens developed an alternate system based on two gyros to
determine attitude and follow a fixed-plane pitch program. The
Siemens system was favored up to mid-1942 due to its production
simplicity. Siemens also had a greater manufacturing and
technical capacity. But its large size made it unresponsive to
the rapid pace of development. A surplus Siemens guidance
system, or an American copy, was used for the Explorer I launch
in 1958.

A third system by Askania-Moeller was briefly put into
development, but quickly abandoned.

* The Kreiselgeraete Sg 70 platform was designed as a replacement
for the Sg 66 to eliminate aluminum gimbals in
1944-1945. Kreiselgeraete's development labs were evacuated to
Sudetenland in 1943 after air raids in Berlin, impacting
development after that date.

Thanks for that. So 2 questions:

1. How is stability provided at low velocity / start, when air flow is
negligible?

2. Did the V2 use an electrical guidance system?

When I come some digitally controlled system, I'm always amazed at the
ingenuity of those that made the system before microprocessors, using
analogue control.

Jeff Findley wrote:
When you stick the engine at the bottom, you can often get away
without much of a control system, by adding some fins to the back,
which insures that the center of pressure is below the center of
gravity.

One can insure a rocket - for a price - but that does not ensure that
the fins will work (Sorry, could not resist

rick jones
--
The computing industry isn't as much a game of "Follow The Leader" as
it is one of "Ring Around the Rosy" or perhaps "Duck Duck Goose."
- Rick Jones
these opinions are mine, all mine; HP might not want them anyway...
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...