Date: 26 Nov 88 21:06:57 GMT
From: ka9q.bellcore.com!karn@bellcore.com  (Phil Karn)
Subject: Re: chemical rockets/exotic launch schemes

Contrary to Henry's assertion, liquid rocket fuels are not necessarily safer
to handle than solid fuels. Modern solid fuels are surprisingly stable; you
can hold a lit match to bulk solid fuel, and chances are it will not ignite.
(Nevertheless, the consequences of undesired ignition are nasty enough that
I would not recommend using a match as a light source while inspecting the
interior of a SRB...)

It takes what is essentially a cascaded series of small, quick burning
rocket engines to reliably ignite the solid rocket motors used on the
shuttle. The NASA Standard Initiator (NSI, essentially a detonator) ignites
a pyro booster pellet of BKNO3, which starts an ignitor initiator (a small
rocket engine) about 18cm long. The ignitor initiator in turn ignites the
main ignitor motor, another, larger, rocket engine about 91cm long. Its
flame shoots down the entire length of the SRB for a hundred milliseconds or
so. The igniter contains 64 kg of solid propellant -- reliable ignition
requires a big flame!

One major drawback to almost all liquid rocket fuels is that they are either
extremely toxic or extremely cold. Sometimes both. This is not the case with
solid fuel.

Phil

Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: SRB's
Date: Fri, 26 Nov 1999 17:06:40 GMT

In article <81l2a70200g@enews1.newsguy.com>, kenb <kenb@spectrawho?.ca> wrote:
>...I'm aware that small solid fuel rockets are often fueled with a
>hollow center to increase the surface area of the combustion, resulting in
>the fuel burning outwards from the center as opposed to burning strictly
>from bottom to top, resulting in higher pressures at the nozzle.  Do the
>shuttle SRB's employ this technique to any degree?

Yes, they do.  Almost all large solid motors are core-burners, burning
outward from a central hole, for a number of reasons; the discovery of
this approach was a major breakthrough in solid-motor design.  Not least
among the advantages is that the unburned fuel insulates the walls from
the flame heat.

As others have already explained, the hole is carefully shaped to get the
desired thrust history.

>I'm also curious about
>how we go about lighting these monsters up before launch.

The way you light a big solid is with a smaller solid.  In the case of the
shuttle SRBs, each has an igniter up at the top, essentially a smaller
solid rocket -- only about 6ft high! -- which fires down the center hole
briefly to get things started.  Those are in turn ignited by essentially
the same method, and after one or two more stages you reach the point
where electrical igniters fire small pyrotechnic charges to get the whole
thing started.

>     Another point I'd like to know is just what percentage of maximum
>thrust are they producing when they are jettisoned?

Essentially zero.  They cannot safely be jettisoned while still producing
any significant amount of thrust.  There is still a little bit of thrust,
because a solid motor (especially a big one) actually takes quite a while
to stop burning completely, but it's no longer very significant.

>...Just what determines the exact critical moment to gracefully
>say goodbye to the SRB's?

Thrust tailoff in both of them.  And it had better be almost exactly
symmetrical, or the orbiter is in big trouble; considerable efforts are
made to ensure that the two SRBs have identical burning characteristics.
(For example, corresponding segments on the two SRBs come from the same
production batch.)
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: SRB's
Date: Mon, 29 Nov 1999 03:10:50 GMT

In article <3841d2cf@derwent.nt.tas.gov.au>,
Justin Wigg <justinwigg@yahoo.com> wrote:
>Just how sensitive to flame is the solid fuel?

At 1atm pressure, not very -- I'm told that if you hold a match up to a
piece of it, nothing much happens.  (Not that this is recommended!)  A
fairly convincing igniter is needed.

>I was shocked to see on
>NASA-TV one time what looked like a worker in the VAB working on a fuelled
>SRB segment with a blow-torch!  Could be that my pathetic little RealVideo
>window was playing tricks on me...

As others have noted, *that* is unlikely.  NASA does not particularly like
having live fuel in the VAB -- in the Saturn days, that was forbidden or
very nearly so -- and is quite paranoid about accidental ignition.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)