Space Question
Jun. 11th, 2008 12:18 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png)
This one goes out to the Space Shot lovers out there (Yes,
krjalk, I am looking at you)
So I have been looking at rockets again, specifically multithruster rockets similar to what Armadillo are eventually aiming to build. Now the Soviets played around with these and had no end of problems, most commonly that variences between thrusters couldn't be countered fast enough and the stack tended to fall over. (There was also some pogoing, but I wonder if it was related)
Now the way I have been thinking about it, we're basically balancing a long thin construction on a relatively small vector. It's like balancing a pencil on the tip of your finger. It is inherently unstable so you are having to continuously "tweak" your engine output.
Now what stops you mounting some Peroxide thrusters higher up your stack and rather than trying to balance everything from below, you do your "quick" adjusts from the top of the stack, which means your primary rockets don't need to be anywhere as sensitive in terms of response time?
I am sure there is something I'm missing (The obvious one is that you are putting stresses across the entire stack frame that may make your stack snap in two) but what?
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So I have been looking at rockets again, specifically multithruster rockets similar to what Armadillo are eventually aiming to build. Now the Soviets played around with these and had no end of problems, most commonly that variences between thrusters couldn't be countered fast enough and the stack tended to fall over. (There was also some pogoing, but I wonder if it was related)
Now the way I have been thinking about it, we're basically balancing a long thin construction on a relatively small vector. It's like balancing a pencil on the tip of your finger. It is inherently unstable so you are having to continuously "tweak" your engine output.
Now what stops you mounting some Peroxide thrusters higher up your stack and rather than trying to balance everything from below, you do your "quick" adjusts from the top of the stack, which means your primary rockets don't need to be anywhere as sensitive in terms of response time?
I am sure there is something I'm missing (The obvious one is that you are putting stresses across the entire stack frame that may make your stack snap in two) but what?
no subject
Date: 2008-06-11 03:50 pm (UTC)but i thought it might clarify things somewhat
if we take this back to physics fundamentals.
Firstly, you're right, a rocket is an inherently unstable design.
Ideally, we'd like to apply the accelerating force above the centre of mass.
But in practical terms,
this means pointing the exhaust back at the vehicle.
We could tilt the thrusters outward slightly,
but this translates to wasted thrust, ie: wasted reaction mass.
We could mount the thrusters on arms,
but they'd hafta be strong enough to transmit the force,
and include fuel pipelines - again, all extra mass.
Secondly, you're right, fuselage strength aside,
even small thrusters near the top of a rocket
would add greatly to th leverage available to turn the vehicle.
You'll notice many military missiles
(esp. ones aimed at moving targets)
have puta-controlld canards etc for this purpose
(though, as the rocket and its thrust get larger,
fins have less and less relative effect, of course).
But, are radical manoeuvres and the extra mass they entail
really necessary for a vehicle designed to arc gracefully into orbit?
As Tsiolkovsky pointed out so elegantly,
each extra kilo carried translates exponentially
into reaction mass on the launch pad,
so th engineers ask of each and every gram:
are you necessary?
At core, th answer to yr question is to note
that some things humans find hard, computers find really easy (& vice versa).
I'v seen footage of robot arm/sensor combos
which could do the pencil balancing trick so responsively
you'd swear th thing was glued there.
The calculations aren't that complex,
all it takes is really fast reflexes.
It turns out that a more stable application of force
is simply not necessary in spacebound rockets.
The primary design problem is how to make a machine
which can reliably channel high temperatures and pressures,
and yet be as light as possible.
Cutting this tradeoff too close is what makes rockets go boom.