# Re: [Propulsion] Minimum Isp

```On 25 Oct 2007, at 14:38, rq17zt wrote:

> All really good points.
>
> [EMAIL PROTECTED] wrote:
>> Glad to see new activity here.
>>
>> I did a lot of independent research and calculations on this stuff a
>> few years back, and my conclusion was basically that the propulsion
>> team should think about designing one stage of a hypothetical three
>> stage rocket - that is, a stage with a delta-v of ~3.2km/s when the
>> payload mass is based on the non-propulsion components of previous
>> launch vehicles. Achieving such a thing would go a long way toward
>> gaining credibility, and it'd make one hell of a sounding rocket on
>> its own. A really conservative rough estimate, assuming 50% kinetic
>> energy lost due to drag effects, still puts the rocket at more than
>> 200km altitude.
>
> It would be great to get this kind of calculation up on the Wiki. I'm
> not claiming i currently have time to do it, or that Richard does
> either, but it would be a great reference to give people interested
> in the orbital aspects of the project.```
```
My calculation here was just based on the kinetic energy of an object
that leaves the surface at that speed, and the height of the
corresponding potential energy. Really rough calculation.

>
>
>> An actual three stage orbital LV would benefit from not having each
>> stage contribute an identical amount of delta-v. Specifically, it
>> makes sense for the first stage to be very high acceleration for a
>> short amount of time, and the last stage to be a longer, slower,
>> high-
>> efficiency burn. The reason for this is that high acceleration is
>> desirable when lifting off vertically because you're losing 1 g just
>> to overcome gravity. However, if your mass ratio for that stage mo/mf
>> is, say, 0.5, this means your total accelerating mass is going to
>> drop in half over the burn, and in the absence of throttling (a
>> recommended absence) your acceleration is going to be twice as high.
>> Therefore the limiting factor on the delta-v contributed by any stage
>> is that the initial acceleration must be great enough that the 1g
>> loss is not significant, but the final acceleration must not tear the
>> vehicle apart.
>
> If we do design an LV3, we'll need at least 1/2-good detailed models
> of the ascent environment to make decent trade offs. I've always
> claimed that we're a little fast on LV2. (Too high thrust and too
> short a burn time given our total impulse.) The idea being that by
> slowing down in the lower atmosphere we'd cut our wave drag enough to
> make up for the increased gravity loss, but i think this was based on
> less than a back of the envelope calculation that i've since
> forgotten, so i can't justify the conclusion now.

I'm right there with you. If I recall, there was some FAA red tape
that applied if your burn time was longer than 15 seconds, which is
rather ridiculously short. I say design the rocket the way physics
says you should, and just deal with the paperwork.

Still, you want to figure on leaving the pad around 4 g's at least -
that is, generating 5 g's worth of thrust, for 80% initial flight
profile efficiency. LV2 was well into the double digits, if i
remember correctly.

>
>
>> This leads to a more realistic estimate of perhaps 2 to 2.5km/s
>> delta-
>> v from the first stage, as a design goal. This is doable with an
>> exhaust velocity of 2500m/s and a fuel/total mass ratio of 63%. Still
>> pretty technically challenging, but within reason. Keep the design as
>> simple as possible, consider doing some test launches without the
>> full avionics and recovery package - just having the nose cone pop
>> off and deploy a streamer to spoil the aerodynamics is a lot more
>> reliable than a parachute recovery system deployed by a computer
>> running Java. By now it should be obvious that the Shuttle is the
>> opposite of good design practices in terms of reliability,
>> infrastructure required to launch, frequency of launch, &c.
>
> Sounds credible.
>
> Any next generation design will seek the path to elegant simplicity.
>
> ---
>
> This is off topic, but i don't understand the recovery details you
> mentioned.
>
> What do you envision popping the nosecone?
>
> AFAIK a streamer will stabilize the vehicle, but the descent will be
> too rapid for undamaged recovery.

For propulsion-only tests I wouldn't be too concerned about undamaged
recovery - even with a theoretically perfect launch and recovery, the
thing's still gonna be completely rebuilt between flights. The idea
is, you make it simple enough that you can do 5 test flights per year
instead of just one - and even if 4 of them are spectacular failures,
you're beating the odds. For full-on launches with avionics and
payload, consider using an 'undamaged recovery' system for just the
expensive stuff, and jettison the much heavier, replaceable
propulsion unit (with streamer recovery to avoid pain).

It should be fairly easy to come up with a simple logic circuit
connected to an accelerometer that figures out when to pop the
nosecone... design something that costs ten bucks to build, and then
use two of them. Don't forget to use the absolute value of the
accelerometer's output, just in case someone mounts it backwards
(genesis).

Richard

>
> --------------------------------------------------------------------
> ... in three to eight years we will have a machine with the general
> intelligence of an average human being ... The machine will begin
> to educate itself with fantastic speed.  In a few months it will be
> at genius level and a few months after that its powers will be
> incalculable ...
>                 -- Marvin Minsky, LIFE Magazine, November 20, 1970
>
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