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Comparisons - expend./reusable,heavy/light,theory/practice

Posted by: Ekkehard Augustin - Thu Mar 30, 2006 8:38 am
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Comparisons - expend./reusable,heavy/light,theory/practice 
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Post    Posted on: Tue Aug 15, 2006 2:20 pm
Ekkehard Augustin wrote:
From this it seems that high Isps are not necessaryly not-outweighed by larger tank-weights.
I cannot figure out what that means at all. :?:


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Post    Posted on: Wed Aug 16, 2006 8:43 am
The data available to me and listed in the table seem to indicate that the advantage of higher Isps is combined with a disadvantage consisting of lower densities of the high-Isp-propellants/oxidators. These high-Isp-low-denisty-propellants/oxidators require tanks that are more voluminous than those for propellants/oxidators of lower density but then even lower Isps. This menas that the tanks for the high-Isp-lower-density chemicals require even more material and so are heavier than tanks for other chemicals...

In short - to get higher Isps propellants requiring heavier tanks are needed - and there are four exceptions to that only (at maximum)

By the way - it's a pity that laughing gas/HTPB, LOX/methane and LOX/propane haven't been added to the list under www.bernd-leitenberger.de yet. But I suppose that data from SSO, the other XPRIZE-competitor using LOX/methane aren't available yet and regarding LOX/propane it has to be waited until Air Luanch LLC has ready QuickReach.



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Post    Posted on: Fri Aug 18, 2006 4:30 am
Ekkehard Augustin wrote:
...although I am not yet at the point to look at the tanks one hint here. Larger spherical tanks would have a larger diameter. So the rockets using larger spherical tanks would be wider and thus tend to suffer more resistance by the air in front of it/them. This in turn would increase the required amounts of propellant - which again would increase the tank diameter...


True to some degree, but ascent profiles can be altered to greatly reduce air resistance, at the cost of a small increase in deltaV. ( For example, staying sub-sonic until over 12 km altitude, eliminates most supersonic drag, at a cost of about 200 m/s)

Alternately, supersonic drag is minimized if the length of a rocket is 20m ore more, but it can be a wide as you want.


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Post    Posted on: Fri Aug 18, 2006 6:56 am
Don't forget that for equivalent drag a very wide rocket will need a proportionately longer nose cone, which will involve some extra mass.


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Post    Posted on: Fri Aug 18, 2006 7:08 am
At present it looks to me as if such considerations are hurting the constants that need to be introced to allow for proper comparisons. Plus it is closer to practice than what I am doing at present - yet to early a bit.

May be I later consider such properties theoretically also but I am not sure about that and will think about it.



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Post    Posted on: Wed Aug 23, 2006 1:02 pm
According to www.bernd-leitenberger as well as to other informations I have ve doesn't depend on the propellant not only but - another step closer towards practice - on a particluar property of the engine also.

The larger the difference of the pressure between nozzle entry and nozzle exit the larger the Isp of the engine. But as Bernd Leitenberger explains the weight of the engine increases superproportionally then. And there are more such links and connections.

So this can't guarantee increased capacities regarding weight but reduces it in fact at least sometimes.

The slope of the weight-of-engine/Isp-function is positive because of what I said.

May be that a mix between propellant and engine-Isp is a chance - so I will look if and how I can consider that.

Again a constant is going to have different values - meaning several families of rockets/vehicles again. This too requires some thoughts. Perhaps limited variations around a significant value are acceptable. But even this needs to be checked.



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Post    Posted on: Mon Sep 11, 2006 8:33 am
Hello, WannabeSpaceCadet,

regarding your mentioning of spherical tanks Bernd Leitenberger says that they are fixed on the surrounding structure by one ring only and there are great stress on that part of a stage.

All in all spherical tanks are suboptimal for a stage as a whole according to Bernd Leitenberger. They used if the propellant is got to the engine by pressure.

This is the main reason why I don't consider spherical tanks in particular. Leitenberger says also that cylindrical tanks with round tops and bottoms are a good compromise.

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Post    Posted on: Mon Sep 11, 2006 2:02 pm
Yes, but spherical tanks are also very very handy for tucking into otherwise wasted volume.

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Post    Posted on: Wed Sep 20, 2006 12:15 pm
I may have lost out of sight something and so have to look through this thread - but it seems as if the next step towards practice could be done now.

This next step is to consider the concept of staging now - if I really haven't lost out of sight something.

But this part or extension of theory seems to be changing at present - simply because two companies plus one consortium are replacing the first rocket stage by an airplane as stage and two or three teams or companies are replacing the first stage by a balloon. Because of this I am not sure at present how I will handle that. Additioanlly there is one company that combines reusability with expendability by the concept of staging. ...

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Post    Posted on: Mon Sep 25, 2006 9:20 am
Ekkehard Augustin wrote:
...
regarding your mentioning of spherical tanks Bernd Leitenberger says that they are fixed on the surrounding structure by one ring only and there are great stress on that part of a stage...

Depends on how you fix them. Check out the Armadillo quad vehicle.

Solid Rocket Boosters are also attached at one or two points only, leading to high stress.

But most rockets do use the tank walls as the outside skin as well, so rounded cylinders are often best.


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Post    Posted on: Fri Oct 06, 2006 10:35 pm
One of the sub-Sea Dragon Big Dumb Boosters was the TRW Double Bubble.


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Post    Posted on: Sun Oct 08, 2006 3:47 pm
Just these moments I listed the functions I identified up to now – and I at present can’t find soemthing to be looked for yet:

- Ziolkovsky: e^(Kv/ve) = (Mve + Mlp + Klt + Klr)/(Mlp + Klt + Klr)

    1. modification: e^(Kv/ve) * (Mlp + Klt + Klr) = (Mve + Mlp + Klt + Klr)
    2. modification: e^(Kv/ve) * (Mlp + Klt + Klr) - (Klt + Klr) = (Mve + Mlp)
    3. modification: e^(Kv/ve) * Mlp + (e^(Kv/ve) - 1) * (Klt + Klr) = (Mve + Mlp)
    4. modification: (e^(Kv/ve) - 1) * (Klt + Klr) = (Mve + Mlp) - e^(Kv/ve) * Mlp
    5. modification: (e^(Kv/ve) - 1) * (Klt + Klr) = Mve + (1 - e^(Kv/ve)) * Mlp
    6. modification: depends on the choice if Mve or Mlp or ve should be kept constant for decisions

    The slope too depends on the choice - let's call this function f then the slope is f'.


- expendability/reusability: g(number of launches)
    g' is negative - which means negative slope

    linked to number, duration, intensity etc. of tests.

- heavy/light: h(time)
    h' is positive - positive slope

- expendability/reusability B: i(weight)
    i' is negative - negative slope

- ve: j(Mve)
    j' unknown in this general version

- ve B: k(chamber pressure)
    k' is positive - positive slope

- remainder of vehicle/rocket: l(Klr)
    l' unknown to me presently - I suppose it to be very complex


What’s not read yet is the list of groups I idebtified in two posts at least – so the theoretical considerations aren’t at the point yet I want them to be at.

The criterions for grouping I already can list in this post are:

Density of propellant as one factor ruling the tank weight – function of negative slope

Difference of pressure between nozzle entry and nozzle exit as one factor ruling the engine weight – function of positive slope.

Natural state of propellants:

    gas/gas
    gas/liquid + liquid/gas
    gas/solid
    liquid/liquid
    liquid/solid



Please note – I am still doing theoretical considerations.



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Post    Posted on: Mon Oct 30, 2006 12:41 pm
In between I was looking throught the thread and found the post with the remaining grouping criterion.

In that post I was considering the link/connection between reusability and weight. If the total weight is given and constant then the equipment that makes the vehicle/rocket reusable reduces the maximum of the possible payload weight and propellant weight etc.. If on the other hand the total weight is not given then the weight and size of the reusability equipment will increase with all the other weight. In the other post I already said that this tends to rule out reusable HLLVs the heavier those HLLVs are.

I also said that HLLVs do have decreasing chances to be more economical than reusables as the flight/launch rate increases.

All these issues are suspicions in that post and my last issue in this post is combining a grouping criterion with a function - which shouldn't be done yet.

But this all is a fourth grouping criterion:

Reusability equipment weight ruling the payload weight capacity: a function linking weight to reusability/expendability that has negative slope.

It seem that now all is found that could be found without consideration of the theory/practice-aspect - may be that later it turns out that something has to be added but it seems that now the practice can be considered using the groups and functions. Comparisons are possible by all this now.

May be that I will group the functions also later.



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Post    Posted on: Sun Nov 05, 2006 4:00 pm
It seems that the vehicles to start the practice-part at best by are the following ones:

Falcon 1
Falcon 5
Falcon 9
Soyuz (one at least)
Proton (one at least)

These vehicle would include different weight capacities of reusable vehicles and at least two different weight capacities of expendable vehicles. Also a difference in propellant is included - but all the propellants are within the same group.

I already started to turn the data available comparable - but it's started only.



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