Author Topic: Basic Rocket Science Q & A  (Read 271732 times)

Online DaveS

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Re: Basic Rocket Science Q & A
« Reply #980 on: 05/11/2015 04:23 PM »
Scaled Composites' White Knight One.
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Offline e of pi

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Re: Basic Rocket Science Q & A
« Reply #981 on: 05/11/2015 11:49 PM »
What was the carrier aircraft for the X 37 test drops at EAFB circa 2005/6 ?
WhiteKnight One, the carrier aircraft Scaled built to win the X-Prize with SpaceShipOne.

Offline CitizenSpace

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Re: Basic Rocket Science Q & A
« Reply #982 on: 05/16/2015 08:48 AM »
Well, this kind of seems like the right place to ask this...

So, I'm 14 years old and interested in aerospace engineering, been working on rockets for two years or so now, but never really got too far into the proper gritty stuff of it all (Been just building and flying my own rockets and drawing crappy designs). So to further my knowledge on rocket engines, I'm designing a H202/RP-1, 100N rocket engine.
I've used RPA for getting the parameters of it, and I've been following a couple PDF's along the design process. I've so far calculated all the nozzle parameters (half-angles, nozzle throat area, nozzle exit area) and now I'm up to the combustion chamber volume/diameter/length.
So I'm trying to figure out the characteristic length of the chamber, calculated as followed:
L* = Vc/At
Where Vc is chamber volume and At is nozzle throat area.
The problem I am having lies within Vc. You calculate Vc using the following equation:
Vc = Ac * Lc + Cv
Where Ac is nozzle throat area, Lc is chamber length and Cv is convergent volume.

So the problem is this: How do i calculate chamber length? Only information I can find is that L* is largely determined by past experiments, as Lc is very complex to calculate. To calculate it mathematically you need to use:
Vc = Mf * V * Ts
Where Mf is propellant mass flow rate, V is average specific volume, and Ts is propellant stay time.
I know Mf and I can calculate V, but how do I calculate Ts?! I can't find any formula for it! Complexity and length is not a problem for me, I really don't care whether I have to spend 3 or even 4 hours on the one thing.

Unless of course, someone happens to have some past data on the L* of 100-200N class H202/RP-1 engines.

Thanks for the help in advanced.

Offline R7

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Re: Basic Rocket Science Q & A
« Reply #983 on: 05/17/2015 07:38 AM »
I know Mf and I can calculate V, but how do I calculate Ts?! I can't find any formula for it!

Because there is no neat compact formula for it. Ts is the answer to the question how long does it take for the propellants to properly combust. To mathematically find that out you have to model propellant injection, mixing, atomization and combustion. Things like injector droplet size, droplet vaporization rate and turbulence matter. The choice of propellants is most prominent factor so past experience gets you to the right ballpark.

Quote
Unless of course, someone happens to have some past data on the L* of 100-200N class H202/RP-1 engines.

Huzel'n'Huang to the rescue.

Page 87. HTP/RP-1 L* 60-70 inches including the catalyst bed.
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Offline CitizenSpace

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Re: Basic Rocket Science Q & A
« Reply #984 on: 05/17/2015 07:57 AM »
I know Mf and I can calculate V, but how do I calculate Ts?! I can't find any formula for it!

Because there is no neat compact formula for it. Ts is the answer to the question how long does it take for the propellants to properly combust. To mathematically find that out you have to model propellant injection, mixing, atomization and combustion. Things like injector droplet size, droplet vaporization rate and turbulence matter. The choice of propellants is most prominent factor so past experience gets you to the right ballpark.

Quote
Unless of course, someone happens to have some past data on the L* of 100-200N class H202/RP-1 engines.

Huzel'n'Huang to the rescue.

Page 87. HTP/RP-1 L* 60-70 inches including the catalyst bed.

Aha! You legend, thanks for that! Now I can finally move on with design  :D

Also about the formula, is 'no neat and compact formula for it' mean there can't be one, because of incredibly-hard to model-mathematically variables? Or is it just because it would be very difficult to make one and no can really be bothered when we have past experience at hand. Not saying its bad- just curious.

Also, 1.5m seems a bit long for a 100N engine...
« Last Edit: 05/17/2015 08:27 AM by CitizenSpace »

Offline R7

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Re: Basic Rocket Science Q & A
« Reply #985 on: 05/17/2015 09:02 AM »
Also about the formula, is 'no neat and compact formula for it' mean there can't be on, because of incredibly-hard to model-mathematically variables? Or is it just because it would be very difficult to make one and no can really be bothered when we have past experience at hand.

According to the revised Huzel'n'Huang computer models do exist to estimate the L* but even they have trouble modelling the mixing properly and have to assume things like droplet size. AFAIK the hardness is in the required computational power to get meaningful results out of the model, not that the math is not known. And because working values are known for common propellants there's no pressing need to try to compute it.

Best of luck to your project! Keep us posted how it goes.
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Offline R7

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Re: Basic Rocket Science Q & A
« Reply #986 on: 05/17/2015 09:22 AM »
Also, 1.5m seems a bit long for a 100N engine...

Note that even though it looks like 'length' it is really just another form to express the stay time. The propellants don't know nor care whether they go into 100N engine or 1000,000N engine, the proper mixing and combustion takes about the same time in both cases.

Smaller engines tend to have higher combustion chamber contraction ratio than larger engines in order to achieve required L* without being impracticaly long.
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Offline CitizenSpace

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Re: Basic Rocket Science Q & A
« Reply #987 on: 05/17/2015 09:29 AM »
Also, 1.5m seems a bit long for a 100N engine...

Note that even though it looks like 'length' it is really just another form to express the stay time. The propellants don't know nor care whether they go into 100N engine or 1000,000N engine, the proper mixing and combustion takes about the same time in both cases.

Smaller engines tend to have higher combustion chamber contraction ratio than larger engines in order to achieve required L* without being impracticaly long.

Thanks a bunch, I was getting myself confused with actual length and L* :D Sorry about that. Thanks for all the help, its been great.

Offline msat

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Re: Basic Rocket Science Q & A
« Reply #988 on: 07/10/2015 01:48 AM »
How are g forces on rockets kept to acceptable levels, at least with humans on-board, when your upper stages have reasonably high T/W and mass ratios but the engines have limited or no throttling? As one example, I'd imagine that the Falcon 9's second stage acceleration near the end of its burn would be uncomfortably high even at 40% throttle (~70k lbf).

I guess this applies to any of the stages involved in getting you to orbit but not necessarily any additional stages once you're already there.

Offline R7

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Re: Basic Rocket Science Q & A
« Reply #989 on: 07/10/2015 12:38 PM »
With the stage, trunk, Dragon V2, payload and people you hit maybe 3g, hardly too uncomfortable.
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Offline kraisee

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Re: Basic Rocket Science Q & A
« Reply #990 on: 07/10/2015 01:08 PM »
R7, unless they do some fairly serious throttling (and lose quite a bit of performance as a result) I think Dragon crews are more likely to see close to 5g at the end of both stage burns.

That is still nothing to worry about for eyes-in flight.   I'll always remember that a few weeks before he passed away aged 84, Skylab 4 astronaut Bill Pogue told me that he could still have easily flown that Saturn rocket up to 4.4g! :)


msat, normally during the design of a new launch system, the stages will be (roughly) sized so that as each stage approaches burnout, the total mass of that nearly-dry stage, the stages above it and the payload together mass the desired amount to produce a maximum peak acceleration.

For example, say a hypothetical first stage had 1,000,000 lbf vacuum thrust.   If the vehicle should not exceed 5g accelerations at any point in the flight (and assuming no throttling), then the first stage burnout mass, upper stages and payload together should mass right around 1/5th of the thrust, in this case no less than 200,000 lb.   That way the vehicle simply can't exceed 5g.

Don't assume this is a hard and fast rule, but it should help when trying to figure out the basics.

In practice, all vehicles play around with this balance in order to get the most out of their specific propellants and propulsion systems, especially if they are capable of throttling their engines.   Atlas-V is a particularly good example of how much it is possible to play around with your optimization.

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Offline msat

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Re: Basic Rocket Science Q & A
« Reply #991 on: 07/10/2015 04:32 PM »
Ah! Thanks, kraisee. That makes sense.

This actually brings up a point I haven't heard the SSTO folks talk about. Using conventional technology, even if one was willing to accept the abysmal payload payload capability, the associated accelerations near the end of it's burn would be extreme unless the propulsion system consisted of numerous identical engines, most of which would get shut down throughout its flight, or incorporate smaller sustainer engines. Having just a few [large] engines would not be possible, assuming a practical lower limit for throttlability. Does that sound right?


Offline gbaikie

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Re: Basic Rocket Science Q & A
« Reply #992 on: 07/11/2015 07:36 AM »
Ah! Thanks, kraisee. That makes sense.

This actually brings up a point I haven't heard the SSTO folks talk about. Using conventional technology, even if one was willing to accept the abysmal payload payload capability, the associated accelerations near the end of it's burn would be extreme unless the propulsion system consisted of numerous identical engines, most of which would get shut down throughout its flight, or incorporate smaller sustainer engines. Having just a few [large] engines would not be possible, assuming a practical lower limit for throttlability. Does that sound right?

Sounds about right to me.
Of course if the returning stage is more massive as it needs wings and/or keeping rocket fuel to de-orbit
it lessens it.
Also if using some sort of assisted boost- like a mothership [though it's not strictly SSTO] one lessen need for as much initial thrust.

And what talking about does not apply for launch vehicles leaving Mars or the Moon- for number reasons- main reason is roughly need 9 rocket fuel to lift 1 payload from Earth. The Moon 1 rocket fuel to 1 payload, and Mars about 2 rocket fuel to 1 payload. Another reason is the low gravity loss for Moon and Mars.
« Last Edit: 07/11/2015 07:37 AM by gbaikie »

Offline R7

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Re: Basic Rocket Science Q & A
« Reply #993 on: 07/11/2015 08:27 AM »
The 3g figure was for stated 40% Mvac throttle. But kraisee is right, they probably floor it all the way up.

Having just a few [large] engines would not be possible, assuming a practical lower limit for throttlability. Does that sound right?

You need to define the practical lower limit. LEM descent engine demonstrated 10% throttle using moving pintle. YouKnowWhoX is already using pintle injector, adding a moving sleeve to increase throttleability (is that a word?) wouldn't be a huge step.

Another question is would you need to use the engines for landing too. In that case just a few large ones are problematic.
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Offline msat

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Re: Basic Rocket Science Q & A
« Reply #994 on: 07/11/2015 10:14 AM »


And what talking about does not apply for launch vehicles leaving Mars or the Moon- for number reasons- main reason is roughly need 9 rocket fuel to lift 1 payload from Earth. The Moon 1 rocket fuel to 1 payload, and Mars about 2 rocket fuel to 1 payload. Another reason is the low gravity loss for Moon and Mars.

Yeah, Mars and the Moon are much more conducive to SSTO than the Earth .

The 3g figure was for stated 40% Mvac throttle. But kraisee is right, they probably floor it all the way up.

Having just a few [large] engines would not be possible, assuming a practical lower limit for throttlability. Does that sound right?

You need to define the practical lower limit. LEM descent engine demonstrated 10% throttle using moving pintle. YouKnowWhoX is already using pintle injector, adding a moving sleeve to increase throttleability (is that a word?) wouldn't be a huge step.

Another question is would you need to use the engines for landing too. In that case just a few large ones are problematic.

I guess should have looked it up before I said anything. I didn't expect S2 + Dragon to mass ~20k lbs!

And no, "throttleability" doesn't appear to be a word, but it should be!  ;)

Apparently AJ has managed to get a version of the RL-10 to throttle down to 8%, but it was a bit of a challenge from the sound of it. Compared to the LEM descent engine, I don't know if the difficulty was mainly associated with the injector type, or the use of turbopumps. 

I had sort of expected that a moving pintle/sleeve was necessary to throttle an engine with that kind of injector. Does _____X "simply" reduce the output pressure from the turbopump to achieve their engine throttling?

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Re: Basic Rocket Science Q & A
« Reply #995 on: 07/12/2015 10:05 AM »
Merlin is a pintle engine.

Cheers, Martin

Offline msat

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Re: Basic Rocket Science Q & A
« Reply #996 on: 07/12/2015 01:00 PM »
Merlin is a pintle engine.

Cheers, Martin

Yeah, I was aware of that, but R7 made it sound like it's fixed position, so throttling is accomplished by other means.

Offline R7

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Re: Basic Rocket Science Q & A
« Reply #997 on: 07/12/2015 03:11 PM »
Merlin is a pintle engine.

Cheers, Martin

Yeah, I was aware of that, but R7 made it sound like it's fixed position, so throttling is accomplished by other means.

Haven't ever heard anything suggesting Merlin pintle would move so assuming it doesn't. No need really for 40% throttle, even conventional injectors can do that. Lowering turbopump pressure is enough.

RL-10 benefits from hydrogen and expander cycle. No GG nor preburner so there's no worry about combustion stability in those things (they have injectors too). Gas-liquid mixing in the main injector provides faster/better mixing than liquid-liquid. Gas-gas is the best, Raptor for the win!
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Offline Hoonte

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Re: Basic Rocket Science Q & A
« Reply #998 on: 07/23/2015 10:27 AM »
My head sometimes askes me the strangest questions :-)

What rocket/booster will have made the overall largest cumulative thrust.
So taken the thrust of 1 type of rocket/booster e.g. Titan II and multiply its thrust by number of launches.

It's a silly question but I guess the proton might be the nr 1 there..

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Re: Basic Rocket Science Q & A
« Reply #999 on: 07/23/2015 01:16 PM »
Wouldn't Soyuz be right at the top of the list, just by sheer number of launches...
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