Author Topic: Rocket Engine Q&A  (Read 382964 times)

Offline Sesquipedalian

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Re: Rocket Engine Q&A
« Reply #720 on: 07/19/2016 03:42 pm »
Why are engines usually "chilled down" prior to startup?  This strikes me as exactly the wrong thing to do.  When the engine actually starts, it immediately gets extremely hot, going through a substantial change in temperature in a fraction of a second.  Chilling down the engine would seem to drastically increase the amount of temperature change it undergoes.  Most materials don't like wild and abrupt temperature swings; they will often fracture or deform or otherwise suffer substantial degradation.  Anyone who has placed a hot glass on a cold counter and seen it shatter will be familiar with this.

Obviously, since engines don't fracture upon startup from the thermal shock, there's a link in the sequence that I'm missing.  Perhaps "chilldown" doesn't mean chill down in the usual sense?

Offline PahTo

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Re: Rocket Engine Q&A
« Reply #721 on: 07/19/2016 04:23 pm »
Why are engines usually "chilled down" prior to startup?  This strikes me as exactly the wrong thing to do.  When the engine actually starts, it immediately gets extremely hot, going through a substantial change in temperature in a fraction of a second.  Chilling down the engine would seem to drastically increase the amount of temperature change it undergoes.  Most materials don't like wild and abrupt temperature swings; they will often fracture or deform or otherwise suffer substantial degradation.  Anyone who has placed a hot glass on a cold counter and seen it shatter will be familiar with this.

Obviously, since engines don't fracture upon startup from the thermal shock, there's a link in the sequence that I'm missing.  Perhaps "chilldown" doesn't mean chill down in the usual sense?


Hi Ses,
You actually answered your own question.  Materials don't like abrupt change, hence a controlled (slow) introduction of cryos to condition the engine components to the temps at which they'll be operating (read:  passing massive quantities of propellants at start up/through run cycle).

Offline Sesquipedalian

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Re: Rocket Engine Q&A
« Reply #722 on: 07/20/2016 08:09 pm »
I don't follow.  Rockets are fueled over a long period of time (30 minutes or so, at least, based on the latest SpaceX launch), which means that the cryogenic components should reach cryogenic temperature gradually.  But rocket engines go from ambient temperature to red-hot in a matter of seconds.

Offline PahTo

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Re: Rocket Engine Q&A
« Reply #723 on: 07/20/2016 08:27 pm »
I don't follow.  Rockets are fueled over a long period of time (30 minutes or so, at least, based on the latest SpaceX launch), which means that the cryogenic components should reach cryogenic temperature gradually.  But rocket engines go from ambient temperature to red-hot in a matter of seconds.

The only portions of a rocket engine that get hot are (mostly) the combustion chamber and nozzle, and preburners as applicable (frictional heating of bearings for the turbo machinery, etc. should be considered separate of this conversation).  Each of these hot components usually cycle cryos around them to keep them from melting (this also serves to pre-heat said props for (in part) helping to drive turbo machinery).  Chill down is for piping, stators and vanes, other turbo hardware, valves, valve housings, etc.    There are some excellent write ups on SSME/RS25 (or used to be, haven't been there in years) on NASA sites.  I'm not a rocket scientist, and have learned most of what I know from googling and reading manuals and such.  This site helps too!  :) 

Offline nicp

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Re: Rocket Engine Q&A
« Reply #724 on: 07/27/2016 07:03 pm »
A couple of book I've read explain the cancellation of the E-1 engine due to it being a 'dead end'. That description never seems to be backed up. A dead end because LR-87 was closer to ready? Or - as I sometimes read it - not big enough with the F-1 on the horizon?

Perhaps there just were no stages that needed it, but I see no great reason why E-1 was fundamentally a dead end. A shame it didn't fly.

Any opinions ?
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Offline Jim

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Re: Rocket Engine Q&A
« Reply #725 on: 07/27/2016 07:14 pm »
It was OBE.  With the "re engining" of the Saturn with the H-1 precursor and success of the LR-87, there was no need for it.
« Last Edit: 07/27/2016 07:16 pm by Jim »

Offline baldusi

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Re: Rocket Engine Q&A
« Reply #726 on: 09/23/2016 03:35 pm »
Highly technical question, but here it goes: should steam generators like RD-107/8 be considered different than traditional gas generators? On the same line of thought, Gamma rocket should be considered staged combustion?

Offline Proponent

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Re: Rocket Engine Q&A
« Reply #727 on: 09/24/2016 12:51 pm »
Well, I remember reading Wernher von Braun's popular book Space Frontier long ago.  IIRC, the four cycles he described were pressure-fed, gas-generator (by which he meant monoprop gas-generator), bi-propellant (i.e., what we call GG around here), and topping (regenerative).  So he seemed to regard it as a separate cycle.

Offline baldusi

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Re: Rocket Engine Q&A
« Reply #728 on: 09/24/2016 06:37 pm »
Well, I remember reading Wernher von Braun's popular book Space Frontier long ago.  IIRC, the four cycles he described were pressure-fed, gas-generator (by which he meant monoprop gas-generator), bi-propellant (i.e., what we call GG around here), and topping (regenerative).  So he seemed to regard it as a separate cycle.
I haven't been able to read that book. I don't quite understand what does he refers as to with topping. And science has really advanced a lot since then. You have closed expander, bleed expander, staged combustion, full flow staged combustion, tap off, electrically pumped and any combination of the above.
With a modern view, the difference is quite minimal. You still take some of the mass of the liquids on board and make hot gas to drive the turbopumps. I'm wondering if writing a whole article for it or just an entry on the Gas Generator cycle on Wikipedia. But I haven't been able to find exactly the quote that considers them different. Now, if I only had access to a copy of that book. NASA perhaps?

Offline Proponent

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Re: Rocket Engine Q&A
« Reply #729 on: 09/24/2016 11:10 pm »
"Topping cycle" was his term for closed expander.  Now that you mention it, he also described the tap-off cycle, referring to it as "thrust-chamber bleed" (mind you, I'm recalling something I read years ago as a child, and could be making a mistake).  The book was a compilation of articles that von Braun had written for the magazine Popular Science.

The ancient (1961) Handbook of Astronautics identifies precisely the same four cycles: bipropellant, monopropellant, topping and thrust-chamber bleed.

By the way, I see that Sutton & Bilbarz (8th ed.) use "topping cycle" as a synonym for "closed cycle."   On gas generators, unlike von Braun, they regard (p. 225) the monoprop cycle as simply an obsolete variant of the gas-generator cycle.
« Last Edit: 09/24/2016 11:21 pm by Proponent »

Offline Prober

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Re: Rocket Engine Q&A
« Reply #730 on: 10/01/2016 05:12 pm »
Rocket Engine Plumbing


« Last Edit: 10/01/2016 05:13 pm by Prober »
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Offline haywoodfloyd

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Re: Rocket Engine Q&A
« Reply #731 on: 02/26/2017 11:54 am »
I heard a report on Daily Planet on the Discovery Channel that said that CO2 was produced along with water vapour when the RS-25 was test fired.
Is this true?

Offline Jim

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Re: Rocket Engine Q&A
« Reply #732 on: 02/26/2017 12:04 pm »
I heard a report on Daily Planet on the Discovery Channel that said that CO2 was produced along with water vapour when the RS-25 was test fired.
Is this true?


no

Offline nacnud

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Re: Rocket Engine Q&A
« Reply #733 on: 02/26/2017 12:06 pm »
RS-25 burns Oxygen and Hydrogen.

Where would the carbon come from?

Offline haywoodfloyd

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Re: Rocket Engine Q&A
« Reply #734 on: 02/26/2017 12:17 pm »
RS-25 burns Oxygen and Hydrogen.

Where would the carbon come from?

My thoughts exactly.

Offline nacnud

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Re: Rocket Engine Q&A
« Reply #735 on: 02/26/2017 12:40 pm »
If they are thinking about pollutants maybe there is some NOx produced?

Offline Proponent

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Re: Rocket Engine Q&A
« Reply #736 on: 02/26/2017 05:45 pm »
Since the exhaust is hydrogen rich and hot, I imagine a number of compounds involving H, N and O would form, possibly including some nitric acid (HNO3).
« Last Edit: 02/26/2017 05:46 pm by Proponent »

Offline haywoodfloyd

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Re: Rocket Engine Q&A
« Reply #737 on: 02/26/2017 07:03 pm »
Since the exhaust is hydrogen rich and hot, I imagine a number of compounds involving H, N and O would form, possibly including some nitric acid (HNO3).

But not CO2, right?

Offline Proponent

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Re: Rocket Engine Q&A
« Reply #738 on: 02/27/2017 08:34 am »
No -- like nacnud said a few posts up, where would the carbon come from?

Offline gospacex

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Re: Rocket Engine Q&A
« Reply #739 on: 02/27/2017 02:14 pm »
Since the exhaust is hydrogen rich and hot, I imagine a number of compounds involving H, N and O would form, possibly including some nitric acid (HNO3).

Exhaust is not hot. It's actually circa ~100 Celsius - because a well-designed engine converts almost all thermal energy (random motion) into energy of the *directed* stream of gas.

On impact into "motionless" atmosphere, some reheating occurs, but I don't know how efficient it is. It might be too low to dissociate N2.

 

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