Author Topic: Rocket Engine Q&A  (Read 284669 times)

Offline R7

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Re: Rocket Engine Q&A
« Reply #740 on: 02/27/2017 02:47 PM »
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.

RS-25 exhaust is about 1150K because the area ratio is still limited by sea level ambient pressure. Still below thermal NOx level which is about 1800K. Reheat happens in the shock diamonds, but AFAIK at that point there's not much mixing between plume and ambient air so there's no or just little N present.
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Offline Robert Willis

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Re: Rocket Engine Q&A
« Reply #741 on: 08/03/2017 03:26 PM »
Engines designed to burn liquid hydrogen, such as RD-0120 & RD-0146 have been extensively test fired running on liquid methane with little modification. RD-701 was actually capable of switching back & forth from kerosene to hydrogen in flight! Seeing as Raptor was originally planned to burn LH2, how difficult would it be to produce such an engine with a high degree of component commonality with the CH4 burning model currently under development? NASA buying a few dozen of these for an improved SLS at a fraction of what AR charges per unit for RS-25 would be a helpful source of funding for SpaceX. Please correct me if I'm wrong, but I would guess that an LH2 fueled Raptor would have lower thrust, but higher ISP than the CH4 powered Raptor baseline. Can anyone out there do some rough calculations/estimates?

Doubtless the Raptor will drastically less expensive than the RS-25; NASA is doling out one point six billion for a mere six new engines to Aerojet-Rocketdyne. Would a hydrogen burning raptor not make a drastically more cost effective RS-25 replacement for SLS applications?

Offline nicp

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Re: Rocket Engine Q&A
« Reply #742 on: 09/03/2017 11:04 AM »
I've just read the Wikipedia article on the J-2. It mentions that the J-2S would have used a de Laval nozzle.

For quite some time I had assumed all rocket engines used de Laval nozzles, though on a few occasions (looking at photographs of an F-1 for example) I did wonder, but put the seeming lack of convergent/divergent form to camera angle or perspective.

The implication is that some rocket engines do not use a de Laval nozzle, and that the (more efficient) J-2S would have.

So my questions are...

Why use - or not use - a de Laval nozzle?
Can there be a disadvantage in using one?
Does a non-de Laval nozzle achieve choked flow (surely it must?)


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Offline robert_d

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Re: Rocket Engine Q&A
« Reply #743 on: 09/03/2017 12:48 PM »
What in general is required to make an engine air-startable and restartable?
What makes a design such as the SSME harder to accomplish this? Hope to find some commonality to similar questions regarding the Raptor development in that thread. Thanks.

Online brickmack

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Re: Rocket Engine Q&A
« Reply #744 on: 09/03/2017 02:43 PM »
I've just read the Wikipedia article on the J-2. It mentions that the J-2S would have used a de Laval nozzle.

J-2 definitely had a de Laval nozzle. Wikipedia just says that to distinguish J-2S and J-2T, since the latter used an aerospike

Offline Jim

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Re: Rocket Engine Q&A
« Reply #745 on: 09/05/2017 08:22 PM »
What in general is required to make an engine air-startable and restartable?
What makes a design such as the SSME harder to accomplish this? Hope to find some commonality to similar questions regarding the Raptor development in that thread. Thanks.

The ability to get the engine parameters into the start box and provide energy to start the engine.

SSME was a head start engine, it relied on the pressure generated by the weight of the propellants.  It also was constantly conditioned by ground sources.  Both of these are hard to do in a free fall at over 100kft.

Don't need to worrying about the Raptor, it will be designed for ir-startable and restartable, just like the Merlin.  There is nothing special that needs to be done.

Online brickmack

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Re: Rocket Engine Q&A
« Reply #746 on: 09/06/2017 01:48 AM »
Addressing *re*-startability, regardless of location (ie, if you've got an engine that stays on the ground and want to restart it), since Jim already covered the main points of air start. A lot of engines, particularly older ones, did things to themselves during startup and shutdown that would make it very difficult to restart them without serious maintenance. Valves would be opened with pyrotechnics, and then closed again at the end of the burn in the same manner. Thermal stresses could also seal valves in one position. Pumps, in engines that had them, might be spun up by small solid rockets. Ignition might use hypergolic or pyrophoric injection (often in burst discs rather than normal plumbing), or pyrotechnics/small solids. All of these issues would require at least replacing several easily-accessible parts between firings (some such engines could actually fire multiple times, but usually only a very small number), if not a significant disassembly.

So, main points anyway: pneumatic or electromechanical valves, temperature-safe parts, hypergolic/pyrophoric ignitor fluid injected through normal reusable plumbing (or, if feasible, use electrical ignition and avoid the problem completely), and use compressed gas/cryogenic fluid expansion/electromechanical means to spin up turbopumps

Offline darkenfast

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Re: Rocket Engine Q&A
« Reply #747 on: 09/06/2017 06:14 AM »
All good facts, but then: wasn't the SSME the first choice for Aries I Upper Stage?  Didn't the people who pushed the SSME for the Upper Stage know this?  How were they going to deal with this?

Offline PahTo

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Re: Rocket Engine Q&A
« Reply #748 on: 09/06/2017 02:35 PM »
All good facts, but then: wasn't the SSME the first choice for Aries I Upper Stage?  Didn't the people who pushed the SSME for the Upper Stage know this?  How were they going to deal with this?

I answered earlier with a snarky "money" post, then had a sip of coffee and my brain kicked in.  I believe the J-2X was slated for the upper stage, not SSME.  Even still, there was a ton of money on an engine that now will likely never see in-space action.

Offline Welsh Dragon

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Re: Rocket Engine Q&A
« Reply #749 on: 09/08/2017 08:45 AM »
Nope, the original concept had an airstart SSME.

Offline wolfpack

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Re: Rocket Engine Q&A
« Reply #750 on: 09/09/2017 01:14 AM »
What is the cleaning procedure for RP-1 fueled engines between static fires and flights/re-flights? Is it still a trichloroethylene flush?

Offline JAFO

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Re: Rocket Engine Q&A
« Reply #751 on: 01/07/2018 09:41 PM »
Ok, bear with me, and I'm probably going to set the record for the dumbest question in the history of the forum, but here's a question I missed from Freshman Rocketry 1.

When I was building Estes rockets a long time ago I accepted the premise that you press the button, the solid ignites, and the force of the escaping gases exiting the motor produce an opposite force against the "top" of the engine, making the rocket go up. For a solid motor, it seems pretty straightforward.

But what about a liquid fueled engine? Hot gases go out the back, but what are they "pushing" against? The engine bell? The thrust chamber? Whatever they're pushing against, how is that force transmitted to the vehicle? In the case of the Falcon, how complicated is it to transfer the thrust of 9 engines to the first stage? If they're pushing against the engine bell/nozzle, how strong is that thing? We all rest our models on them, but I doubt they're strong enough to hold the mass of even an unfueled vehicle in real life.


Thanks for your patience,
« Last Edit: 01/07/2018 09:42 PM by JAFO »
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Offline Bernie Roehl

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Re: Rocket Engine Q&A
« Reply #752 on: 01/07/2018 10:04 PM »
Rocket engines actually don't "push" against anything. It's all about conservation of momentum.

The rocket+propellant are initially at rest. Expelling propellant in one direction means the rest of the system has to move in the other direction so that the net momentum remains at zero. Momentum is mass times velocity, so the more mass you expel and the faster you expel it, the faster you go in the opposite direction.

The classical rocket equation is Vrocket = Vexhaust x ln(M/m), where M is the mass of the rocket+propellant and m is the "dry weight" or the mass of the rocket by itself without the propellant.

Notice that I say "propellent", which is not quite the same as "fuel". Fuel is what provides the energy, propellant is the reaction mass. In a chemical rocket, those two are basically the same thing -- the reaction mass is the byproduct of burning the fuel (with an oxidizer).

In a nuclear thermal rocket, the propellant is usually hydrogen and the energy comes from a reactor.

In an ion engine, the reaction mass is something like Xenon and the energy comes from a solar or nuclear electric source which accelerates the propellant ions using electric fields.

And so on.


Offline Jim

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Re: Rocket Engine Q&A
« Reply #753 on: 01/09/2018 12:21 AM »
Rocket engines actually don't "push" against anything. It's all about conservation of momentum.


Wrong, they do,  the combustion chamber

Offline Proponent

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Re: Rocket Engine Q&A
« Reply #754 on: 01/09/2018 04:04 PM »
Rocket engines actually don't "push" against anything. It's all about conservation of momentum.


Wrong, they do,  the combustion chamber

Two sides of the same coin.  The usual expression for the thrust generated by a rocket engine is

      qc + (pe - pa)Ae ,

where q is the mass flow rate, c is the exhaust velocity, pe is the pressure at the nozzle exit, and pa is the ambient pressure.  This is usually derived by appealing to conservation of momentum for the first term and then, rather unconvincingly in my opinion, adding the second term to account for pressure differences.  But it is also possible to derive the whole expression by considering nothing but the pressures, both internal and ambient, on the combustion chamber and nozzle.  In this case the pressure term arises quite naturally.

Offline the_other_Doug

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Re: Rocket Engine Q&A
« Reply #755 on: 01/09/2018 06:27 PM »
I think we're getting lost in semantics, here.  The original question, I believe, was asking where in the rocket is the force applied by the engine's escaping gasses -- commonly just called thrust -- transferred into the structure of the rocket as a whole.

Jim's right -- the escaping gasses apply force against the side of the combustion chamber opposite from the hole in the chamber that lets the gasses escape.  That force (thrust) is transferred into the body of the rocket by firmly attaching the combustion chambers to a thrust bulkhead, so to speak -- a portion of the structure of the rocket built heavily enough to withstand the force applied through the backsides of the combustion chambers and safely, with no structural failures, transfer that force through to the entire rocket's mass.

Without a strong enough thrust bulkhead, a rocket would act like a model rocket were the builder forgot to glue the engine stop ring into the engine mount.  The engine would, while it was thrusting, just pass up through the rocket's structure and out the top...  :o
-Doug  (With my shield, not yet upon it)

Offline Lars-J

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Re: Rocket Engine Q&A
« Reply #756 on: 01/09/2018 08:13 PM »
I think we're getting lost in semantics, here.  The original question, I believe, was asking where in the rocket is the force applied by the engine's escaping gasses -- commonly just called thrust -- transferred into the structure of the rocket as a whole.

Jim's right -- the escaping gasses apply force against the side of the combustion chamber opposite from the hole in the chamber that lets the gasses escape.  That force (thrust) is transferred into the body of the rocket by firmly attaching the combustion chambers to a thrust bulkhead, so to speak -- a portion of the structure of the rocket built heavily enough to withstand the force applied through the backsides of the combustion chambers and safely, with no structural failures, transfer that force through to the entire rocket's mass.

Without a strong enough thrust bulkhead, a rocket would act like a model rocket were the builder forgot to glue the engine stop ring into the engine mount.  The engine would, while it was thrusting, just pass up through the rocket's structure and out the top...  :o

Exactly... In image 1 (Merlin 1C for Falcon 1), you can see such a thrust structure. The four beams in this case transfers the load from the engine to the tanks and rocket structure.

Image 2 is the Merlin 1D, which just uses a plate. (top of engine and thrust vector actuators connect to the plate) That plate is bolted to the octaweb, which transfers the combined load to the rocket.
« Last Edit: 01/10/2018 12:35 AM by Lars-J »

Offline Proponent

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Re: Rocket Engine Q&A
« Reply #757 on: 01/10/2018 02:05 AM »
... the escaping gasses apply force against the side of the combustion chamber opposite from the hole in the chamber that lets the gasses escape.

At points within the chamber or nozzle where the internal pressure exceeds the ambient pressure, diverging sections (including the top of the combustion chamber) contribute to thrust while converging sections detract.  Where ambient pressure exceeds internal pressure, as is the case in the tail end of an over-expanded nozzle, it's the other way around.

Offline nicp

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Re: Rocket Engine Q&A
« Reply #758 on: 01/30/2018 07:50 AM »
I was wondering recently what kind of injector Raptor might have, and vaguely assumed it might be another pintle.
But then I remembered (I think this is right) that the fuel and oxidizer are both fully in the gaseous phase before hitting the main combustion chamber. This may also be true of BE-4.

As I recall one advantage of pintle injectors is you can get 'free' film cooling from fuel hitting the chamber walls. Which presumably is not going to happen with fully gaseous propellants.

But then (presumably) even a classic waterfall injector is going to have to be designed a little differently for gaseous propellants.

So what does a gaseous injector look like?
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Tags: Injector Raptor BE-4