Author Topic: Rocket Engine Q&A  (Read 229282 times)

Offline R7

  • Propulsophile
  • Senior Member
  • *****
  • Posts: 2738
    • Don't worry.. we can still be fans of OSC and SNC
  • Liked: 943
  • Likes Given: 663
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.
AD·ASTRA·ASTRORVM·GRATIA

Offline Robert Willis

  • Member
  • Posts: 5
  • Canada
  • Liked: 0
  • Likes Given: 0
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

  • Member
  • Posts: 26
  • UK
  • Liked: 6
  • Likes Given: 67
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?)


Where's my Guinness?

Offline robert_d

  • Full Member
  • **
  • Posts: 256
  • Liked: 31
  • Likes Given: 94
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

  • Full Member
  • **
  • Posts: 227
  • USA
  • Liked: 69
  • Likes Given: 9
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

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31158
  • Cape Canaveral Spaceport
  • Liked: 9398
  • Likes Given: 297
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

  • Full Member
  • **
  • Posts: 227
  • USA
  • Liked: 69
  • Likes Given: 9
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

  • Member
  • Full Member
  • ****
  • Posts: 705
  • Liked: 344
  • Likes Given: 662
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?

Online PahTo

  • Full Member
  • ****
  • Posts: 1487
  • Seattle
  • Liked: 136
  • Likes Given: 381
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

  • Full Member
  • *
  • Posts: 196
  • Liked: 187
  • Likes Given: 25
Re: Rocket Engine Q&A
« Reply #749 on: 09/08/2017 08:45 AM »
Nope, the original concept had an airstart SSME.

Online edkyle99

  • Expert
  • Senior Member
  • *****
  • Posts: 12576
    • Space Launch Report
  • Liked: 3384
  • Likes Given: 562
Re: Rocket Engine Q&A
« Reply #750 on: 09/08/2017 08:08 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?
The ESAS report specified that the second stage would be powered by "[t]he RS-25 .... an expendable version of the current SSME, modified to start at altitude."

Then there was this paragraph.

"Altitude-Start SSME
A 1993 Study (NAS8-39211) and a 2004 Marshall Space Flight Center (MSFC) Study examined
the Block II engines for altitude start. Both studies determined altitude start will require
minor changes, but is considered straightforward. Specialized testing for certification to the
environment will be required. Development and certification of altitude start for the Block
II RS-25d engine is needed. The cost estimate is based on contractor provided information,
which included SSME historical actuals, vendor quotes, and estimates. It also assumes the
Shuttle Program continues to pay the fixed cost of infrastructure through Shuttle termination."

 - Ed Kyle
« Last Edit: 09/08/2017 08:16 PM by edkyle99 »

Online wolfpack

  • Full Member
  • ****
  • Posts: 663
  • Wake Forest, NC
  • Liked: 86
  • Likes Given: 2
Re: Rocket Engine Q&A
« Reply #751 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?

Online edkyle99

  • Expert
  • Senior Member
  • *****
  • Posts: 12576
    • Space Launch Report
  • Liked: 3384
  • Likes Given: 562
Re: Rocket Engine Q&A
« Reply #752 on: 09/09/2017 03:04 PM »
Another thought on air-start SSME.  As I recall, the problem wasn't that it would have been hard to convert the engine into an air-start machine.  The problem was that it would have been very, very difficult, if not impossible, to turn it into a re-startable engine, which would have been needed for Ares V upper stage.  Not having that ability forced the J-2X decision, which caused a loss of performance, etc.

 - Ed Kyle
« Last Edit: 09/10/2017 02:34 PM by edkyle99 »