Author Topic: Rocket Engine Q&A  (Read 382913 times)

Offline Remes

  • Full Member
  • ****
  • Posts: 434
  • Germany
  • Liked: 246
  • Likes Given: 142
Re: Rocket Engine Q&A
« Reply #500 on: 08/04/2013 03:51 pm »
Scaling up rocket engines seeems to be difficult/costly. I'm not talking about changing the cycle or increasing chamber pressure, only about increasing thrust. I wonder why this is the case and to what extent higher thrust engines are more expensive in development as well as in production.

More thrust@same pressure->more fuel/oxidizer:
- bigger thrust chamber, bigger injector, completely new geometry
- TP/GG redesign
- bigger fuel/oxidizer lines (perhaps throughout the whole stage)
- valves needs to be bigger, have higher flow forces, needs stronger actuation, different actution->maybe different power stages
- different volumes/flows/geometry->different dynamics, perhaps different oscillation behaviour
- Cooling needs to be redesigned (different surface/volume ratio)
- All the control loops need to be investigated tuned (if it is not open loop control, in that case all the orifices&Co needs to be tuned)
- Some adaptation in software
- ...

Actually you are building a new engine and you have to recertify it. Some basic things like materials used, supplier, etc. remain the same, but actually most of the machining/tooling/parts/... changes.

Edit:
aaaaand:
- higher thrust-> do you have to change the thrust transfer structure of the LV?
- Do the tanks have to modified?
- do you need new/stronger TVC-actuators?
- what about the engine gimbal?
- Does the ascent profile have some undesired side effects? (I was thinking of the first DIV and the cavitation problem. This was not really a engine redesign and fits only partially to this topic. But it's a good example for sideffects).

Some of these points are not that critical if you improve the thrust by a few percent. At higher improvement rates the points get more and more critical.
« Last Edit: 08/04/2013 04:08 pm by Remes »

Offline R7

  • Propulsophile
  • Senior Member
  • *****
  • Posts: 2725
    • Don't worry.. we can still be fans of OSC and SNC
  • Liked: 992
  • Likes Given: 668
Re: Rocket Engine Q&A
« Reply #501 on: 08/05/2013 07:43 am »
I've been thinking, about using different propellant for the TP of the GG (like the RD-107/8), but on the expander cycle case. If you were to do a bleed expander rocket, wouldn't using He to expand and move the TP give better performance? The Heat of vaporization of H2 is 0.904kJ·mol−1, while the He's is 0.0829 kJ·mol−1. If I understand that right, it would allow for a lot extra thrust for a given heat output.

Strangequark has some nice relevant posts on expander/cooling issues

http://forum.nasaspaceflight.com/index.php?topic=16279.msg986192#msg986192

http://forum.nasaspaceflight.com/index.php?topic=30910.msg1006206#msg1006206

http://forum.nasaspaceflight.com/index.php?topic=31040.msg1009889#msg1009889

In short, specific heat capacity (Cp) matters more and hydrogen is in it's own league. Also remember the He is getting more and more expensive, also difficult to store efficiently (AIUI rockets don't store it liquefied but compressed and chilled in supercritical form).
AD·ASTRA·ASTRORVM·GRATIA

Offline baldusi

  • Senior Member
  • *****
  • Posts: 8356
  • Buenos Aires, Argentina
  • Liked: 2539
  • Likes Given: 8273
Re: Rocket Engine Q&A
« Reply #502 on: 08/05/2013 12:46 pm »
I've been thinking, about using different propellant for the TP of the GG (like the RD-107/8), but on the expander cycle case. If you were to do a bleed expander rocket, wouldn't using He to expand and move the TP give better performance? The Heat of vaporization of H2 is 0.904kJ·mol−1, while the He's is 0.0829 kJ·mol−1. If I understand that right, it would allow for a lot extra thrust for a given heat output.

Strangequark has some nice relevant posts on expander/cooling issues

http://forum.nasaspaceflight.com/index.php?topic=16279.msg986192#msg986192

http://forum.nasaspaceflight.com/index.php?topic=30910.msg1006206#msg1006206

http://forum.nasaspaceflight.com/index.php?topic=31040.msg1009889#msg1009889

In short, specific heat capacity (Cp) matters more and hydrogen is in it's own league. Also remember the He is getting more and more expensive, also difficult to store efficiently (AIUI rockets don't store it liquefied but compressed and chilled in supercritical form).
I had confused heat of vaporization with heat capacity. Sorry.

Offline fatjohn1408

  • Full Member
  • ***
  • Posts: 325
  • Liked: 17
  • Likes Given: 13
Re: Rocket Engine Q&A
« Reply #503 on: 08/05/2013 01:39 pm »
Hello I am wondering the following:

Does the difference in Isp for an engine under vacuum conditions and under sea level conditions solely arise from the pressure related term (Pe-Pa)*Ae?

If so, does that mean that the mass flow of an engine is constant throughout flight? Just to refresh my memory.

Regards

Offline MP99

Re: Rocket Engine Q&A
« Reply #504 on: 08/05/2013 06:08 pm »
Hello I am wondering the following:

Does the difference in Isp for an engine under vacuum conditions and under sea level conditions solely arise from the pressure related term (Pe-Pa)*Ae?

If so, does that mean that the mass flow of an engine is constant throughout flight? Just to refresh my memory.

Regards

I assume the generator on a GG engine would also be affected same way, IE pumping efficiency is reduced. I guess that would also reduce efficiency (assuming the GG compensates to keep Pc the same).

Cheers, Martin

Offline Galactic Penguin SST

Re: Rocket Engine Q&A
« Reply #505 on: 08/15/2013 06:44 pm »
What rocket engines using hydrogen peroxide/kerosene are there in current service? How good would they perform as a restartable upper stage engine?

(I'm asking because China is apparently making a new multi-restartable upper stage based on a 35 kN thrust H2O2/RP-1 closed cycle engine and plans to put it into service next year)
Astronomy & spaceflight geek penguin. In a relationship w/ Space Shuttle Discovery. Current Priority: Chasing the Chinese Spaceflight Wonder Egg & A Certain Chinese Mars Rover

Offline baldusi

  • Senior Member
  • *****
  • Posts: 8356
  • Buenos Aires, Argentina
  • Liked: 2539
  • Likes Given: 8273
Re: Rocket Engine Q&A
« Reply #506 on: 08/15/2013 07:08 pm »
What rocket engines using hydrogen peroxide/kerosene are there in current service? How good would they perform as a restartable upper stage engine?

(I'm asking because China is apparently making a new multi-restartable upper stage based on a 35 kN thrust H2O2/RP-1 closed cycle engine and plans to put it into service next year)
The Soyuz's first stage use the combination to run the turbopump.

Offline baldusi

  • Senior Member
  • *****
  • Posts: 8356
  • Buenos Aires, Argentina
  • Liked: 2539
  • Likes Given: 8273
Re: Rocket Engine Q&A
« Reply #507 on: 09/05/2013 01:30 pm »
Am I right in my understanding that in the expander cycle rockets is purely liquid? In other words, the expander part works by upping the pressure of the liquid but avoiding the phase change? Thus, the injector is liquid-liquid (as the Russians say)?

Offline strangequark

  • Full Member
  • ****
  • Posts: 1072
  • Co-Founder, Tesseract Space
  • San Francisco, CA
  • Liked: 226
  • Likes Given: 12
Re: Rocket Engine Q&A
« Reply #508 on: 09/05/2013 07:12 pm »
Am I right in my understanding that in the expander cycle rockets is purely liquid? In other words, the expander part works by upping the pressure of the liquid but avoiding the phase change? Thus, the injector is liquid-liquid (as the Russians say)?

I'm stretching a bit out of my expertise, but I believe expanders typically run supercritical. In short, once it gets to the injector, the fuel will be a "gas", but the process to get there does not involve a sharp phase change (like boiling). As for the pressure, it will actually drop through the cooling jacket. The energy increase is expressed in the temperature of the fluid, not the pressure.

Offline baldusi

  • Senior Member
  • *****
  • Posts: 8356
  • Buenos Aires, Argentina
  • Liked: 2539
  • Likes Given: 8273
Re: Rocket Engine Q&A
« Reply #509 on: 09/05/2013 08:22 pm »
Am I right in my understanding that in the expander cycle rockets is purely liquid? In other words, the expander part works by upping the pressure of the liquid but avoiding the phase change? Thus, the injector is liquid-liquid (as the Russians say)?

I'm stretching a bit out of my expertise, but I believe expanders typically run supercritical. In short, once it gets to the injector, the fuel will be a "gas", but the process to get there does not involve a sharp phase change (like boiling). As for the pressure, it will actually drop through the cooling jacket. The energy increase is expressed in the temperature of the fluid, not the pressure.
I forgot, you have pressure drop and temperature goes up. Silly me. So, if critical point of H2 is (33K,1.3MPa), as long as you keep it above 1.3MPa, you don't have nasty things like bubbles/cavitation, right? My question is, then, by the time it arrives at the turbine, how's energy transferred? I can understand that a gas expands and since the other side is incompressible you trade movement (which in a gas is temperature or pressure?) for potential energy (higher pressure) on the pump side. In the case of super critical H2, does it behave like a gas by the time it reaches the turbine?

Offline Robotbeat

  • Senior Member
  • *****
  • Posts: 39270
  • Minnesota
  • Liked: 25240
  • Likes Given: 12115
Re: Rocket Engine Q&A
« Reply #510 on: 09/05/2013 08:25 pm »
Supercritical works like a gas. It is compressible.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline strangequark

  • Full Member
  • ****
  • Posts: 1072
  • Co-Founder, Tesseract Space
  • San Francisco, CA
  • Liked: 226
  • Likes Given: 12
Re: Rocket Engine Q&A
« Reply #511 on: 09/05/2013 11:45 pm »
I forgot, you have pressure drop and temperature goes up. Silly me. So, if critical point of H2 is (33K,1.3MPa), as long as you keep it above 1.3MPa, you don't have nasty things like bubbles/cavitation, right? My question is, then, by the time it arrives at the turbine, how's energy transferred? I can understand that a gas expands and since the other side is incompressible you trade movement (which in a gas is temperature or pressure?) for potential energy (higher pressure) on the pump side. In the case of super critical H2, does it behave like a gas by the time it reaches the turbine?

Right, no bubbles, boiling or any of that mess. So, turbine power is mass flow*specific heat*T_inlet*(1-[pin/pout]^((1-k)/k).

So, what's that mean? If the pressure drop across the turbine is held constant, then for a given amount of gas, the hotter it is, the more power you get out of it. This is how the expander cycle works. The energy is in the temperature, rather than the pressure, so to speak.

This is true for all cycles, actually. Pressure will never be higher than it is at pump outlet, but temperature will increase substantially.

Offline ClaytonBirchenough

  • Full Member
  • ****
  • Posts: 777
  • ~ 1 AU
  • Liked: 34
  • Likes Given: 348
Re: Rocket Engine Q&A
« Reply #512 on: 09/06/2013 10:52 pm »
Does anybody know how much the ISP of a rocket engine changes throughout its atmospheric flight? Is there any quick way to calculate this? For example, if a rocket engine is sea-level optimized, what would how much would its ISP change at 10 km, 25 km, etc.? Is there a table for this?
Clayton Birchenough

Offline Robotbeat

  • Senior Member
  • *****
  • Posts: 39270
  • Minnesota
  • Liked: 25240
  • Likes Given: 12115
Re: Rocket Engine Q&A
« Reply #513 on: 09/06/2013 11:19 pm »
Does anybody know how much the ISP of a rocket engine changes throughout its atmospheric flight? Is there any quick way to calculate this? For example, if a rocket engine is sea-level optimized, what would how much would its ISP change at 10 km, 25 km, etc.? Is there a table for this?
Rocket Propulsion Elements, Sutton.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline nimbostratus

  • Member
  • Posts: 99
  • Mainland, China
  • Liked: 8
  • Likes Given: 13
Re: Rocket Engine Q&A
« Reply #514 on: 10/10/2014 05:19 pm »
Below I list data of some 1st stage liquid hydrogen engine:

                                           combustion pressure                                area ratio

vulcain 1                               100bar                                                        45

vulcain 2                                117bar                                                       58

le-7                                         127bar                                                       52

le-7a                                      120bar                                                       52

rs-68                                        97bar                                                       21.5

Here comes my question, how comes that similar engines have so different area ratios, I mean,is rs-68 properly designed?

Thanks in advance.

BTW,  why is there not a engine section for such a specialized forum?



« Last Edit: 10/10/2014 05:47 pm by nimbostratus »
Wonders in the desert

Offline kevin-rf

  • Elite Veteran
  • Senior Member
  • *****
  • Posts: 8823
  • Overlooking the path Mary's little Lamb took..
  • Liked: 1318
  • Likes Given: 306
Re: Rocket Engine Q&A
« Reply #515 on: 10/10/2014 05:42 pm »
Remember the RS-68 was designed for cost. Hence the use of an ablative nozzle instead of a regeneratively cooled nozzle. I wonder if it had something to do with manufacturing costs.

If the other two engine families where not also fist stage engines, I would say it had something to do with a sealevel optimized engine, but that is not the case here.

Could it be the weight of a larger nozzle would have negated the ISP gain?
If you're happy and you know it,
It's your med's!

Offline sdsds

  • Senior Member
  • *****
  • Posts: 7201
  • “With peace and hope for all mankind.”
  • Seattle
  • Liked: 2050
  • Likes Given: 1962
Re: Rocket Engine Q&A
« Reply #516 on: 10/10/2014 05:50 pm »
Just a guess? Thrust. The low area ratio implies a big throat. Have you compared the propellant mass flow of the engines?
— 𝐬𝐝𝐒𝐝𝐬 —

Offline R7

  • Propulsophile
  • Senior Member
  • *****
  • Posts: 2725
    • Don't worry.. we can still be fans of OSC and SNC
  • Liked: 992
  • Likes Given: 668
Re: Rocket Engine Q&A
« Reply #517 on: 10/10/2014 05:54 pm »
If the other two engine families where not also fist stage engines, I would say it had something to do with a sealevel optimized engine, but that is not the case here.

Look beyond the engines to the LVs which use them. Ariane and H-II have solid boosters which do most of the lifting at low altitudes. Thus you don't have to worry about core engine sea level Isp so much. Situation is different with Delta IV, good sea level performance is required from RS-68.

sea level Isps:

Vulcain 1: 326s
Vulcain 2: 318s
LE-7: 349s
LE-7A: 338s
RS-68: 365s
AD·ASTRA·ASTRORVM·GRATIA

Offline baldusi

  • Senior Member
  • *****
  • Posts: 8356
  • Buenos Aires, Argentina
  • Liked: 2539
  • Likes Given: 8273
Re: Rocket Engine Q&A
« Reply #518 on: 10/11/2014 12:08 am »
Vulcain 1/2 and LE-7/A are, basically, sustainer engines. When you use a booster augmented h2/lox core (like Ariane 5, H-II or STS did), you want to optimize for vacuum performance. When you need an engine to propel the whole stack alone (like RS-68 might do) you want to focus more on sea level performance.

Offline strangequark

  • Full Member
  • ****
  • Posts: 1072
  • Co-Founder, Tesseract Space
  • San Francisco, CA
  • Liked: 226
  • Likes Given: 12
Re: Rocket Engine Q&A
« Reply #519 on: 10/11/2014 12:26 am »
Vulcain 1/2 and LE-7/A are, basically, sustainer engines. When you use a booster augmented h2/lox core (like Ariane 5, H-II or STS did), you want to optimize for vacuum performance. When you need an engine to propel the whole stack alone (like RS-68 might do) you want to focus more on sea level performance.

Yeah, it's all about the operating envelope. According to the Ariane V user's manual, the core engine cutoff is at an altitude of about 250km, and burns for over 500 seconds for a typical GTO trajectory. It needs to do well in vacuum. The Delta IV on the other hand has MECO occur at 121km, after 245 seconds, and a third of that time is below 10km altitude. The time averaged altitude is much lower for the RS-68.

 

Advertisement NovaTech
Advertisement Northrop Grumman
Advertisement
Advertisement Margaritaville Beach Resort South Padre Island
Advertisement Brady Kenniston
Advertisement NextSpaceflight
Advertisement Nathan Barker Photography
1