8 comments

• Wayne

  February 25, 2016 at 11:14 am

  Do we know what fuel & oxidizer this engine uses?
  –hopefully awaiting the SpaceX launch this afternoon.
• Dick Eagleson

  February 25, 2016 at 3:46 pm

  According to Wikipedia, the ISRO CE-20 is a gas generator cycle LOX-LH2 engine that produces 45,000 lbf thrust (80% more than an AJR RL-10C; 40% of a Blue Origin BE-3) with a vacuum Isp of 443. Looks to be a nice piece of work.
• wayne

  February 25, 2016 at 4:35 pm

  Dick
  Thanks!
  Maybe you can tell me the difference between the Merlin vacuum engine and a “regular” Merlin engine? that is, how they actually work. (Merlin uses kerosene & LOX, correct)
  -T-minus 15 minutes for the SpaxeX SES-9 launch!
• Edward

  February 25, 2016 at 7:30 pm

  Generally, an engine intended for use from sea level up to the upper atmosphere has a ratio of the area of the exit of the bell to the throat of the engine (expansion ratio https://en.wikipedia.org/wiki/Rocket_engine_nozzle#Optimal_shape ) as around 10:1. This gives better thrust in the lower atmosphere, but at higher altitudes the exhaust tends to spread out. You may have seen this phenomenon in Saturn V videos, and it can be seen in other rocket launch videos.

  An engine intended for use in the vacuum of space (or the upper atmosphere, where pressure is low) has a ratio closer to 100:1, as this helps direct the thrust better.

  https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#Merlin_Vacuum_.281C.29

  Performance is the main driver. The specific impulse (Isp) is the usual measure of efficiency, the amount of fuel needed to produce each pound of thrust. It is given in seconds, as in the number of seconds the engine takes at a 92,500 lb thrust to burn 92,500 lb of propellant. Higher numbers are better, as less fuel is needed to move the rocket to where it needs to go.

  Then there is the aerospike (or linear aerospike) engine, a cool concept which looks more like an inside out engine than a regular rocket engine. As far as I know, it has not yet flown.
  https://en.wikipedia.org/wiki/Aerospike_engine
• wayne

  February 26, 2016 at 2:12 pm

  Edward
  Thank you! Appreciate you aggregating that info. I can actually follow all that.

  Yes, have wondered about exhaust spread & have seen that in NASA films. I sorta thought it was an artifact of the long-range camera angle, but now that you describe it, “it’s all clear to me now!” — atmospheric pressure & engineering design requirements.

  — do you happen to know if there is a way to avoid “maximum dynamic pressure?” Or is that a set combination of physical-states that just occur no matter what? I mean, does a model rocket for example, have a “max-q?”
  –does that pressure value dictate a minimum required amount of structural strength? (I assume some components are over or under-engineered depending on requirement.)
  –is the required escape-velocity larger at sea-level compared to altitude? If you could launch from Mount Everest for example, would that take less energy?
  — if a rocket accelerates to less-than the escape velocity, it will eventually arc back down somewhere- correct?

  Just wondering out loud & thanks again for the info.
• Edward

  February 26, 2016 at 4:19 pm

  Wayne asked: “do you happen to know if there is a way to avoid ‘maximum dynamic pressure?’ Or is that a set combination of physical-states that just occur no matter what? I mean, does a model rocket for example, have a ‘max-q?'”

  You will always have a max-q. No matter how slowly you climb out of the atmosphere, there will be some point where the dynamic pressure is the greatest. The solution is to design for it. The Space Shuttle, for example, reduced the main engine thrust for a while. Payload shrouds are designed to withstand the pressure. I have even seen rockets that light some of their strap-on solid rockets mid-air, apparently in an attempt to increase acceleration after the max-q point.

  For model rockets, max-q will usually be at engine burnout. The “Estes” rockets do not get high enough for the atmosphere to get thin, but there are some model rocketeers who make larger, higher flying rockets, but they have to get far from civilization to fly them (e.g. Nevada desert).

  Escape velocity is dependent upon altitude. For instance, if you are already in a circular orbit, escape velocity is the orbital velocity times the square root of two. Low Earth Orbit has a circular orbital speed of around 17,500 miles per hour, escape velocity around 25,000 miles per hour. The geostationary orbit that communication satellites often use has a speed of around 6,800 miles per hour, escape velocity around 9,600 miles per hour.

  Accounting for atmosphere in determining escape velocity is difficult, as the atmospheric drag depends upon the aerodynamics of the item being launched. However, the general concept is that the higher altitude gives less drag and a slightly slower escape velocity, so Mount Everest would be a better launch pad for using less energy to get to orbit, with a big caveat*.

  At less than escape velocity, the rocket may be in orbit in an elliptical path (this will happen if it started in orbit), or it may be suborbital and come back down to Earth, which is what happens to first stages, as they do not make it to orbit.

  Air-dropping a rocket, as Orbital ATK does with their Pegasus, Virgin Galactic does with SpaceShipTwo, and StratoLaunch wants to do, takes advantage of several things: 1) not carrying an oxidizer to (9-ish mile) altitude, 2) better efficiency of jet engines and wing efficiency over rocket engines, 3) starting at a lower-density atmosphere 4) flexibility in launch latitude. This is somewhat similar to launching from a high mountain.

  For more on orbital mechanics and space travel (not so much launch, though), try this tutorial from NASA’s JPL:
  http://solarsystem.nasa.gov/basics/index.php

  Chapter 4 also discusses gravity assist, or “slingshot” mechanics, which is difficult to explain without sketches.

  * As it turns out, it is best to launch from the latitude that corresponds with the angle of the orbit that you want, where angle means the angle at which it crosses the equator. This is a bit counterintuitive, because one would think that launching at the equator gives the best kick, but to get into a 60 degree orbit requires more fuel to go north than you saved from the 1,000 mile per hour head start. It is best to launch from Russia for that 60 degree orbit.

  To change the angle of the orbit’s plane takes a lot of energy, proportional to twice the sine of the angle change. Thus launching from Mount Everest would be better for 28-ish degree orbits. It might also be OK for launching a rocket to escape velocity for a trip to somewhere else in the solar system.
• wayne

  February 26, 2016 at 6:48 pm

  Edward my Man!
  -Beautiful! Thank you for your effort. (You’re an Engineer aren’t you?!)
  I’m actually following the logic pretty well, so none of it is lost on me.
  -I’ve always loved this stuff –but spent 30 years in Mental Health (Psych degree) & only now am I able to go back to really learn “science” in detail, inside & out.
  (Been hooked on Space ever since I saw Apollo 8 launch. Absolutely Fantastical!)
  Totally had “escape-velocity” confused– appreciate the clarification.
• wayne

  February 26, 2016 at 7:10 pm

  Edward–
  GREAT tutorial at the JPL website– thank you very much! (Going to task my grand-daughter to this, as well.)
  –I have been watching a lot of the von Karman lectures on the JPL YouTube page–will have to explore the JPL website, a little more closely.
  Thanks again.