ELF110-Space  Wk1 Part B

About 1 hr

 

In ELF110 Wk1 Part B we will take a break from the tutorial and do some experiments exploring  why rockets are built the way they are.

 

First, we will explore the idea of a flight envelope.   The “envelope” is the combination of speed, altitude and maneuvering that are safe for a particular vehicle.   For rockets the envelope can be limited by strength of the vehicle to resist aerodynamic or acceleration forces, or its resistance to heat from friction with the atmosphere.   Exceed these limits, and you may have an “unscheduled disassembly” event.

 

Let’s start with the idea of a flight envelope.  Here is the Game Set up:

 

  1. Start New
  2. Set mode to Sandbox
  3. Call this game “Wk1 Part B1”:
  4. Go to the VAB, we are building a rocket (yay!)
  5. Select the Mk1 Command Pod, the same Pod used in the tutorial
  6. Next select the Kerbodyne S3-72000 Tank.  It’s big.
  7. Finally the S3 KS-24×4 “Mammoth” Liquid Fuel Engine.  It’s big too.
  8. Title your rocked “Overpowered”
  9. Save, and head to the Launch Pad
  10. Turn the SAS on, tap “Z” to bring the throttle to 100% and hit space bar to launch.  Go straight up.
  11. What happened? Why?
  12. Hit “Revert to Launch” and launch again, and note the g-force meter in the lower right ark of the Navball.  1g is normal gravity, 2g is 2x gravity, so you feel like you weigh twice as much. How many g(s) did “Overpowered” hit?  Could a crew function or survive that that kind of acceleration? What about the rest of the rocket? (even if there were no heat issue)
  13. Is it possible for a rocket to be over powered?
  14. Thank your Kerbel for their sacrifice (or did you sneak a parachute on there)
  15. Exit to the main menu

 

Next we will examine one of the distinctive features of modern rocketry, staging.  Start a new game as follows:

 

  1. Start New
  2. Set mode to Sandbox
  3. Call this game “Wk1 Part B2”:
  4. Head back to the VAB, we are building another set of rockets to illustrate the reason rocket are staged
  5. Select the Mk1 command pod
  6. Attach 3 FL-T200 fuel tanks below your command pod.  This is our payload, and attachment point for solid rocket motors.
  7. Attach 4 TT-38K radial decouplers symmetrically
  8. Put 4 RT-10 ‘Hammer’ solid rocket boosters on the decouplers.  Note they are 3.5 tons each.
  9. Name this vessel “4SRB – single stage” and save.    Move to the launch pad.
  10. Set SAS on, and launch.  Go straight up.
  11. Watch the altimeter and write down the maximum altitude.  It should be around 11,000m. This is our single-stage benchmark.

 

  1. Head back to the VAB, we are building a identical rocket, but with 2 of the solid rocket boosters staged.
  2. Select the Mk1 command pod
  3. Attach 3 FL-T200 fuel tanks below your command pod.  This is still our payload, and attachment point for solid rocket motors
  4. Attach 4 TT-38K radial decouplers symmetrically
  5. Put 4 RT-10 ‘Hammer’ solid rocket boosters on the decouplers.  Note they are 3.5 tons each
  6. Set the staging so 2 opposing rockets fire and decouple, then the other 2 fire then decouple (left click to select or deselect parts).  
  7. Name this rocket “4SRB – two stage” and save
  8. Launch with SAS on.  Go straight up.
  9. Record the maximum height of your launch.  It should be over 30,000 meters,
  10. Both rockets have the same launch weight, and the same amount of thrust from the 4 Solid Rocket Motors.  Why does the staged rocket go almost 3x higher?

 

So staging works.  In the example we just did, the solid fuel motors were all identical.   But there is another reason to stage.

 

  1. Start New
  2. Set mode to Sandbox
  3. Call this game “Wk1 Part B3”:
  4. Head back to the VAB, we are building another set of rockets to illustrate another reason rocket are staged
  5. Select the Mk1 command pod
  6. Attach 1 FL-T200 fuel tanks below your command pod.
  7. Add a LV-909 “Terrier” Liquid Fuel Engine
  8. Add a TR-18A Stack Decoupler
  9. Add another  FL-T200 fuel tank
  10. Finally add a LT-T45 Swivel Liquid fuel engine
  11. Call this vehicle “Swivel on the bottom” save it, and move to launch
  12. Set Z to max throttle, SAS on, and launch.   Go straight up.
  13. Record your max altitude.  It should be a bit over 71,000 meters… just into Kerbel Space.

 

  1. Now head back to the VAB.  We are building an identical rocket, but with the same  engines in a different order.
  2. Select the Mk1 command pod
  3. Attach 1 FL-T200 fuel tanks below your command pod.
  4. This time the top stage will be the  LT-T45 Swivel Liquid fuel engine instead of the Terrier
  5. Add a TR-18A Stack Decoupler
  6. Add another  FL-T200 fuel tank
  7. Finally add a LV-909 “Terrier” Liquid Fuel Engine
  8. Call this vehicle “Swivel on the top” save it, and move to launch
  9. Set Z to max throttle, SAS on, and launch.  Go straight up…. Or try too.
  10. What happened? Stage when the first stage is out of fuel.
  11. Record your max altitude. Did you clear 30,000 meters?  ½ the altitude achieved with the swivel on the bottom?
  12. Head back to the VAB.  Mouse over the “Terrier” and “Swivel” engines.  Note the Mass (weight) of each engine. Note the Thrust of each engine.      Why are there two numbers for Thrust?  Hint: (ASL) is “At Sea Level”  and (Vac.) is vacuum (like space).    The Terrier produces 4 times more thrust in a vacuum that it does at sea level. The Swivel does not  even produce twice the sea level thrust in a vacuum. So, the Terrier is great in space, but it doesn’t even have the power to get our vehicle off the pad.
  13. Hit the right mouse button [RMB] to expand the engine data.   Note the “Engine Isp:” this is the Specific Impulse of the engine, a measure of how much thrust the engine produces for a unit of fuel.   A higher Isp is better. Note the “Swivel” has a higher Isp ASL, and the “Terrier” has a higher Isp in a vacuum.  The Terrier is the more efficient engine in space, but it produces too little thrust ASL, and is very inefficient ASL.  
  14. These are pretty realistic numbers for Isp.  The NASA Space Shuttle SRBs (Solid Rocket Boosters) have an Isp around 250 seconds, while the hydrogen liquid fuel motors have an Isp of 450.  Note the Space Shuttle liquid fuel motors produce about 1900kN, and there are three of them, so net they are much larger (but not much more efficient) than the vehicle we just built.  It’s always nice to have a little reality in your games.

 

So there is another reason to stage beyond getting rid of dead-weight as you ascend.   Rocket motors are engineered for particular total thrust levels, and operating environments.  The high thrust motor that is great for getting you off the launch pad at Sea Level is not an efficient motor in space.   Staging lets you use the best motor for the flight phase you are in, generally high-thrust motors that produce good power at sea level on the bottom stage, and lower thrust motors that produce high Isp in a vacuum on the top stages.