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 Post subject: ElectronPosted: Sun, 11-01-15, 13:18 GMT

Joined: Tue, 04-09-07, 2:32 GMT
Posts: 429
Location: South Korea
Here is a new addon I'm making of New Zealand's new Electron rocket.
Electron is a small rocket for launching 100 kg-class payloads to LEO: http://www.rocketlabusa.com
They're aiming to make their first launch in 2015, which if it succeeds would not only make New Zealand the 11th nation to perform an orbital launch with its own rocket, but would also start a trend of "smaller cheaper faster" in the launch industry.

Here is the full stack:
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electron-stack.png [ 268.07 KiB | Viewed 2699 times ]

This is the 2nd stage (no payload yet):
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electron-stage2.png [ 243.71 KiB | Viewed 2699 times ]

All textures come from the Rocket Lab web site.
I used Blender for 3d modeling.
The aim is to model a full mission, such as a small spacecraft delivery to a translunar trajectory. I don't know if that's realistic though. I'm currently trying to get NASA's GMAT to compile on my Mac so that I can generate some interesting trajectories.

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 Post subject: Re: ElectronPosted: Fri, 23-01-15, 4:42 GMT

Joined: Tue, 04-09-07, 2:32 GMT
Posts: 429
Location: South Korea
To simulate the atmospheric phase of an Electron launch, I tried using OpenRocket, a Java GUI originally designed for simulating model rocket launches.
However the limitations of using a model rocket simulator to simulate a 10.5-ton orbital launch vehicle quickly became apparent:
Attachment:

ELECTRON-openrocketscreen.png [ 84.87 KiB | Viewed 2645 times ]

In the above screen, I roughly sketched out the two stages of the Electron vehicle using the measurements in my Blender model.
As Electron will use composite materials for its hull, I specified carbon fibre for my OpenRocket model as well.
There is no publicly available data on the thrust curve of the Rutherford engines powering the Electron, so I took the highest thrust motor O8000 available on ThrustCurve.org and scaled up the maximum thrust to match the peak thrust figure quoted for Rutherford on the Rocket Lab web site (146 kN for the 9-engine 1st stage cluster, or 16.2 kN/motor, and 18 kN for the 2nd stage vacuum motor). I also set the burn times for the engines to 114 seconds, which is about how long ESA's Vega burns for its 1st stage, and increased the propellant masses until the wet mass of the rocket was in the ballpark of the quoted figure of 10.5 metric tons.

This gets Electron up to about 1.6 km altitude and nearly Mach 0.4, but without active GNC (guidance, navigation and control), the LV starts to "dance" around and tumble out of control and hits the ground after only 70 seconds in the air. Here is a GNU Octave (Matlab) plot of the simulated flight profile from OpenRocket:
Attachment:

ELECTRON-noguidance.png [ 131.22 KiB | Viewed 2645 times ]

The problem is that all model rocket simulator programs like OpenRocket assume that you are using appendages such as fins to passively stabilize the rocket and cannot simulate active stabilization using a GNC system. Still, it's quite interesting to see how long such a large rocket can stay aloft even without stabilization.

To be able to simulate active control, a different program is required. Apparently JSBSim, which is also used by the open source flight sim FlightGear, can allow you to model a rocket GNC system. This is not surprising, as the designer of JSBSim is Jon S. Berndt, a NASA rocket dynamics engineer. My plan is to try this next.

Last edited by dirkpitt on Thu, 29-01-15, 11:32 GMT, edited 1 time in total.

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 Post subject: Re: ElectronPosted: Mon, 26-01-15, 10:04 GMT

Joined: Tue, 04-09-07, 2:32 GMT
Posts: 429
Location: South Korea
JSBSim still requires that I specify aerodynamic data before I can realistically simulate atmospheric flight. These could be the following:

• Drag coefficient for different Mach numbers (CD)
• Side force coefficient (CY, or alternatively, normal force coefficient CN since a rocket is symmetric about its center)
• Lift force coefficient (CA)

Normally, one would need to go about building a scale model and run a CFD sim or perform measurements in a hypersonic wind tunnel. However, I found a free program (unfortunately Windows-only, with no source code) called RASAero that can calculate various aerodynamic parameters through a wide range of Mach numbers in the specific case of a rocket with a nose cone and fins.

The following is the main screen of the program where I input the dimensions of the Electron rocket. As there are no fins on Electron, I set 0.0001 for the fin dimensions (since setting them to 0.0 would cause an exception). Also the program can display a sketch of the rocket:
Attachment:

RASAero_mainscreen.jpg [ 92.01 KiB | Viewed 2613 times ]

Attachment:

RASAero_rocketpic.jpg [ 32.52 KiB | Viewed 2613 times ]

This is an example of the plot output of drag coefficient versus airspeed (Mach number). You will notice the drag increasing dramatically just before Mach 1 (speed of sound). This illustrates the "sound barrier" nicely. There are two graphs: power-off is a simulation for gliding (unpowered) flight and power-on includes effects from the engine such as the rocket exhaust plume.
Attachment:

RASAero_plotscreen.jpg [ 116.01 KiB | Viewed 2613 times ]

The next step is to copy and paste numbers from RASAero into JSBSim.

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 Post subject: Re: ElectronPosted: Thu, 29-01-15, 5:58 GMT

Joined: Tue, 04-09-07, 2:32 GMT
Posts: 429
Location: South Korea
JSBSim is an open source flight dynamics simulator. It can simulate the flight of balloons, gliders, prop planes, jets, rocket-powered jets, and rockets. Importantly, I can program in GNC (guidance, navigation, control) logic to perform active stabilization during flight. JSBSim is a console program that takes xml files as input and outputs csv files (which can be plot in Matlab or Excel), linked to Simulink, or even stream output via telnet for remote "telemetry."

A JSBSim model requires aircraft, engine, and script definitions.
This is how I structured the Electron flight model in JSBSim:

Code:
aircraft/
Electron.xml      <-- Rocket geometry, aerodynamic parameters (from RASAero), and engine config
NZ01.xml          <-- Parameters of (imaginary) launch site in New Zealand

aircraft/Systems/
ElectronControlSystem.xml             <-- GNC parameters
ElectronGuidanceExecutive.xml       <-- Mission clock, guidance modes
ElectronFirstStageEffectors.xml       <-- First stage engine gimbal definition
ElectronSecondStageEffectors.xml   <-- 2nd stage engine gimbal definition

engine/
Rutherford.xml
Rutherford-nozzle.xml
Rutherford_vac.xml
Rutherford_vac-nozzle.xml

scripts/
Electron.xml       <-- Defines wind speeds, rocket staging, console output

I based the file organization and GNC structure on the Jupiter-246 concept model available in JSBSim, but otherwise everything was done nearly from scratch.

The masses of each major rocket part such as payload shroud, body tubes, engines, were specified as pointmass elements inside the mass_balance section of the aircraft definition, aircraft/Electron.xml. Only cylindrical and spherical (solid or hollow) shapes can be specified, so it ends up being an approximation of the geometry. The dimensions of each part are fairly well defined from Rocket Lab's web site, and I used masses previously estimated when trying OpenRocket.

Engines are first defined in engine files (the engine and nozzle are separately defined in JSBSim). Here are example engine and nozzle files for the Rutherford engine. The Isp was guessed from other high-performing Kerosene liquid engines, and the mass flow rate $\dot{m}$ was calculated using the relation $Isp=F/\dot{m}g$ where F is the thrust of the engine given on the Rocket Lab web site (146.6 kN peak, or 16.3 kN/engine) and g is the acceleration due to gravity 9.8 m/s^2. The mixture ratio 2.6 is a standard oxidizer to propellant mixture ratio for LOX/kerosene. Incidentally, LOX/Kerosene is the same proven combination used on the Saturn V moon rocket and SpaceX's Falcon 9.

Code:
<?xml version="1.0"?>
<rocket_engine name="Rutherford">
<isp>                   350.0 </isp>
<maxthrottle>           1.00  </maxthrottle>
<minthrottle>           0.40  </minthrottle>
<propflowmax unit="LBS/SEC"> 10.4625 </propflowmax>
<mixtureratio> 2.6 </mixtureratio>
</rocket_engine>

Code:
<?xml version="1.0"?>
<nozzle name="Rutherford Nozzle">
<!-- area = Nozzle exit area, sqft. -->
<area unit="FT2">  0.209  </area>
</nozzle>

This NASA web site gives a nice introduction to the concept of specific impulse.

Tanks are specified in the aircraft definition file, by giving the types (FUEL/OXIDIZER), locations, capacities, and drain locations. Tanks are "hooked up" to engines by specifying the tank number as feed elements in each engine.

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 Post subject: Re: ElectronPosted: Thu, 29-01-15, 10:32 GMT

Joined: Tue, 04-09-07, 2:32 GMT
Posts: 429
Location: South Korea
Aerodynamic coefficients are specified in the aerodynamics section in the aircraft definition file.
Coefficients must be defined per axis; for Electron, I specified DRAG (axial) and LIFT (pitch/normal) axes.
Normally, I would also need to define the SIDE (yaw) axis, but due to symmetry about the longitudinal axis, SIDE is equivalent to LIFT and there is no need to define both.

DRAG force is a factor of the following:
• Dynamic pressure (JSBSim property: aero/qbar-psf): pressure of air exerted on moving rocket; increases with speed
• Cross-sectional area (JSBSim property: metrics/Sw-sqft): for a cylindrical rocket, equal to the circular cross sectional area
• Angle of attack alpha (JSBSim property: aero/alpha-rad): angle between wind and rocket, related to pitch angle
• Mach number (JSBSim property: velocities/mach)

RASAero was able to plot the drag coefficient CD versus alpha at Mach 0.1, 0.5, 1.1, 2.0, 5.0.
It was also able to plot CD versus Mach number from 0 to 25.
I defined tableData elements in JSBSim for these two plots. The first plot has two independent variables alpha and Mach number so the table is two-dimensional with alpha increasing from top to bottom, and Mach number increasing from left to right.
The second table is just CD with Mach number increasing from top to bottom. I took the "power-on" sim values (exhaust plume effects included) as the rocket will likely be thrusting through most of its flight path.
Attachment:

plotCDpon.png [ 18.55 KiB | Viewed 2584 times ]

LIFT force, like DRAG, is also a factor of dynamic pressure, cross-sectional area, angle of attack, and Mach number.
I defined another two-dimensional table with independent variables alpha and Mach number from a RASAero plot of lift coefficient CL.
Attachment:

plotCLalpha.png [ 23.2 KiB | Viewed 2584 times ]

Next I will define the GNC parameters for stabilizing the rocket during flight.

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 Post subject: Re: ElectronPosted: Thu, 29-01-15, 11:06 GMT

Joined: Tue, 04-09-07, 2:32 GMT
Posts: 429
Location: South Korea
Let's take a break from aerodynamics, and see if we can improve the 3D model of Electron.
Christophe (ElChristou) has offered his designer skills and has done a much better job at creating a more detailed model of the rocket!
Notice details such as straps holding the tanks to the underside of the second stage, rivets on the bottom of the first stage, and cleaner textures:
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unnamed.jpg [ 101.19 KiB | Viewed 2580 times ]

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unnamed-3.jpg [ 120.64 KiB | Viewed 2580 times ]

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unnamed-2.jpg [ 166.13 KiB | Viewed 2580 times ]

The one remaining issue is the part joining the payload and the second stage. This is a closeup from Rocket Lab's web site:
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A3.jpg [ 43.79 KiB | Viewed 2580 times ]

The golden cube is definitely the payload, but Christophe and I can't figure out what the rest of the parts are.
The red outlined part below the payload is probably an upper stage kick motor (a rocket motor that would fire to boost the payload into its final orbit). But that's just a wild guess, and we don't know what the black hexagonal structure that is masking part of the upper stage(?) is. Is it part of the second stage? What are the tanks inside the hexagonal structure? etc.

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 Post subject: Re: ElectronPosted: Fri, 06-02-15, 5:46 GMT

Joined: Tue, 04-09-07, 2:32 GMT
Posts: 429
Location: South Korea
I sent an email to Rocket Lab last week asking about the upper stage.
I addressed the email to the general media inquiry address at Rocket Lab, but imagine my surprise when the CEO replied personally!
Quote:
From: Peter Beck <email address redacted>
Date: Fri, Jan 30, 2015 at 7:51 AM
Subject: Rocket Lab

Hi Dawoon,

Nice work!
It's no fun if we answer all your questions, but you are on the right track......

Pete

--
Peter Beck
CEO

ROCKET LAB LTD.

Apart from the fact that it was very cool to get a personal email from the head of a rocket company and that we know that we're on the right track, let's recap what our plan is so far.
On top of stage 2 is a hexagonal stand. There is a ring on top of this stand through which a upper stage kick motor is inserted before launch. And on top of the upper stage is the payload:
Attachment:

stage2_and_upperstage.png [ 58.89 KiB | Viewed 2487 times ]

Christophe our designer decided to etch the big letters ELECTRON and the NZ logo onto the rocket instead of using textures.
While this turned out to be much more challenging in terms of uv mapping, this allowed us to get very clean results even at high zoom without using huge textures:
Attachment:

electron-sci-nz-smooth.jpg [ 58.72 KiB | Viewed 2487 times ]

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electron-sci3.jpg [ 89.18 KiB | Viewed 2487 times ]

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electron-sci-stack.jpg [ 146.15 KiB | Viewed 2487 times ]

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 Post subject: Re: ElectronPosted: Fri, 06-02-15, 6:39 GMT

Joined: Tue, 04-09-07, 2:32 GMT
Posts: 429
Location: South Korea
Let's talk launch trajectory design.
The basic steps followed by many orbital launch profiles are the following:

1. Vertical Launch: Pitch angle (angle from horizontal)=90 degrees
2. Pitchover Maneuver: Rocket is tilted slightly and so is no longer vertical, initiating the gravity turn
3. Gravity Turn: Rocket pitch angle is increased so that the rocket flies a curved path and the angle of attack is nearly zero, minimizing aerodynamic stresses at the expense of not gaining altitude as rapidly
4. Max Q Throttling: Engines are throttled down while passing through maximum dynamic pressure ("Max Q") to reduce aerodynamic stresses; usually happens at around 70 seconds into flight just prior to hitting the sound barrier
5. Engine Cut-Off: Often abbreviated to MECO when referring to main engine cut-off, engines are shut down once fuel is used up and thrust falls below a certain level, or deliberately to coast
6. Staging: Stages that have spent fuel are jettisoned to reduce mass
7. Payload shroud (fairing) Jettison: Shroud protecting the payload at the top of the rocket is jettisoned early to reduce mass once altitude is high enough and air is thin enough
8. Payload/Upper stage Deployment: Payload is deployed by mechanical means (e.g., springs) or given a boost to a different orbit by an upper stage kick motor

Here is a great long-exposure photograph of the OCO-2 launch by Rick Baldridge illustrating the entire launch flight path including the characteristic curvature of the gravity turn. (Visit the web page to learn more)

Some rockets may also perform a Roll program during launch to roll the rocket to a certain orientation about its central axis. For example, the Space Shuttle rolled over to make sure its wings faced up during the gravity turn and reduce aerodynamic loads. Here is a gif illustrating it from NASA TV coverage of STS-129:
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output_b5LYwh.gif [ 90.46 KiB | Viewed 2484 times ]

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 Post subject: Re: ElectronPosted: Sat, 18-04-15, 19:27 GMT

Joined: Tue, 04-09-07, 21:55 GMT
Posts: 749
Location: N 42.38846 W 83.45456
news article

from: "The New Zealand Herald"

_________________
"I don't pitch Linux to my friends, I let Microsoft do that for me."
Using OpenSUSE 42.1 & Scientific Linux 6.7

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 Post subject: Re: ElectronPosted: Mon, 20-04-15, 23:30 GMT

Joined: Tue, 04-09-07, 2:32 GMT
Posts: 429
Location: South Korea
Yes, the electric turbopump is interesting. A Tesla's worth of batteries (85 kWh, 0.5 metric tons, mass of motors is probably significant but not included in this simple calculation) could power the 1 MW turbopump for about 300 seconds, which is usually enough for the first stage and also according to my calculations part of the second.

Here is a paper on comparing electric turbopumps to "classical" gas-generator cycles. Electric turbopumps can result in lower structural mass in certain circumstances, potentially offsetting the cost of the batteries:
http://www.dima.uniroma1.it:8080/STAFF2/jpp12r3.pdf

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