# Am I supposed to modify the gravitational constant with scale and why do fps & time scale changes cause my orbit to break?

I am using Unity3D and writing C#

• Earth Mass: 5.972e+22
• Earth Velocity: [0,0,0]
• ISS Mass: 4194.55
• Starting ISS Velocity: [76.50, 0, 0]
• ISS Altitude: 4
• Gravitational Constant: 6.67408e-15

All values are the real life values / 100, as I am scaling things down. (ex. Gravitational Constant was 6.67408e-11 / 11 = 6.67408e-15.

Here is the code...

void Gravity(Gravity body) {
float dSquared = (body.transform.position - transform.position).sqrMagnitude;
float force = (body.G * shipMass * body.planetMass) / dSquared;
Vector3 forceDirection = (body.transform.position - transform.position).normalized;
Vector3 forceVector = (forceDirection * force);
shipVelocity += forceVector * Time.deltaTime;

transform.position += shipVelocity * Time.deltaTime;
}


In order to make this work I need to set the gravitational constant to: 1.59508e-21

Here is an image of the orbit, the pink line is past not future path (the calculated orbital elements shown are most likely wrong):

Here is an image of the orbit after I speed up time:

Edit 1:

Using this formula for acceleration and the first part of Russell Borogove's answer I was able to achieve a circular orbit using these values:

• Earth Mass: 5.972e+22
• Earth Velocity: [0,0,0]
• ISS Mass: 4194.55
• Starting ISS Velocity: [0.0765, 0, 0]
• ISS Altitude: 4
• Gravitational Constant: 6.67408e-24 (Shifted -15 then +2 orders of magnitude)

and this code:

void Gravity(Gravity body) {
float dSquared = (body.transform.position - transform.position).sqrMagnitude;
float M = body.planetMass;
Vector3 forceDirection = (body.transform.position - transform.position).normalized;
float g = G * M / dSquared;
Vector3 forceVector = (forceDirection * g);
shipVelocity += forceVector * Time.deltaTime;
}


Edit 2: Added gravity calculations using Russell Borogove's accumulator script below...

void Update() {
Gravity(orbitee);
ApplyForce();
}

void ApplyForce() {
transform.position += shipVelocity * Time.deltaTime;
transform.Rotate(transform.forward * shipTorque * Time.deltaTime);
}

void Gravity(Gravity body) {
accumulator += Time.deltaTime;
while (accumulator > 0.0f) {
float dSquared = (body.transform.position - transform.position).sqrMagnitude;
float M = body.planetMass;
Vector3 forceDirection = (body.transform.position - transform.position).normalized;
float g = G * M / dSquared;
Vector3 forceVector = (forceDirection * g);
shipVelocity += forceVector * simtime;
accumulator -= simtime;
}
}


Edit 3: Updated Orbital Elements Script (Probably not 100% correct)

    //Mass of satellite
float m1 = shipMass;

//Mass of planet
float m2 = orbiting.planetMass;
float M = m2;

//Relative Position Vector
Vector3 r = FindRelativePosition(orbitee.transform.position, transform);

//Relative Velocity Vector
Vector3 v = shipCurVel;
//#!# add relative to planet velocity

//Specific Angular Momentum
Vector3 h = Vector3.Cross(r, v);

//Standard Gravitational Parameter
float µ = G * (m1 + m2);
//float µ = G * M;

//Eccentricity Vector
Vector3 evec = (Vector3.Cross(v, h) / µ) - (r / Vector3.Magnitude(r));

//Eccentricity
float e = Vector3.Magnitude(evec);

//Vector to Ascending Node #!#
Vector3 n = Vector3.Cross(new Vector3(0, 0, 1), h);

//True Anomaly
float t = Mathf.Acos((Vector3.Dot(evec, r)) / (e * Vector3.Magnitude(r)));
if (Vector3.Dot(r, v) < 0)
t = (2 * Mathf.PI) - t;

//Eccentric Anomaly
float E = 2 * Mathf.Atan(Mathf.Tan(t / 2) / Mathf.Sqrt((1 + e) / (1 - e)));

//Longitude of Ascending Node (2D)
float Ω = 0;

//Inclination (2D)
float i = 0;

//Argument of Periapsis
float ω = Mathf.Atan2(evec.y, evec.x);
float ωdegrees = ω * (180 / Mathf.PI);
//float ω = Mathf.Acos((Vector3.Dot(n, evec)) / (e * Vector3.Magnitude(n)));

//Mean Anomaly
float MA = E - (e * Mathf.Sin(E));

//Semi-Major Axis
float a = 1 / ((2 / Vector3.Magnitude(r)) - (Mathf.Pow(Vector3.Magnitude(v), 2) / µ));

//Apoapsis
float ap = a * (1 + e);

//Periapsis
float pe = a * (1 - e);

//Orbital Period
float T = (2 * Mathf.PI) * Mathf.Sqrt(Mathf.Pow(a, 3) / µ)/60;


Most recent edit: fixed issue I was having with eccentricity being incorrect - was calculating relative position wrong.

• You're using ship mass to compute force, but not dividing by mass when you apply it as acceleration (a quick check, 1.595 (mantissa of your G) x 4194 (mass of ship) yields about 6700 (matching mantissa of correct G) so that's likely the problem). It's generally easier to skip computing force and just compute acceleration, which is independent of ship mass. And again you're mixing up 100m and 100km units. – Russell Borogove Jun 16 '16 at 3:30
• I will give that a try and report back. So the velocity should be 0.0765 instead of 76.5? – Wafer Jun 16 '16 at 3:37
• It's probably okay if you get the right number of zeros in your G. – Russell Borogove Jun 16 '16 at 3:40
• Does G change with scale or is it something that should be constant? I had to make it significantly smaller compared to the change in all the other parameters. – Wafer Jun 16 '16 at 3:46
• Of course it does. Elaborated in answer. – Russell Borogove Jun 16 '16 at 3:57

Big G is 6.67408e-11 m3 kg-1 sec-2.

So if you uniformly use 100km units instead of m units, that's 5 magnitudes different, so G has to change by 15 magnitudes because the linear dimension is cubed.

Mass units are 100kg, G uses -1 of those, so G goes the other way by 2 magnitudes.

I have no idea what you're doing with seconds. If seconds are seconds (and it takes 90 minutes to complete an orbit), then there's no change to G. If you're using some other conversion factor, then double the number of zeros you're shifting seconds by to change G.

As for the instability, that's inevitable when you do time-step simulations of continuous functions. You can split your Unity Update time slice into some small fixed quantum and run your simulation function several times for each game frame:

float accumulator = 0.0f;
float simtime = 0.001f;

void Update( void )
{
accumulator += Time.deltaTime;
while (accumulator > 0.0f)
{
SimulateGravityForTime( simtime );
accumulator -= simtime;
}
}


This runs your sim once for each millisecond of time elapsed in the frame. That should keep things pretty stable. The accumulator is there to retain fractional milliseconds from frame to frame. You'll spend more CPU time doing repeated computations, but a modern computer should be able to afford it easily.

Regarding the units, if I were in your shoes I would separate the view (based on Unity transforms) from the model (the physics simulation). This would allow you to run the simulation entirely in standard units, eliminating much confusion and reducing the likelihood of errors. Inside the physics model you'd always use m/kg/s units and keep track of positions in vectors that are not part of Unity transforms. Once per frame you'd carefully convert to Unity units, and set the positions of the transforms.

• +1 for great answer, +1 for including a little code, -1 for aiding and abetting creative and individualized physical units without even a little caveat about what can go wrong down the road. :) Even inches vs centimeters almost killed Hubble. – uhoh Jun 16 '16 at 5:27
• I think I've been as clear in the discussions I've had with OP (in comments in this and the previous question) that paying attention to the units is important. That said, I'll add a bit about view-model separation. – Russell Borogove Jun 16 '16 at 13:20
• Good - I'll start working on a way to change my vote to +2 which your answer deserves. It involves integer overflow and something about row hammering and then some tachyons, but I'm just not there yet. (humor) – uhoh Jun 16 '16 at 13:28
• @RussellBorogove I've finished the gravity and used your accumulator script here. When I convert T to minutes you can see that it matches the 90m orbital period here. I have edited my post to include the orbital elements script. Does there appear to be anything wrong with my orbital elements? I'm sure the AP and PE are incomplete or wrong as I need to modify them by radius to get numbers that are close, or the magnitude of r in order to get accurate numbers at t: 0. – Wafer Jun 17 '16 at 3:01
• The 2 in Atan2 in computer math libraries refers to two arguments, not multiplying by 2. One-argument Atan can't tell the difference between ( + / + ) and ( - / - ) inputs, which is important in some trig applications -- I don't know if this is one of them. The second of your options looks right, the first needs a 2 *. If your results are sane for half the orbit and insane for the other half, you need Atan2. – Russell Borogove Jun 17 '16 at 4:24