SpaceX Starship : First Flight : Starbase, TX : 20 April 2023 - DISCUSSION

[edzieba]

Let me restate my question. My understanding and paraphrasing of the claim is: If a body in free fall travels more than halfway around the globe without impacting, it will always make it all the way around (ignoring drag, etc.). My question is, is that a true statement?

No. You can have a trajectory that just clips the surface at perigee and is therefore suborbital, and there is no requirement for that to be no more than 1/2 the diameter of the orbited body.

However, the difference in delta-V between a suborbital trajectory that gets halfway around the planet before entering the atmosphere and an orbital trajectory is so small that you may as well produce that extra zephyr of a fart and go fully orbital. And with the practical requirements to avoid entering the atmosphere and that your initial launch trajectory starts your orbit from somewhere past perigee (because we launch with actual rockets rather than an instantaneous velocity impulse), you have to put a lot of work into trajectory shaping to avoid orbiting if you want to ballistically re-enter that far downrange.

[Yggdrasill]

My searching has failed me, but back during the discussion of "orbital" vs. "3/4 of the way around the Earth" someone made a statement that anything that goes more than halfway around the globe has to be "orbital". (I'm assuming this claim is only talking about a ballistic segment of the flight.)

I discussed that a bit here: https://forum.nasaspaceflight.com/index.php?topic=52134.msg2238213#msg2238213

Edziebas explanation above is good.

[aero]

Little change of subject, but shouldn't an orbital Starship be a named vehicle? Lindberg flew the "Spirit of Saint Louis," and all of the space shuttles were named, most aircraft are named, all boats and so on.

Maybe this need to be its own topic thread?

[RAN]

Little change of subject, but shouldn't an orbital Starship be a named vehicle? Lindberg flew the "Spirit of Saint Louis," and all of the space shuttles were named, most aircraft are named, all boats and so on.

Maybe this need to be its own topic thread?

I would personally really like them to have names  very Culture-esque. no idea whether it will happen though

[niwax]

Little change of subject, but shouldn't an orbital Starship be a named vehicle? Lindberg flew the "Spirit of Saint Louis," and all of the space shuttles were named, most aircraft are named, all boats and so on.

Maybe this need to be its own topic thread?

I would personally really like them to have names  very Culture-esque. no idea whether it will happen though

In a Culture way I thought they might want to give themselves names and this is literally what happened on the first try:

[Surfdaddy]

With an atmosphere, the difference of "orbital" vs. "suborbital" might be a bit vague. Is there a specific recognized hard cutoff here? For example, you could have an "orbit" where perigee is say 30,000 feet above earth. That would work without an atmosphere, but would still burn up/reenter due to the atmosphere. Whereas if no atmosphere, could orbit just fine with that perigee.

I believe the low point of an Apollo moon orbit was quite low by earth standards, due to no atmosphere to worry about.

[Joffan]

The more important distinction to me is the energy of the trajectory. Given suitable eccentricity, the same energy that could produce orbit can be used for a suborbital trajectory. If this is the case here, I would be happy to call this an orbital demonstration launch. And the same thoughts apply perhaps with even more force to the re-entry - if the energy profile of re-entry matches some possible orbital re-entry ,which it inevitably will do, it's a demonstration of orbital re-entry.

[rakaydos]

Little change of subject, but shouldn't an orbital Starship be a named vehicle? Lindberg flew the "Spirit of Saint Louis," and all of the space shuttles were named, most aircraft are named, all boats and so on.

Maybe this need to be its own topic thread?

I would personally really like them to have names  very Culture-esque. no idea whether it will happen though

Icarus flew too close to the sun and plummeted to earth. Daedelus survived the flight.

Sounds like the first two test ships?

[daedalus1]

Daedalus.

[su27k]

Little change of subject, but shouldn't an orbital Starship be a named vehicle? Lindberg flew the "Spirit of Saint Louis," and all of the space shuttles were named, most aircraft are named, all boats and so on.

Maybe this need to be its own topic thread?

Old threads for this purpose:

ITS (Formerly known as MCT) Names?

BFR/BFS/ITS Naming speculation thread

Providing naming options for individual SpaceX Starships

Since the first few orbital Starships have low chance of surviving the reentry, there's really no need to name them.

[OTV Booster]

Is it safe to leave those raptors on the sea floor? I imagine those would be worth fishing out for some, not necessarily friendly, entity.

It is safe to leave them on the sea bed.
Same goes for the RS-25, the F-1 and even the RD-180. Apart from Jeff Bezos nobody has ever bothered to pick them up from the ocean floor. Not even the Chinese.

Also, why do people always assume that wreckage from engines is enough to reverse-engineer them?
I can tell you that it is not nearly enough to make a working clone.

Sufficient to reverse engineer? No. Necessary? Probably not. Helpful? Yes. How many times has somebody here in NSF, especially the wiser more experienced members, itched to get their hands on an engine to verify a design speculation?


Worthwhile? Can only be decided by those with an interest and the capability.

[OTV Booster]

My searching has failed me, but back during the discussion of "orbital" vs. "3/4 of the way around the Earth" someone made a statement that anything that goes more than halfway around the globe has to be "orbital". (I'm assuming this claim is only talking about a ballistic segment of the flight.)

IANARS, but naïvely this makes some intuitive sense to me—any trajectory that would intersect the Earth/planet/whatever would do so less than halfway around, right? But I didn't see any followup discussion, and I was wondering if that is really the case. Thanks for any elucidation!

If it reenters naturally after 3/4 of circumference, then it's suborbital. If it has to use engines to return (slow down), then it's orbital.

Let me restate my question. My understanding and paraphrasing of the claim is: If a body in free fall travels more than halfway around the globe without impacting, it will always make it all the way around (ignoring drag, etc.). My question is, is that a true statement?

Let me take a stab at this. First, some simplifications. Earth is a perfect sphere, and it has no atmosphere.


The only reasons rockets initially go 'up' is clear the atmosphere. With no atmosphere it is more efficient to launch horizontal which is tangent to the surface.


Let's throw in two more simplification. First, the planet is not rotating. Second, instead of a rocket that adds impulsel over time, let's shoot the gizmo out of a cannon so all the impulse comes all at once. Shoot the gizmo. It hits some distance out. Remember, this is taking place on a sphere, so 'some distance out' is also some distance around the planet. This is suborbital.


Increase the firing charge and it goes further, but still hits. Keep increasing the firing charge and it can go further. It can go half way around, 3/4 around, 99.99999% around. This is all suborbital. On that last shot, the gizmo hits a little ways behind the cannons breach, but it didn't make it all around.


Increase the charge a bit more, move the cannon out of the way after firing so it doesn't get hit, and the gizmo will make it all the way around and pass through where the cannon was. It will keep going and will repeat it's trajectory. It is in orbit.


As a refinement, if the gizmo has a rocket engine, it can fire this at the high point of its orbit. This will be exactly 180deg from the cannon. If the rocket engine fires exactly right, the orbit will go from elliptical to circular with a radius exactly the same as that high point where the engines fired. As a point of interest, this is known as a Hohmann transfer orbit.


Rockets don't get all their impulse at one time and they have atmospheric issues. This muddies up any answer to your question but the above simplifications hopefully get to the heart of it.


Touching on an issue implied by this whole line of questions and putting the atmosphere back into play, the closer the gizmo gets to a full orbit, the closer the reentry velocity will be to orbital reentry velocity.


Warning: IAABAARS (I Am At Best An Amateur Rocket Scientist)

[Greg Hullender]

It can go half way around, 3/4 around, 99.99999% around.

Uh, actually, no. Look at the equation for an orbit: radius = p/(1 + ecc * cos(theta)) where radius is the distance from the center of the Earth, p is a constant (the semi-latus rectum of the orbit, but that's not important), ecc is the eccentricity, and theta is the angle between the cannon and the current position of the shell.

At 180°, the shell is at its apex (the perigee). But because cos(-theta) = cos(theta), the orbit is symmetrical. (As if I needed to prove that.) :-) As a result, if it does not touch the ground in the first 180°, it will not touch the ground at all until it reaches the launch point.

[cdebuhr]

It can go half way around, 3/4 around, 99.99999% around.

Uh, actually, no. Look at the equation for an orbit: radius = p/(1 + ecc * cos(theta)) where radius is the distance from the center of the Earth, p is a constant (the semi-latus rectum of the orbit, but that's not important), ecc is the eccentricity, and theta is the angle between the cannon and the current position of the shell.

At 180°, the shell is at its apex (the perigee). But because cos(-theta) = cos(theta), the orbit is symmetrical. (As if I needed to prove that.) :-) As a result, if it does not touch the ground in the first 180°, it will not touch the ground at all until it reaches the launch point.

This.  To clarify a bit more, using and slightly extending OTVBooster's simplifying assumptions (no atmosphere, no planetary rotation, perfectly smooth and spherically symmetric planetary body, no mascons, no external gravitational perturbations all impulse delivered instantaneously at the moment of launch, and the "vehicle" is a point-like particle ... talk about spherical cows!), suborbital means the periapsis is below the planetary surface, while if periapsis is above the surface, you're orbital.  That's pretty much it.  Note that given the assumptions above, if you fire your projectile exactly tangent to the surface, then the point of launch will be either the periapsis, in which case you're orbital, or apoapsis, in which case you're not and the projectile will auger into the surface immediately.  True tangent launch on a suborbital trajectory that goes anywhere is actually impossible.  If you want to launch a suborbital projectile that goes halfway, 3/4, or even 99.99999% of the way around, you can either fire surface-parallel at non-zero elevation, or fire from the surface with an elevation angle raised amount above the surface-tangent.

[soyuzu]

But because cos(-theta) = cos(theta), the orbit is symmetrical. (As if I needed to prove that.) :-) As a result, if it does not touch the ground in the first 180°, it will not touch the ground at all until it reaches the launch point.

Exactly, because orbit is symmetrical, if it touches the ground at some theta between 90-180°, then it must have travelled another theta above ground BEFORE the point of the cannon. So the angular distance between these two intersections with ground is 2theta, a value greater than 180°

[steveleach]

But because cos(-theta) = cos(theta), the orbit is symmetrical. (As if I needed to prove that.) :-) As a result, if it does not touch the ground in the first 180°, it will not touch the ground at all until it reaches the launch point.

Exactly, because orbit is symmetrical, if it touches the ground at some theta between 90-180°, then it must have travelled another theta above ground BEFORE the point of the cannon. So the angular distance between these two intersections with ground is 2theta, a value greater than 180°

There used to be a clear distinction between orbital & suborbital , a wide gulf between the two. A space launch vehicle was one or the other. But now SpaceX have, as they have with other industry terms like "flight proven", thrown themselves directly into the grey area and forced us to probe concepts that never had a clear boundary. This is what has lead us to strained definitions involving spherical cows in a vacuum.

I suspect this whole discussion is just a sign that we're all bored because not a lot is happening at the moment.

[ppb]

In a Culture way I thought they might want to give themselves names and this is literally what happened on the first try:

Is Nomadd aware of this? That would be an entirely fitting name for SN20.

[cdebuhr]

But because cos(-theta) = cos(theta), the orbit is symmetrical. (As if I needed to prove that.) :-) As a result, if it does not touch the ground in the first 180°, it will not touch the ground at all until it reaches the launch point.

Exactly, because orbit is symmetrical, if it touches the ground at some theta between 90-180°, then it must have travelled another theta above ground BEFORE the point of the cannon. So the angular distance between these two intersections with ground is 2theta, a value greater than 180°

There used to be a clear distinction between orbital & suborbital , a wide gulf between the two. A space launch vehicle was one or the other. But now SpaceX have, as they have with other industry terms like "flight proven", thrown themselves directly into the grey area and forced us to probe concepts that never had a clear boundary. This is what has lead us to strained definitions involving spherical cows in a vacuum.

I suspect this whole discussion is just a sign that we're all bored because not a lot is happening at the moment.

While I suspect that your assessment of what this discussion really signifies is spot on, discussion of the "spherical cow" ideal case isn't totally useless.  Ideal examples sit at the boundaries of what the physics will allow in theory, even in the most absurdly idealized case, and thus form the fundamental foundation upon which we layer the all the obnoxious intrusions of physical reality.  They provide the starting point.  Show me any high school physics text, and I'll show you a tome completely dedicated to the study of spherical cows.  What physics student hasn't studied ideal harmonic oscillators based on mythical ideal springs?

All that said, I'm sure we'll all be much happier when rockets start flying again and we have something real to talk about!

[cwr]

But because cos(-theta) = cos(theta), the orbit is symmetrical. (As if I needed to prove that.) :-) As a result, if it does not touch the ground in the first 180°, it will not touch the ground at all until it reaches the launch point.

Exactly, because orbit is symmetrical, if it touches the ground at some theta between 90-180°, then it must have travelled another theta above ground BEFORE the point of the cannon. So the angular distance between these two intersections with ground is 2theta, a value greater than 180°

There used to be a clear distinction between orbital & suborbital , a wide gulf between the two. A space launch vehicle was one or the other. But now SpaceX have, as they have with other industry terms like "flight proven", thrown themselves directly into the grey area and forced us to probe concepts that never had a clear boundary. This is what has lead us to strained definitions involving spherical cows in a vacuum.

I suspect this whole discussion is just a sign that we're all bored because not a lot is happening at the moment.

It seems to me that none of this discussion about sub-orbital versus orbital is because the participants
haven't read the SpaceX FCC application whose link is at the beginning of this thread.

In the "Flight Profile" section it says:

"The Starship Orbital test flight will originate from Starbase, TX. The Booster stage will separate
approximately 170 seconds into flight. The Booster will then perform a partial return and land in the
Gulf of Mexico approximately 20 miles from the shore. The Orbital Starship will continue on flying
between the Florida Straits. It will achieve orbit until performing a powered, targeted landing
approximately 100km (~62 miles) off the northwest coast of Kauai in a soft ocean landing."

I'm surprised that the Soviet FOBS launches of the 60s wasn't mentioned in the discussion.
The payloads achieved orbit and were de-orbited before completing a full orbit, Just like Gagarin's
Vostok flight.

Carl

[meekGee]

But because cos(-theta) = cos(theta), the orbit is symmetrical. (As if I needed to prove that.) :-) As a result, if it does not touch the ground in the first 180°, it will not touch the ground at all until it reaches the launch point.

Exactly, because orbit is symmetrical, if it touches the ground at some theta between 90-180°, then it must have travelled another theta above ground BEFORE the point of the cannon. So the angular distance between these two intersections with ground is 2theta, a value greater than 180°

There used to be a clear distinction between orbital & suborbital , a wide gulf between the two. A space launch vehicle was one or the other. But now SpaceX have, as they have with other industry terms like "flight proven", thrown themselves directly into the grey area and forced us to probe concepts that never had a clear boundary. This is what has lead us to strained definitions involving spherical cows in a vacuum.

I suspect this whole discussion is just a sign that we're all bored because not a lot is happening at the moment.

I think it's been explored often in ICBM circles, and is the original meaning of "suborbital".

The "Suborbital" that VG and BO/NS are doing is very very very suborbital.  It's what a sounding rocket does.  Calling it "suborbital" is charitable.  I'm saying it because on the grand scheme of Earth, you can just approximate it with a parabola, since the gravity vector doesn't have time to shift.  A true suborbital trajectory is elliptical.

... and, I like the energy definition of suborbital better, since there are orbital-energy trajectories that still intersect the surface of the Earth, and from a vehicle capability point of view, are still orbital.  (e.g. "suborbital around the moon", as folks here often point out) - all that's gone with an energy-based definition.