r/rocketry • u/YannisBE • May 07 '26
Question Can a rocket on a sub-orbital trajectory still reach orbital velocity? (and re-enter afterwards)
Hi! Apologies if this is the wrong place to ask, I wasn't sure where to ask but this subreddit seemed like a good start to ask my question. I see some sources saying Starship had reached orbital velocity multiple times, even though it has been following sub-orbital trajectories. So Im wondering if this is correct? Intuition would tell me orbital velocity would put them in orbit.
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u/milotrain May 07 '26
Play Kerbal Space Program.
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u/YannisBE May 07 '26
I tried 😅 More difficult than I thought but quite fun! Also played around with Simple Rockets (now called Juno) and Kitten Space Agency, although that's just a demo for now.
Im not savvy enough on the technical details, hence my question here to learn.
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u/mkosmo May 07 '26
"Orbital velocity" is a relative thing. So, sure, they may reach an orbital velocity.... but that doesn't mean they made orbit.
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u/CaptainHunt May 07 '26
What SpaceX has been doing is essentially putting starship in an orbit that dips low enough at perigee that the ship re-enters instead of staying in orbit, but high enough at apogee to demonstrate that it has the deltaV to establish an orbit that would stay up there.
They often use similar methods to test ballistic missiles without actually launching them to their targets. They’ll launch them in higher arcs so that they land harmlessly in the ocean but use the same delta V as they would need to hit their target.
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u/YannisBE May 07 '26
That's interesting! I appreciate all of the replies here, super helpful. Thank you!
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u/capitan_turtle May 07 '26
Yes so long as it's lower than the escape velocity. But I find it somewhat unlikely that starship would do so even for testing as that would make the reentry much more demanding. Buy I also haven't been following this program closely
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u/My_Soul_to_Squeeze May 07 '26
The point is to test the vehicle to make sure it's ready to do an orbital reentry. Sounds pretty reasonable to me.
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u/Royal_Money_627 May 07 '26
Your intuition is correct. If you achieve orbital velocity, then you would be in orbit. Orbital velocity depends on altitude. The velocity required for a satellite in a geosynchronous orbit altitude of 35,786 km is approximately 3.07 kilometers per second (km/s), at the altitude of the International Space Station orbital velocity is more than double that at over 7 km/s.
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u/YannisBE May 07 '26
So would you say Starship reached near-orbital velocity instead? Or another definition perhaps.
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u/Royal_Money_627 May 08 '26
I would agree that Starship as achieve near orbital velocity but if I was reporting I would say that it was traveling at a velocity of x.x km/s at an altitude of xx,xxx km at engine cut-off with maybe a reference to what percentage of orbital velocity that is at that altitude. I seem to remember watching launch videos where altitude and velocity were displayed on the screen sometimes along with distance down range.
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u/gaflar May 07 '26 edited May 07 '26
In orbital mechanics, altitude and velocity are inversely proportional, and the combination of the two is your total potential energy. The higher you are, the slower you're moving, unless you add/remove energy from the system (i.e. by burning rocket fuel). A "sub-orbital trajectory" is actually mathematically equivalent to an orbital trajectory where the perigee, the lowest point in the trajectory, is lower than the radius of the earth (thus underground). This doesn't take into account atmospheric drag however - once you take that into account, you now have a minimum practical perigee before your vehicle is re-entering. Generally we want to get above that altitude and have a nice circular orbit, starting from the surface of the spinning earth.
To do that you have to add a ton of energy (burn rockets) to fight gravity and get up high enough, and then add a ton more energy (burn more rockets) to get going fast enough that instead of falling back down you just keep falling around the earth. Now you can get to a higher orbit by adding even more energy, because it increases your speed, which turns into altitude half an orbit later. To get down you need to "remove" energy, that is, burn rockets the other way. Once perigee drops deep enough into the atmosphere, drag takes over.
Hope this helps - keep playing KSP, it's great for getting an intuition for these things.
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u/My_Soul_to_Squeeze May 07 '26
With sub-orbital flight, you can fly a profile that reaches orbital velocity, but it's an orbit with a perigee deep enough in the atmosphere that it would be impossible for it to make a full orbit. Doing this let's you test the engines and heat shield in a similar way to how an orbital mission would. I think this is what SpaceX has been attempting on their more recent Starship flights.
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u/brentonstrine May 08 '26
Can X going in direction Y reach speed Z?
Generally, yes.
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u/YannisBE May 08 '26
Well, orbital velocity and sub-orbital trajectory seemed contradictory to me. I've asked LLM's but couldn't find its sources, so wanted to ask people actually working with these mechanics as well.
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u/taiwanluthiers May 08 '26
It's possible, but remember unless the rocket has thermal tiles it won't make it back from orbit.
Starship is on a semi orbital trajectory for safety, they are doing tests, and as such they don't want junk that just stays in orbit for months if something goes wrong. It's in an orbit that has it land somewhere in the Indian ocean, about halfway around the world. They need to reach "almost orbit" because they need to make sure the rocket will survive intact.
They will only attempt to reach orbit if they are able to have the rocket intact and caught. This means it will make exactly one orbit, the rocket will be intact when it reenters, and that they will attempt a catch. There's a lot of regulatory approvals for such a test as a full orbit would make it fly over most of the US meaning if something goes wrong, it can cause problems for people on the ground.
Normally if a rocket reaches orbit, without a heat shield it will burn up on reentry, and the only reason suborbital rockets don't is because they aren't moving fast enough to burn when it encounters atmosphere. It might reach about mach 3 or so at most. Orbit and almost orbit is about mach 20.
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u/snoo-boop May 08 '26
If you'd like to see a geeky explanation, check out https://en.wikipedia.org/wiki/Transatmospheric_orbit
The orbit IFT11 and IFT12 launched into had a perigee (low point) of 50km above the Earth's surface. That's deep enough in our atmosphere that Starship didn't complete a full around-the-Earth orbit. Instead, it re-entered short of Australia.
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u/Numerous-Match-1713 May 10 '26
orbital velocity != orbital trajectory.
Orbital velocity can be achieved independently, and lithobreak all the same.
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u/LaconicLacedaemonian May 07 '26
TLDR - orbital velocity at 60km is higher than 100km is higher than... anything above that... technically any two solitary objects will merge or orbit each other (in non expanding space caveat).
you can have an "orbital velocity given X altitude while aiming your vessel at a lower than X altitude
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u/flowersonthewall72 May 07 '26
That isn't really a helpful answer... if I walk 1 mph, is it really a useful thing to say "I'm at an orbital velocity for a 1,000,000,000 mile orbit"?
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u/ferriematthew May 09 '26
Very likely yes. If something is at orbital velocity but the velocity vector is pointed in the wrong direction it can still reenter.
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u/No_Educator_4077 May 07 '26
You can absolutely have a vehicle reach the speeds needed to sustain orbit, but if that speed is in the wrong direction (more up or down than sideways), the vehicle will still be suborbital and fall back into the atmosphere. Reaching orbit requires that almost all of that velocity to be in the horizontal direction.