Prograde or retrograde?!?!

[JIMMY_the_DOG]

What's up kerbonauts! I had sent an unkerbaled probe to Jool and then enter Jool's SOI! Ok? What's the question you ask? Well, when I begun to do a retrograde (or so I thought) burn, it acted like I was burning prograde (my velocity was increasing) but I looked at my map, it was normally circularizing! Which one is correct? Why? Thanks for your help!

Edited September 26, 2020 by JIMMY_the_DOG
Accidentally hit enter!

[jimmymcgoochie]

It’s still possible to gain orbital velocity when burning retrograde, if your thrust is low and the parent body has a large and/or powerful gravity well. Jool has the biggest gravity well of any planet and a really high orbital velocity, so it’s unsurprising that the rather low acceleration your engine can produce (especially for smaller engines and even more so for ion thrusters) is being outmatched by gravitational acceleration as you fall towards periapsis. The important thing is that your retrograde burn is making you slow down relative to just flying on a ballistic trajectory, which is why you’re capturing into orbit.
Your speed will stop increasing when you get close to periapsis and will start dropping once you’re past it.

You can see the same phenomenon if you try to decelerate a heavy vessel using RCS when you’re about to re-enter on Kerbin; your periapsis will still drop even though your orbital velocity is increasing, because it’s increasing by less than it would with no thrust applied.

[king of nowhere]

you see it also when you aerobrake on returning to kerbin; in the beginning your speed increases even though the atmosphere is slowing you.

specifically, orbiting jool is very fast, over 9000 m/s, and you gain speed very quickly when closing in to periapsis. it's easy to gain speed faster than you are burning.

then again, it's always worth checking that the rocket is pointed correctly.

Edited September 26, 2020 by king of nowhere

[Vanamonde]

Got a screenshot from map view? 

[JIMMY_the_DOG]

Unfortunately no, but I see why I'm accelerating. I was using the atomic rocket motors and I had a decently sized craft... I will supply a screenshot replica (because I reverted the mission because I had no clue what on kerbin was happening.) Thanks for your help!

[jimmymcgoochie]

17 hours ago, JIMMY_the_DOG said:

Unfortunately no, but I see why I'm accelerating. I was using the atomic rocket motors and I had a decently sized craft... I will supply a screenshot replica (because I reverted the mission because I had no clue what on kerbin was happening.) Thanks for your help!

You reverted the mission? From JOOL!? 

Yep, NERVs are on the weedy side for their size and especially mass leading to a low TWR and thus low acceleration, so descending into Jool’s gravity well with a few of those on a biggish ship will lead to outright velocity increasing as you get lower despite the retrograde burn.

[Fierce Wolf]

Isn't this a case of "probe core placed in reverse, that makes you believe you are burning in one direction when it's actually the opposite"? I had my good share of those

[Spricigo]

7 hours ago, Fierce Wolf said:

Isn't this a case of "probe core placed in reverse,

If that was the case the orbit  wouldn't "normally circularize". Another clue was the burn time decreasing (maybe not seen by the OP while panicking).

@jimmymcgoochie nailed it. Acceleration due gravity was  higher than acceleration due engine thrust.

 

 

[Dr. Kerbal]

If your confused on which ones Retograde or Prograde (which It sound like you are) well Retrograde () will slow you down. Prograde will make you faster. 

If that’s not corosponding in the game you may have a bug.

[Spricigo]

54 minutes ago, Dr. Kerbal said:

If your confused on which ones Retograde or Prograde (which It sound like you are) well Retrograde () will slow you down. Prograde will make you faster. 

Moot point, since / is tied to  control point, not necessarily the thrust. Is perfectly normal  to slow down while the control point is pointed prograde (just ask anyone that used a VTOL plane). Let alone when, as was the case for the OP, there are other forces involved.

[JIMMY_the_DOG]

On 9/27/2020 at 2:17 PM, jimmymcgoochie said:

You reverted the mission? From JOOL!? 

Ahhh don't worry... I'm not afraid of wasting a 'lil time!

On 9/27/2020 at 2:17 PM, jimmymcgoochie said:

Yep, NERVs are on the weedy side for their size and especially mass leading to a low TWR and thus low acceleration, so descending into Jool’s gravity well with a few of those on a biggish ship will lead to outright velocity increasing as you get lower despite the retrograde burn.

I think this is the right answer. I might of rolled my eyes when Werner von Kerman began to scream :"ZHE NERVEZ ARE NAT TO BE USED LIKE ZHE GOLIATH" 

On 9/29/2020 at 4:54 PM, Dr. Kerbal said:

If your confused on which ones Retograde or Prograde (which It sound like you are) well Retrograde () will slow you down. Prograde will make you faster. 

I know my basics... I'm really sure i was doing the right thing hence the fact that the map orbit was acting like I was burning prograde, again, I think that @jimmymcgoochie has the right answer. Thanks!