# Ways to decelerate satellites

1. Oct 12, 2012

### Smileyxx

How can decelerating force would be applied to satellite in order to slow it down?

2. Oct 12, 2012

### PhysicoRaj

By using retro rockets. A small rocket's stream of exhaust fired in the direction of the motion can provide thrust in the opposite direction, slowing it down.
http://www.astronoo.com/images/sondes/curiosityParachuteMotoriseMars.jpg
Consider this. The motion is downwards and the firing is downwards causing upward thrust which slows down the rate of descent.

3. Oct 12, 2012

### Smileyxx

Yes bro i get it now perfectly. thanks!! :)

Last edited by a moderator: Oct 12, 2012
4. Oct 12, 2012

### Drakkith

Staff Emeritus
Typically by chemical rockets using on board fuel. However some are allowed to simply decelerate due to friction with the Earth's atmosphere until they burn up on re-entry.

5. Oct 12, 2012

### Pkruse

I've been in the space and aerospace business for a long time, and I've never heard the term "retro-rockets." Every program I've worked called them TVC or TVCS for "thrust vector control system."

6. Oct 12, 2012

### Drakkith

Staff Emeritus
Welcome back to the world of "popular science". We like our things retro here. Including our rockets.

7. Oct 12, 2012

### Smileyxx

Ohhk i get it now. but i read somewere that if satellite travels too slowly,it will fall down towards the earth surface or if it goes too fast it will move out to higher orbit.it has something to do with formula v^2=GM/r where G is gravitational constant.can you tell me whats the reason behind it?

Last edited by a moderator: Oct 12, 2012
8. Oct 12, 2012

### SHISHKABOB

that formula is for a satellite in a circular orbit, which is a simplification because orbits are actually elliptical

but, what is also says is that the radius of the orbit is *determined* by the velocity of the object (and ofc the mass of what it's orbiting)

so by accelerating and decelerating in the direction of motion, a satellite can change the radius of its orbit.

It's a bit more complicated than that because orbits are elliptical and depending on where you accelerate in the orbit (at the furthest point from the earth or the closest point; apogee or perigee) you will change your apogee or perigee or something like that.

the basic thing is that if a satellite is at a constant velocity, its orbit will stay the same. If its velocity is changing, then the orbit will change.

9. Oct 12, 2012

### Bandersnatch

Think of when you're throwing a rock. It travels in a ballistic trajectory and then hits the ground. Now imagine you throw it harder. Perhaps hard enough to go beyond the horizon(large guns can do that. For example, in WWI the Germans used one to shell Paris from ~120 km away). As it flies beyond the horizon, it lands farther than it would were the Earth a flat surface. In other words, the curvature of the planet and the ballistic trajectory intersect farther away.
You could imagine throwing a rock hard enough that it would never hit the ground. It's ballistic trajectory would never intersect with the Earth's surface. As the gravity would try to pull the rock down, it would still travel so fast, that it'd keep "missing" the Earth. It's ballistic trajectory would turn into an ellipse - an orbit.
In a way, all ballistic trajectories are parts of elliptic orbits that protrude above Earth's surface.
So, a satellite is just an object like that very fast rock. It constantly falls towards Earth, but it's got just enough sideways velocity to keep on missing it.

Now, if you've got a satellite that's already in orbit, and you decelerate it, then it's orbit might shrink enough to graze the upper atmosphere, slowing it down even more and eventually hitting the ground. You took away enough of it's sideways(aka tangential) velocity that allowed it to keep missing the surface.
If you accelerate it, on the other hand, the ellipse of its orbit will grow larger, taking it further away from the surface of the Earth. With enough extra velocity added, it might escape the Earth's gravitational pull completely.

If you're interested in this topic, I'd recommend playing a bit with the free game called Orbiter( http://orbit.medphys.ucl.ac.uk/ ). It's a great way of getting the hang of the orbital mechanics. It comes with a manual describing the basic orbital manuevers and the physics behind them.

10. Oct 12, 2012

### Staff: Mentor

That's odd. The term "thrust vector control system" implies to me something completely different than "retro-rocket" (and even odder to me that you haven't heard it!). Are you sure the term retro-rocket isn't simply an anachronism? A "retro-rocket" would have been a rocket engine with the sole purpose of slowing the spacecraft down for re-entry/landing. In early spacecraft, it would have been an engine dedicated to that purpose. In the space shuttle, though, that task was performed by the multi-purpose OMS engines.

The term "TVCS" though looks to me like a name for a control system, not a name for an engine. Ie, the space shuttle's OMS engines would contain a TVCS. And I don't think the Mercury retro-rockets even used thrust vectoring! Note, I'm not an aerospace professional, but the term "retro-rocket" is so pervasive in early space program media, I have a hard time with what you are saying.

Though I realize wiki can have limitations, I would have expected the article on the subject to list a proper name if one existed: http://en.wikipedia.org/wiki/Retrorocket

Googling finds the term retro-rocket in actual sceintific literature: http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0843181

Last edited: Oct 12, 2012
11. Oct 13, 2012

### PhysicoRaj

Retro rockets are used to slow down the movement at that particular time. The major difference is that the TVCS can vary the thrust for any purpose (i.e., changing acceleration, or velocity or even direction). But a retro rocket is the kind that "mainly" decelerates (it can also have directional deceleration, but it is 'deceleration').

12. Oct 13, 2012

### Smileyxx

. Yeah i get it. Thanks:)

13. Oct 13, 2012

### Smileyxx

I get your point completely and game is quite fun tbh:D

14. Oct 15, 2012

### manojr

You may be interested to know about gravitational slingshot which can alter speed of the satellite. Used mainly to gain speed, but can be used to reduce speed just like Messenger spacecraft on the way to Mercury did Venus flyby to reduce speed.

15. Oct 20, 2012

### BobG

It's an anachronism used to make things perfectly clear to the average person by indicating what direction the rocket is firing relative to motion.

In reality, you could have only one thruster powerful enough to make significant changes to the orbit. When the thruster is facing one way, it's a 'rocket' that increases the size of the orbit. Rotate the spacecraft 180 degrees and fire the same thruster and it's a 'retro-rocket'.

In fact, 'normal' would be to only have one thruster powerful enough to make drastic changes to the orbit (circularize your transfer orbit, for example).

An on-orbit satellite would usually also have several weak thrusters with the most powerful of the weak thrusters making minor adjustments to the orbit and perhaps some even weaker thrusters to control rotation of the satellite. Or one set of thrusters and you just fire them for a short period to control rotation and fire them for long periods to control the orbit. The main point being that most spacecraft have several thrusters since you don't want to rotate sensors on the satellite away from the Earth when adjusting the orbit - hence the 'system' part.

But, following that line of thought, for a geosynchronous satellite, I guess you could refer to the East facing thrusters as retro-rockets. But most people don't. We just call them East facing thrusters. You fire them and the satellite drifts to the West (relative to the surface of the Earth). You fire the West facing thrusters and the satellite drifts to the East.

(And just so we don't appear to be total dweebs, we have sophisticated names for our thruster packages, too, but I'd have to look at our Power Point slides to remember what they are.)

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