Single Burn to Orbit?

[Rdivine]

Quick question here. Why are most rocket launches able to perform a single burn and reach a circular, low earth orbit? In KSP, whenever we try to launch a rocket, the apoapsis is always way ahead of the spacecraft, and we have to cut the engines off so that the apoapsis doesn't increase any further. However, launches such as the space shuttle, orbcomm-2, and most launches only perform a single burn, and do not reignite their second stage to circularize their orbit. Why is it so?

 

I also realized that some rocket launches such as Jason-3 reignite their second stages, but why not just perform a single burn to orbit like other rockets?

[Guest]

Because TWR, I think.

When I started playing KSP 64K, my rockets had to be bigger than they would have to be in stock KSP in order to have more delta-V. I think that the decreased thrust-to-weight ratio might also influence the burn-to-orbit aspect, but maybe it's also because the planet was bigger.

[Spaceception]

2 minutes ago, Rdivine said:

Quick question here. Why are most rocket launches able to perform a single burn and reach a circular, low earth orbit? In KSP, whenever we try to launch a rocket, the apoapsis is always way ahead of the spacecraft, and we have to cut the engines off so that the apoapsis doesn't increase any further. However, launches such as the space shuttle, orbcomm-2, and most launches only perform a single burn, and do not reignite their second stage to circularize their orbit. Why is it so?

 

I also realized that some rocket launches such as Jason-3 reignite their second stages, but why not just perform a single burn to orbit like other rockets?

I'm not 100% Sure, so don't quote me, but I think they have really efficient gravity turns, precise engine burns, and are burning below the horizon (Sometimes), so they can go into orbit almost immediately after getting into space, I haven't done that in KSP, but I have made really shallow gravity turns, and have a really long arc, where I go really far, I don't go into orbit (No fuel), but maybe that's how they do it.

[Bill Phil]

They generally burn for quite some time, going PAST the apoapsis sometimes, while still burning. The upper stages rely on the momentum they received from the first stage to fling themselves onto the proper trajectory, and then burn from separation to insertion, but generally at a very low TWR.

[Nothalogh]

Proper throttle management for the gravity turn

[magnemoe]

4 hours ago, Aperture Science said:

Because TWR, I think.

When I started playing KSP 64K, my rockets had to be bigger than they would have to be in stock KSP in order to have more delta-V. I think that the decreased thrust-to-weight ratio might also influence the burn-to-orbit aspect, but maybe it's also because the planet was bigger.

This, I sometimes only get an tiny burn to circulate if the TWR of upper stage is low, typical then using the LV-N of interplanetary ships as upper stage. 
Think I have been down to an 30 m/s adjustment burn. 

On the other hand if you have an high twr on upper stage you might end up with 1 km/s circulation with mechjeb.

[rudi1291]

Single burns to LEO/LKO are possible with proper gravity turns and throttling the engine down. KSP provides time to apoapsis, that needs to reach 0 at the time you make orbit. (if you dont care about efficency, you can also burn 'below the horizon')

However, if the orbit is to high (GTO for example) an extra circularization burn is more efficent, because the spacecraft has to travel further until it reaches its final altitude.

[Kaos]

My KSP rockets also burn often continuously until the orbit is circular. Especially when the upper stage has a low TWR for full reusable interplanetary missions.

[winged]

5 hours ago, Aperture Science said:

Because TWR, I think.

Because of TWR compared to velocity needed to make orbit. In real life you need >9000 m/s of delta V to reach LEO and with the same TWR or even bigger (because most of the rockets in real world have much bigger max TWR than in stock KSP) you simply don't have a time to wait for Ap (unless your launch profile is very inefficient)

Another thing: look at Earth with 6000km radius and around 200 km LEO altitude. Now look at Kerbin with 600km radius and 70km LKO altitude - quite high compared to planet size, isn't it? Two different planets with two different optimal launch profiles.

I'm RSS player and I'm always doing continuous burn in order to circularize orbit, but in stock KSP I simply can't - unless I'm doing ridiculously shallow burn (which is dangerous). Kerbin it's just a different planet with different optimal launch profile.

 

EDIT: I've forgotten about the engines. They're not infinitely restartable- in fact most of them have only one ignition so single burn is much more desirable.

Edited January 30, 2016 by winged

[GoSlash27]

Doing a single burn to orbit is fairly easy, even in KSP. You just manage the apoapsis with radial- in instead of throttle.

 They prefer single burns IRL because it's more reliable (ullage and possible ignition failure).

Best,

-Slashy

[Nuke]

ive launched into a direct circular orbit before. is usually because my rocket is underpowered and i almost dont go to space, but manage to squeeze in anyway. usually i launch with such an op first stage that its hard not to over shoot your apo target.

[InsaneDruid]

Its harder in KSP because our orbital velicity is rather tiny while athmosphere is rather large compared to real world proportions. Real rockets spent much longer just pushing nearly sideways without the excessive vertical velocity resulting from the early parts of the trajectory after coming out of the denser athmosphere.

In ksp, either you take a rather shallow trajectory so that your vertical speed isnt that much after you reached around >40000m or you can also just dip the nose under the horizon and kill the vertical speed this way, after you are out of the denser atmo.

Currently i experiment whith scaling my proton down in performance and if i do this by decreasing the ISP of the engines to match a burn time of around 0.64 of the original rocket (thrust is original*0.64^3) rather than increasing the weight of the engines, then she nearly does the single burn to orbit automatically. Just a sliight nose-down here and then to adjust the time to APO and booom.

[Hcube]

It's also in part because upper stages' TWR is anemic compared to the stages we are used to make in KSP. Upper stages rely a lot on first stages IRL, and burnes for a very long time whereas in KSP it will usually have a relatively high TWR.  In fact it's totally possible to do a 1burn perfect circular orbit in KSP if you throttle down a lot and stay 5-10 seconds away from apoapsis during the final part of the ascent. 

It's pretty much a combination of the differences in Isp, planet size, rocket design and TWR ^^

[cantab]

I've done plenty of KSP launches where I burn all the way. It comes from having a sufficiently low TWR upper stage.

[Mad Rocket Scientist]

Because real life is different from KSP.  Here's a couple of things that influence single burn or immediate start up of second stage to orbit:

Most engines only have 1 start, especially on the first stage, this means that you're wasting fuel to shutdown it early.

Second stages have very low TWR, often less than 1.  This means that you must burn as early as possible so you don't pass ap.

 

[Delta_8930]

Well, in KSP the only practical application of this is a highly elliptical transfer orbit.

[magnemoe]

49 minutes ago, Delta_8930 said:

Well, in KSP the only practical application of this is a highly elliptical transfer orbit.

No, I and many other has done launches who only require small adjustments to circulate. 

First you need far larger speeds in real world this require far longer burn times. Back before 1.0 I did som Jool missions, escape back to orbit with either giant asparagus or balloon then rockets. 
The upper atmosphere part is more like an rocket launch from Earth. All was single burn to orbit even without planning for it.

I commonly use the interplanetary LV-N stage as upper stage to do the end of gravity turn, because of the low TWR the circulation burn is 50 m/s or less.

During the stock Jool mission I had to launch a lot of fuel tankers to top up the giant Jool accent rocket, I was tweaking then to get more fuel up and ended with 20 m/s for circulation. 
In practice I adjusted Pe for intercepting the huge ship. 
In real world they do far more calculations making it easy to set up single burns. 

[wumpus]

Note that this doesn't apply at all to the shuttle.  The shuttle ditches the tank at 114km (ISS for example is about 400km up).  Circularization burns are done by completely different engines (the OMS engines).  This isn't much of a burn, but it is needed.

On 1/30/2016 at 11:21 PM, Nothalogh said:

Proper throttle management for the gravity turn

Not too likely.  I don't think many of these ships have throttles to manage (the shuttle could emulate such things by pre-packing the SRBs to burn at different rates).

[worir4]

On 1/30/2016 at 4:21 AM, Nothalogh said:

Proper throttle management for the gravity turn

I thought that most lower stage rockets don't have throttleable engines. 

Less re-starts of engines = less likely to fail ignition. 

Edited February 1, 2016 by worir4
I meant lower stage (thanks @magnemoe)

[magnemoe]

31 minutes ago, worir4 said:

I thought that most rockets don't have throttleable engines. 

Less re-starts of engines = less likely to fail ignition. 

Think most have some throttle if nothing else to manage imbalance its however narrow Falcon 9 70% is seen as high. Upper stage is usually restartable, you need it for multiple payloads and for higher orbits 
Lower stages are not as it has no purpose 

[Mad Rocket Scientist]

2 hours ago, wumpus said:

Note that this doesn't apply at all to the shuttle.  The shuttle ditches the tank at 114km (ISS for example is about 400km up).  Circularization burns are done by completely different engines (the OMS engines).  This isn't much of a burn, but it is needed.

Not too likely.  I don't think many of these ships have throttles to manage (the shuttle could emulate such things by pre-packing the SRBs to burn at different rates).

I think the SSMEs could actually throttle to something like 65-70%

[Nothalogh]

3 hours ago, wumpus said:

Note that this doesn't apply at all to the shuttle.  The shuttle ditches the tank at 114km (ISS for example is about 400km up).  Circularization burns are done by completely different engines (the OMS engines).  This isn't much of a burn, but it is needed.

Not too likely.  I don't think many of these ships have throttles to manage (the shuttle could emulate such things by pre-packing the SRBs to burn at different rates).

 

3 hours ago, worir4 said:

I thought that most lower stage rockets don't have throttleable engines. 

Less re-starts of engines = less likely to fail ignition. 

Wait, what?

Go watch some launch video with commentary, they always throttle back as they approach max Q, and also as they approach stage burnout to manage TWR

[RuBisCO]

Well I have done it before, it is a matter of low TW upperstage, and a very well designed launch profile. You definitely get closer to experiencing this if you play X6.4 or X10.

[fredinno]

5 hours ago, Delta_8930 said:

Well, in KSP the only practical application of this is a highly elliptical transfer orbit.

Or, if you have a really bad upper stage with a low TWR.

48 minutes ago, Nothalogh said:

 

Wait, what?

Go watch some launch video with commentary, they always throttle back as they approach max Q, and also as they approach stage burnout to manage TWR

Not all of them do, though. Most nowadays probably do to simplify aerodynamics.

[Rybo]

I read through some of the comments but not all so I'm not sure if someone has mentioned this or not. Another important factor to consider is atmospheric behavior. In Kerbin the atmospheric pressure density is more-or-less linear from launch until about 60km. It's also completely independent of temperature and AFAIK doesn't simulate winds aloft. 

IRL that is NOT the case and those play a major factor in the single-burn profiles for launches, especially the Atlas V's that I'm most familiar with. Because of Earth atmospherics, every single launch profile is unique with respect to roll program execution, throttle settings throughout the burn, alpha biased and subsequent closed loop steering windows, MECO and sep timing, etc etc. It's pretty much a necessity to constantly burn and constantly make in flight vector corrections to stay centered on the profile.