How to get to orbit with only one burn of each stage?

[Budgie]

As I understand it, real-world rockets do not burn until the Ap is at the desired altitude, then circularise the orbit, as we do in KSP. This would require multiple restarts of complex engines that may not be built to be ignited more than once. They continually burn from the ground into orbit, and once in orbit, they use engines capable of multiple restarts to put themselves into the required orbit for their payload.

How is this done in KSP, ensuring that the orbit I inject myself into is clean and circular?

Edited October 6, 2015 by Budgie

[KerBlammo]

you will still need a circularization burn even if you burn all the way to orbit, albeit a smaller one than the "lob it out of the atmosphere and then burn to orbit" approach. The shuttle used its Orbital Maneuvering System to circularize its orbit after launch (OMS-2 burn).

They key to doing a continuous burn to initial orbit is not to over power the first & second stage. If you're seeing any heating effects during launch you're probably going too fast.

[Cairol]

I don't know if I understand you correctly.

In KSP you can design your rocket in a way so each stage has a single purpose. A single burn.

Example:

Stage 0: SRBs for the first couple km and then -> dropped

Stage 1: LFO Motor to lift the rocket to designated AP and then -> dropped

Stage 2: Coast to circularization burn and burn at AP till curcularized, then -> dropped

Stage 3: Transfer Stage for interplanetary missions or return stage for orbital operations around Kerbin

To get a perfectly circular orbit in one go, you need as much impulse as possible at AP.

Technically, you will never be 100% circular in one burn because of this.

And for real world rockets:

I'm pretty sure only the transfer stage of a rocket is ignited more than once. If anything. Normally it's only the return stage for manned missions.

Even the space shuttles build-in main engines (SSME) are only used for the ascent as far as I know.

After that, they use the smaller OMS engines which are capable to do multible burns for circularization and re-entry as well as RCS thrusters for small maneuvers.

[iBeej]

Somebody can correct me if i'm wrong, but I believe SpaceX's Falcon 9 is primarily used to bring the vehicle to a general "target apoapsis" (altitude) and then they stage it and use the 2nd stage to circularize, putting the Dragon on course for a rendezvous with the ISS. So not even that spacecraft does it in one go.

[DeMatt]

...The Space Shuttle did do that - burn main engines and SRBs until AP was in the right altitude and PE was mostly there, coast to AP, then burn OMS engines to finish circularizing.

Anyways, it boils down to proper design and gravity turn.

Relative to Kerbin, KSP's engines are more efficient and powerful than real-life engines are relative to Earth. Because of this, it's easy to stick together a rocket that "mostly" works, and then just brute-force past the "mostly" part. Real-life rockets don't get that option (they cost lotsa money, yo!), so they design them to be "precisely" capable instead of "definitely" capable. Also, the thrust of real-life solid rockets varies over time, and is designed to maintain roughly correct acceleration; KSP solids maintain constant thrust, making them more difficult to design for an entire flight, but easier for the "off the launch pad" part.

Performing an ideal gravity turn is equally important. Essentially, you start your rocket turning from the vertical at the launch pad, it burns continuously (barring staging actions) and turns continuously (due to gravity pulling the nose down), and it reaches the horizontal when it reaches apoapsis and thus performs the last bit of circularization with the last bit of thrust. Us KSP players are generally less than skilled at making this turn, so we take advantage of the lack of cost and build bigger, less efficient, rockets to work around our piloting inadequacies.

[The Aziz]

Well.. unless you are using SRBs, you can control your throttle, right? (Yah I know that it's not how it works IRL but..) Then when you are close to your desired Ap, set throttle to a very minimum, then set in on 100% to circularize. I guess..

[Budgie]

...The Space Shuttle did do that - burn main engines and SRBs until AP was in the right altitude and PE was mostly there, coast to AP, then burn OMS engines to finish circularizing.

Anyways, it boils down to proper design and gravity turn.

Relative to Kerbin, KSP's engines are more efficient and powerful than real-life engines are relative to Earth. Because of this, it's easy to stick together a rocket that "mostly" works, and then just brute-force past the "mostly" part. Real-life rockets don't get that option (they cost lotsa money, yo!), so they design them to be "precisely" capable instead of "definitely" capable. Also, the thrust of real-life solid rockets varies over time, and is designed to maintain roughly correct acceleration; KSP solids maintain constant thrust, making them more difficult to design for an entire flight, but easier for the "off the launch pad" part.

Performing an ideal gravity turn is equally important. Essentially, you start your rocket turning from the vertical at the launch pad, it burns continuously (barring staging actions) and turns continuously (due to gravity pulling the nose down), and it reaches the horizontal when it reaches apoapsis and thus performs the last bit of circularization with the last bit of thrust. Us KSP players are generally less than skilled at making this turn, so we take advantage of the lack of cost and build bigger, less efficient, rockets to work around our piloting inadequacies.

Thanks all! This is all valuable knowledge. Marked as "Answered". I've played a bit of RO and saw that the upper stage engines are efficient, but have a TWR < 1 sometimes. Perhaps this would explain my assumption.

I'd argue, for the point of arguing, that the Space Shuttle was a very unique rocket and shouldn't be pointed at to show what's normal.

[Racescort666]

Well.. unless you are using SRBs, you can control your throttle, right? (Yah I know that it's not how it works IRL but..) Then when you are close to your desired Ap, set throttle to a very minimum, then set in on 100% to circularize. I guess..

To some degree yes, real rockets can throttle. SSMEs could throttle to like 65% which is pretty good for a real life rocket engine but there are many engines that can't throttle that much if at all. The first stage of the Saturn V would shut down the center engine during ascent to reduce the thrust which I understand is a pretty common practice.

Side note, the gif in your signature is awesome.

[MarvinKitFox]

Somebody can correct me if i'm wrong, but I believe SpaceX's Falcon 9 is primarily used to bring the vehicle to a general "target apoapsis" (altitude) and then they stage it and use the 2nd stage to circularize, putting the Dragon on course for a rendezvous with the ISS. So not even that spacecraft does it in one go.

Sort of.

The first stage just lobs the craft to about 50-80km, and about 3km/s. Basically, it gets the second stage out of significant atmosphere, where it can use its more efficient vacuum engine and several minutes to burn to orbital speed

The second stage provides about 85% of the orbital energy.

At staging, the falcon9 is still below 100km altitude, sometimes much lower. But its Apogee is at least 160km, sometimes 400+ (the falcon9 uses rather extreme variety of launch profiles, depending on intended payload orbit, and whether they want to do a boost-back/recovery or not)

[Laie]

Thanks all! This is all valuable knowledge. Marked as "Answered". I've played a bit of RO and saw that the upper stage engines are efficient, but have a TWR < 1 sometimes. Perhaps this would explain my assumption.

Generally speaking, there's two ways to do it:

a) launch to high apoapsis, then have a dedicated stage to cirularise there (not only the space shuttle does it like that)

control time-to-apoapsis by pitch (more common in real life, but harder to do in KSP)

Actually, most vessels follow a combined a)& approach, where the second stage tosses the vessel to a high apoapsis and the third stage circularises, only that the apoapsis is so close and the last stage's TWR is so low that the circularisation burn has to start right away. Also, the eventual altitude may be considerably lower than apoapsis.

If you play RO, I'd like to suggest you try this vessel. Not because it's good (it's a half-assed attempt to replicate Atlas behaviour with "stock" RP-0 parts) but because it comes with instructions (click the "rename vessel" field to see them) and because it gives a good illustration of a realistic launch profile.

[iBeej]

Sort of.

The first stage just lobs the craft to about 50-80km, and about 3km/s.

Wait, that doesn't sound right, unless i'm misunderstanding. The SpaceX site shows the Falcon 9 reaching speeds of Mach 2 at MAX-Q, 78 seconds in to the flight. That roughly translates to 1,534mph or 686m/s. There is no way there are going 3km/s, that's Mach 8.7 lol

Were you meaning total ÃŽâ€v?

Edited October 6, 2015 by iBeej
clarification

[GoSlash27]

To circularize in a single burn without throttling, you pitch to add radial-in once you've established apoapsis. It's a tricky maneuver to execute manually, and it helps if your upper stage is weak.

Best,

-Slashy

[NathanKell]

iBeej: Max Q is far from first-stage burnout.

If you want to fly direct ascents in KSP, you have to unlearn the KSP feitshization of ~moar boosters~. You'll need a first stage that will leave you at about 1km/sec and an apoapsis of maybe 55km, and then a low-TWR second stage if ~km/sec that takes about a minute longer to burn out than your time-to-apoapsis at ignition (maybe only 30s longer, it's been a while).

If you burn that stage purely horizontally, you should hit orbital velocity just as you hit your new perigee of 70km. You can then perform a small (~10m/s) circularization burn half a world away at your desired apogee.

Note that the Shuttle's insertion burn was only a few dozen m/s, tops--it basically achieved orbit on the SSMEs, and tuned that orbit with OMS, you can't really call OMS a third stage in any traditional sense, any more than you can call Soyuz's Service Module a third stage.

[Alshain]

When I launch I hit 75km Ap about the same time I hit 25km Pe. I could do better than that, but I like to drop my last stage so the game will clear it. The trick is to do your gravity turn at the right moment and gradually so you end up going no more than 5degrees on the NavBall and then throttle back and keep your time to apoapsis hovering at about 1 minute till near the end of the burn.

[Budgie]

iBeej: Max Q is far from first-stage burnout.

If you want to fly direct ascents in KSP, you have to unlearn the KSP feitshization of ~moar boosters~. You'll need a first stage that will leave you at about 1km/sec and an apoapsis of maybe 55km, and then a low-TWR second stage if ~km/sec that takes about a minute longer to burn out than your time-to-apoapsis at ignition (maybe only 30s longer, it's been a while).

If you burn that stage purely horizontally, you should hit orbital velocity just as you hit your new perigee of 70km. You can then perform a small (~10m/s) circularization burn half a world away at your desired apogee.

Note that the Shuttle's insertion burn was only a few dozen m/s, tops--it basically achieved orbit on the SSMEs, and tuned that orbit with OMS, you can't really call OMS a third stage in any traditional sense, any more than you can call Soyuz's Service Module a third stage.

Thanks, NK! This is exactly what I was looking for. Makes sense it'd come from a proponent of RO

[DancesWithSquirrels]

TheBigSwede has a video showing an unguided rocket to orbit with continuous burns, see here.

[Gaarst]

I personally find it a lot easier to orbit in a single burn in RSS than in stock Kerbin. I usually stop at a 90x150km orbit (atmosphere is 130km high in RSS) because I like to let my booster stages fall back to Earth instead of leaving space junk, but if the final stage has already been dropped, I circularise in one burn, and end up with a roughly circular orbit around 150-180km.

I think that this is so because you need really high orbital velocities in RSS (7.7km/s), and therefore spend much more time burning horizontally. The thing is, if you built your launcher correctly, most of this horizontal burn occurs close to your final orbit height (0° pitch at around 100km for me) and finishes when you are outside atmosphere. That way, you spend a lot of time with precise control of both your apo height and time to apo, while being really close to it during the whole process (between 1 and 2 min, with an orbital period of 90min): when you finish your burn, you are close to your apoapsis height, and therefore can rise your periapsis to above the ground before the apoapsis starts to drift off in front of you. Then, point a little towards the ground, and you can circularise in one burn.

If you do it well, you can do everything at 100% throttle (real engines don't throttle down a lot), though it is a lot less precise.

In stock KSP, as the speeds are lower, you often end up reaching orbital speed way before being at your orbit height, and therefore it is less easy to circularise in one burn, but still very doable with some practice.

[KerikBalm]

In real life, the ratio of total dV needed vs gravity, is much higher.

If we had to accelerate to ~8km/sec to attain orbit, it would be easier to get it in one go, since you'll be burning nearly all the way to Apoapsis.

Here if you burn all the way to apoapsis, you've way overshot.

And you need a very powerful engine to fight against 1g, but then it would need to be throttled way back to have torbital velocity attained as you're nearly at Ap.

A solution that works in game is a powerful first stage to send your apoapsis high, and then a weak 2nd stage so you spend the whole time to apoapsis burning horizontally.

[Van Disaster]

Low TWR upper stage should do it if you don't give too massive a bump with lower stages. The trick is mostly in getting the staging right - you could practice with a spaceplane with a bunch of different engines to see how it works, use nukes & burn prograde ( or maybe even straight at the horizon ) when your AP is in the 50km-ish range & see if you can arrive at a useful AP with a PE as close as possible.