Why cant the Falcon Heavy fly in a straight line to orbit?

[oiorionsbelt]

This is probably a nob question but, I've always wondered why rockets such as the Shuttle performed the early roll manouver shortly after launch. I noticed the Ares V simulations have the same manouver. Can this not be achieved by orienting the vehicle on the pad?

[Joffan]

I think we've beaten this question almost to death, but I'll just add a link to Jon Goff's assessment of "pop-up" first stages, where the first stage goes straight up and the second stage does all the orbital velocity work.

[Jim]

This is probably a nob question but, I've always wondered why rockets such as the Shuttle performed the early roll manouver shortly after launch. I noticed the Ares V simulations have the same manouver. Can this not be achieved by orienting the vehicle on the pad?

The pads were built for the Saturn V.  Use of the VAB and pad dictated a certain orientation which was not the same as the Saturn V's and hence the need for the roll.

[Jim]

Even if they did, most people simply don't care about things like that when they are in high school, if they don't want to study physics or something.

Orbital Mechanics are beyond what is done in high school.  I had calculus and analytical geometry in high school (but my physics course only used algebra.  We touched on it a little but my courses were the most advanced ones and there were only 30 of us in it out of a class of 600.

[veedriver22]

 Here is the simple explanation.  To be in orbit you need 2 things.   #1 is speed. You need speed and lots of it,  like 17000 MPH.  #2 is Direction.  You must be travelling in a direction that is parallel to the earth surface.

 If you go straight up you can build lots of speed, but your momentum will want to keep going straight up.  You can't just take a right turn.   That's even true with a car.   If you are going very fast and try to make a sharp turn you will spin and the car will pretty much keep going in the direction you were going until the friction of the tires stops you.

 Once you get up to the orbit altitude you can then make a gradual turn to achieve orbit, but to do that you need lots more fuel.  The rocket wants to keep going straight and it takes fuel to change the momentum.  Fuel is cheap, but the problem is its heavy.
 

[rpapo]

Orbital Mechanics are beyond what is done in high school.  I had calculus and analytical geometry in high school (but my physics course only used algebra.  We touched on it a little but my courses were the most advanced ones and there were only 30 of us in it out of a class of 600.

My experience in high school was the same.  If you intended to go on to college, you took the toughest courses available, period.  Most of the others in the high school didn't care to work that hard.

I learned some orbital calculations on my own in college, and bought computer time to play with it.

[strangequark]

... Circular orbits have constant velocity.

I'm sure you meant to say "Circular orbits have constant speed"   

No, he's clearly using a spherical coordinate system.

[Owlon]

Orbital Mechanics are beyond what is done in high school.  I had calculus and analytical geometry in high school (but my physics course only used algebra.  We touched on it a little but my courses were the most advanced ones and there were only 30 of us in it out of a class of 600.

My experience in high school was the same.  If you intended to go on to college, you took the toughest courses available, period.  Most of the others in the high school didn't care to work that hard.

I learned some orbital calculations on my own in college, and bought computer time to play with it.

I think most states in the US require some sort of physics course, and most of those cover the basic concept of orbit using algebra [F=GmM/r^2, v=(GM/R)^(1/2)] --but not much of anything beyond that. Whether most people understand or remember that bit is another question entirely.

[rpapo]

Orbital Mechanics are beyond what is done in high school.  I had calculus and analytical geometry in high school (but my physics course only used algebra.  We touched on it a little but my courses were the most advanced ones and there were only 30 of us in it out of a class of 600.

My experience in high school was the same.  If you intended to go on to college, you took the toughest courses available, period.  Most of the others in the high school didn't care to work that hard.

I learned some orbital calculations on my own in college, and bought computer time to play with it.

I think most states in the US require some sort of physics course, and most of those cover the basic concept of orbit using algebra [F=GmM/r^2, v=(GM/R)^(1/2)] --but not much of anything beyond that. Whether most people understand or remember that bit is another question entirely.

That formula was the basis of the program I was playing with...

[Soralin]

I like newton's cannon for a simple understanding of orbits:

http://en.wikipedia.org/wiki/Newton%27s_cannonball


If you fire a cannonball out of a cannon, it will move to the side, and curve toward the ground, due to gravity.  Get it moving fast enough, and it will be moving so fast to the side, that by the time it would have hit the ground, the ground has curved away from under it, due to the curvature of the Earth, and it misses it.  Since the ground is now in a different direction, it gets pulled in that direction, and again misses hitting it due to the curvature of the Earth, etc.

That's basically just what an orbit is: Falling, but moving so fast to the side that you miss hitting the planet.  The only reason for the height, is to get out of the atmosphere, so that you don't have to deal with air resistance slowing things down.  If you don't have any air, and there aren't any mountains or such in the way, you could get something into an orbit arbitrarily close to the ground.

There is an art, it says, or rather, a knack to flying. The knack lies in learning how to throw yourself at the ground and miss.

[Nindalf]

The only reason for the height, is to get out of the atmosphere, so that you don't have to deal with air resistance slowing things down.  If you don't have any air, and there aren't any mountains or such in the way, you could get something into an orbit arbitrarily close to the ground.

This is correct in principle, but not actually true.  If you get too low around an airless world, you encounter gravitational variations as you travel over it, due to it not being a perfectly uniform sphere.  You wouldn't notice them standing on the surface, but they're enough to make your orbit unstable.

This is a real practical consideration in low lunar orbit, as seen in the example of LADEE:
http://www.gizmag.com/ladee-moon-probe-impact-nasa/31708/

[SVBarnard]

So the first stage is going to literally fly hundreds of miles back to shore? There is no way in hell it has enough fuel to travel that much distance! I am totally confused! There is no way in hell they're ever gonna land back on shore it'll have to be a barge.

Surely I'm mistaken?

[SVBarnard]

Not a big deal, plenty of people ready to educate a new guy in these forums

True.

I also HAD a girlfriend asking me in the middle of some space-related conversation (or monologue as I am usually the only space-geek in the group) that "But how can they drive rovers on Mars because there's no gravity there, because space is weightless place?".

So, no need to tell, soon after this we just had to break up. I mean, really, we had to.

How could someone be so dense? lol i laughed so hard!

[Ludus]

Seriously, read this: https://what-if.xkcd.com/58/

The OP started with an older Elon Musk tweet, this link was a more recent Musk retweet that's precisely the answer to the difference between going into space and going into orbit the OP is asking about.

What Virgin Galactic is promising to do is take passengers straight up into space, which is a heck of a lot easier than taking them into orbit.

[rockettrey]

I love this topic and have also recently posed a related question on it elsewhere in the vast realm that is NSF.   It has also bugged me for years.  Thanks to all the experts out there helping out us orbital mechanics-deprived individuals!   

We all know boost back reduces the amount of payload to orbit. Gravity losses associated with just going vertical also have associated payload reduction, not to mention all the work the 2nd stage now has to do to build up velocity.  Which loss type is greater- gravity v.s. boost back? 

The reason I ask is this- instead of going vertical to whatever orbital altitude is desired (for easy RTLS) and then work on the necessary orbital velocity (which we all now know is really inefficient), why not go vertical to say 500% of the desired orbital altitude (wild guess to make my point), separate the booster stages (or first stage in F9R), and let GRAVITY (along with 2nd stage) help to achieve the necessary orbital velocity?  The boosters/first stage could then RTLS with minimal corrections due to Earths rotation and the important parts would have a nice long downhill ride to pick up speed.

[Burninate]

I love this topic and have also recently posed a related question on it elsewhere in the vast realm that is NSF.   It has also bugged me for years.  Thanks to all the experts out there helping out us orbital mechanics-deprived individuals!   

We all know boost back reduces the amount of payload to orbit. Gravity losses associated with just going vertical also have associated payload reduction, not to mention all the work the 2nd stage now has to do to build up velocity.  Which loss type is greater- gravity v.s. boost back? 

The reason I ask is this- instead of going vertical to whatever orbital altitude is desired (for easy RTLS) and then work on the necessary orbital velocity (which we all now know is really inefficient), why not go vertical to say 500% of the desired orbital altitude (wild guess to make my point), separate the booster stages (or first stage in F9R), and let GRAVITY (along with 2nd stage) help to achieve the necessary orbital velocity?  The boosters/first stage could then RTLS with minimal corrections due to Earths rotation and the important parts would have a nice long downhill ride to pick up speed.

Because velocity is a vector (represented by three quantities, possessing orientation and magnitude), not a scalar speed represented by one quantity.  Achieving extra velocity towards the ground doesn't help you much.

Gravity is accelerating the spacecraft towards Earth, while to raise periapsis, the closest-approach of the orbit (which starts out deep down near the core of the Earth, and which we are trying to get above the atmosphere), we need to thrust perpendicular to that acceleration vector.  The problem with explaining it this way, is that Earth's gravity *does indeed* get weaker with altitude, the higher the vehicle is at apoapsis, the "cheaper" raising periapsis is - and this sounds a lot like your (incorrect) idea.  The thing is, gravity fades out so very, very slowly.  You don't need 500% of the height of a low earth orbit, you need 500,000%, for it to take a lot less velocity to raise periapsis above horizon - and getting there takes extra energy;  Energy you're just going to slowly aerobrake away (itself a problem for things like solar panels) if you want to achieve a low earth orbit.

A direct injection ascent, which skips LEO, is something that would theoretically save some fuel, but seems to be prohibited through some combination of being very, very inconvenient from an operations perspective, and requiring significantly higher thrust engines for more of the mission, which cost a lot more and are heavier in an upper stage than the alternative.

[Damon Hill]

So the first stage is going to literally fly hundreds of miles back to shore? There is no way in hell it has enough fuel to travel that much distance! I am totally confused! There is no way in hell they're ever gonna land back on shore it'll have to be a barge.

Surely I'm mistaken?

Well, there's a performance hit for sure but the (nearly) empty first stage is MUCH lighter than when it started out.  It depends on how far downrange the core travels and its velocity, which will be higher.  The boosters have it much easier as they drain very rapidly with propellant cross-feed to the core and spend most of their ride just gaining altitude for the core rather than downrange velocity.

I sort of think a core stage barge landing is more likely.  There's going to be enough excitement with two boosters coming back for a landing at the launch site, never mind the core making it three.

--Damon

[Owlon]

So the first stage is going to literally fly hundreds of miles back to shore? There is no way in hell it has enough fuel to travel that much distance! I am totally confused! There is no way in hell they're ever gonna land back on shore it'll have to be a barge.

Surely I'm mistaken?

While physics normally manages to make everything about space and rockets difficult, it this case it is helpful in a few convenient ways.

When your second stage separates from the first stage, both stages are still moving upwards and away from the launch site in a parabolic arc. Your first stage doesn't need to "fly hundreds of miles back" per se, it just needs to impart enough velocity change on itself to reverse direction such that it by the time it has reached the peak of it's trajectory and fallen back to land, it has traveled backwards just as far as when stage separation happened.

I don't know off the top of my head what realistic numbers here might look like, but these are probably relatively close: say your first stage is moving away from the launch site at 1000 m/s and upwards at 200 m/s upon stage separation; you might only need to accelerate by 1500 m/s back towards the launch site in order to make it back.

Fortunately, that 1500 m/s velocity change is not actually as difficult as it might seem. Due to the exponential nature of the rocket equation (http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation), the less propellant you have left, the more punch you get out of each kilogram, because you no longer have to push along the weight of the propellant that has already been burned. If you assume the empty F9 first stage masses about 25000 kg, then, using the rocket equation, the last 25000 kg of propellant is enough to impart about 2100 m/s of velocity change.  The first 25000 kg that is burned in the first few seconds of a launch only accelerates the rocket to about 150 m/s, because you have all the fuel, the full second stage, and the payload weighing the rocket down.

I believe it has been said by Elon Musk and/or other SpaceXers in the past that for every 7 kg of mass added to the first stage, the payload to orbit is cut by about 1kg. Saving 25000 kg of fuel for the boost-back and landing reduces the payload by 25000/7 = 3570 kg. That adds up to roughly what the expected payload hit will be: the Falcon 9 is supposed to deliver nearly 17000 kg to LEO, but the SpaceX website only lists it as lifting 13150 kg. 13150kgkg + 3570kg = 16720
(Wow, the numbers I pulled from thin air actually worked!)

The situation with Falcon Heavy is worse. With the Falcon 9, much of the first stage propellant is used fighting gravity just to lift the rocket up; with the Falcon Heavy, the side boosters do that and the core actually picks up a lot of downrange velocity before second stage separation, so it needs much more propellant to boost back to the launch site.

This nifty graphic of another boost-back design that others have posted in the past was helpful to me in visualizing this:

[Burninate]

The situation with Falcon Heavy is worse. With the Falcon 9, much of the first stage propellant is used fighting gravity just to lift the rocket up; with the Falcon Heavy, the side boosters do that and the core actually picks up a lot of downrange velocity before second stage separation, so it needs much more propellant to boost back to the launch site.

FH is designed, really, around the concept of side-booster flyback to the launch complex being very easy.  Falcon 9 is in an uncomfortable situation where the first stage separates late enough that flyback does take significant amounts of fuel.  For the center core?  The center core will, IMO, probably never fly back all the way to the launch complex in the course of normal operations - it's that impractical.  I think SpaceX want to try it for testing purposes, and that's why they were advertising 6T to GTO prices (less than a third of maximum capability).
In normal practice, I think the center core will either be expendable (which essentially triples the payload for the price of a Falcon 9 non-R), or it will land on something like a barge or an uninhabited island, far from humanity and much closer to its natural unpowered landing zone.  In normal operations, then, probably FH will spend less capability on flyback than F9R.

[Aerospace Dilettante]

The Dry Tortugas are about 1500km slightly south of due east from Boca Chica, tailor frickin' made for our purposes I'd think.

Loggerhead Key isn't being used for anything, put it to use.