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

[Jim]

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

Who is "our"?

[ThereIWas3]

I learned orbital mechanics by the seat of my pants, playing the original SpaceWar on a PDP-1.
You quickly learn that you can't thrust straight out of a deep gravity well.

[nadreck]

On the educational issue here, by the time everyone has graduated from high school they have been given the option of being exposed to everything they need to know to go on on their own and learn just about anything. However, to do that you have to be motivated, to do that in a lot of different areas you have to be motivated and more intelligent than average, to do that and go on to make new contributions to human knowledge you need to be motivated, significantly more intelligent than average, and be stubborn about accomplishing what you want to accomplish.

So the motivation part, you will find that levels of motivation vary greatly from person to person irrespective of their intelligence. And within the same person varies across goals and time. So if 1 in 100 people in high school are motivated to learn more about rockets (like I was, enough to be president of the high school rocketry club) only one in 10,000 has a higher IQ than 150 and could go on to make rockets that are new and different than what we have today - if they don't get distracted by a myriad of other things in life.

So there are lots of smart, contributing people in society who don't know much about orbits, but it is not because high schools don't cover it, it is because they aren't interested.

When I was president of the rocketry club we had lots of members who had the patience to sand and polish balsa fins, painstakingly dope them and then align them on the rocket bodies, and even painstakingly paint them to match a particular historical design or futuristic artists impression that Estes or Cox had illustrated the kit with. Then they wondered why I spray painted mine black on one side and white on the other. However most of our members didn't want to learn the physics of it, they just wanted to build and launch rockets. The few of us interested in the physics had no trouble learning about it.

Oh and I recommend both Kerbal Space and Orbiter to any and all.

[The Amazing Catstronaut]

Oh and I recommend both Kerbal Space and Orbiter to any and all.

Kerbal is a wonderful simulator if you'd like a layman's model. Apart from the fact that very few of the physics are scaled accurately (the game is designed to be playable, after all), and appears to have a slightly differing comprehension of mass, relativity, the light barrier, conservation of momentum, newtonian principles, ecetera, it serves as a majestic introduction to the wonders of real-world spaceflight. Barring a few reasonable balance reductions (that can easily be modded back in), such as simplified aerodynamics, aerobreaking and… ah, none-incendary meteoric re-entries,

Orbiter is older than much of the ISS, a number of prominent STS missions and that SpaceX thing we all know and love, yet is still without equal. However, it only models Newtonian physics, and is heavily unpermissive of in game craft design, which is much of Kerbal's thrill.

(I personally advocate Kerbal).

[Avron]

I would think its time to optimise the flight profile to maximise the mass to orbit etc and also constrain the flight path so that the first stage can land back at the pad. Yes its gonna take more gas, and/or the mass to orbit will reduce but the overall efficiency of the vehicle will improve from a cost prospective as gas cost less, bigger rockets cost more, but are very cheep if you can reuse . So yes, there will be gravity losses etc but all in all the efficiency must increase, taking into account that a reused stage overall will lift many times a given flight. all depending on usage. so when the calcs are done, take multiple flights into account, add in the added gas needed at less that 1% of cost and while ur at it add in reduced time to market for any core..  doglegs are us .. straight lines all depend on point of reference

[aceshigh]

So someone please explain to me why the Falcon Heavy cant just fly straight up, directly up, in a straight line to orbit? Seems this would solve their boost back problem?

So the rocket flys diagonally across the sky, it doesn't fly straight up? There must be a good reason for this, please explain?

Oh dear...

This must be trolling.

Please let it be just that.

actually, the amount of people who donīt really stop to think about it, and think that orbit and space are the same thing, and that you could get straight up and somehow "gravity disappears" and stuff like that... I think itīs actually the majority of people. Just another day I was teaching a friend about it. I started making him questions like "if there is no gravity, why does the moon and the artificial satellites stay circling the Earth instead of flying straight away?" to make him THINK about these things, instead of just telling him.

[Donosauro]

[I doubt most high schools all around the world teach even basic orbital mechanics to be honest.

MAYBE Japan, but that's more likely an elective.  Most kids have to pick it up on the streets.

     You know the type; Punk haircuts, wire rimmed glasses, wearing white leather labcoats and sneaker soled boots.  Walking around with kluged together calculators or tricked out laptops, running pirate copis of spreadsheets and databases, while drinking stove top brewed versions of Jolt Cola and overcaffinated Mountian Dew. Using abandoned warehouses and old barges to build and launch rockets that they put together with salavged sheet metal and hand built rocket engines using windshield wiper motors as fuel pumps.

You know... The Rocket Punks...

If I remember correctly, OTRAG actually did use windshield wiper motors to control propellant valves.

[Nilof]

My one size fits all answer to questions like these is to link this excellent blog post by Matt Strassler.

[Lobo]

... I intuitively thought that rockets fly straight upwards to orbit.

If they just flew straight up, they'd fall straight down again -- along with whatever they were carrying.  There are rockets that do exactly that ... they're called sounding rockets.  They never build up much (if any) sideways speed, so they never achieve orbit.  You may find these links helpful:

http://www.braeunig.us/space/orbmech.htm#launch

http://en.wikipedia.org/wiki/Orbit#Understanding_orbits

 

Yes, it would do exactly what SpaceShipTwo will do.  Go straight up and come straight down, without expending all of the energy necessary to gain the 17,500MPH speed nessesary to achieve stable low each orbit.
Getting "up" is only a small part of what's needed.  The Shuttle SRB's essentially got the Shuttle to space, but the SSME's needed to burn the hydrolox in the ET for another several minutes to get going fast enough to achieve orbit, after jettisoning the SRB's.

[Lobo]

Altitude is necessary to get out of the atmosphere and eliminate drag.  Velocity (17,000+ mph) tangental to the Earth's surface is necessary to stay up there, essentially falling at a right angle to Earth's gravity.

The first stage's main job is to get the upper stages/payload up and out of the atmosphere, and started downrange to get going on the velocity.  It might be possible to mostly go straight up to orbital altitude and let the upper stages do nearly all the velocity gain, but I think that's highly inefficient.

I believe there's been talk of a more "lofted" trajectory this way to help the boosters of FH or F9R return to landing site.  I'm not expert on orbital mechanics, but from what I've read being discussed, yea, that's not a very efficient trajectory.  But it might work...to a degree (it won't be "straight" up, but lofted)...with FH as it will have a lot of capacity to spare.  Getting the boosters back as cheaply as possible is more important than wringing every spare ounce of capacity out of the LV. 

[Lobo]

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

If it flies a less efficient lofted trajectory, where it goes more up than over before staging, it's not as far down range or going as fast.  A large part of it's velocity will be a vertical componant that gravity will "brake" for free.  The boost back burn helps slow down the horizontal velocity, as does the atmosphere when it hits is.  Meanwhile the Earth is rotating under the booster, bringing the launch site to the booster essentially.  So by the time it comes down it's much closer to the launch site than you'd otehr wise think.  The boost back burn needs to be long enough to put the booster right over the launch site by the time it fully decends.

For the center core of a FH to return to the launch site, it would need to stage not too long after the boosters, and do more boost back.  Which would really start to kill payload.  However, how much payload capacity do you need?  That's the question.  If a FH sending all of it's cores back to the launch site could put 23mt into LEO and 11mt to GTO (1500m/s to GSO), then it would match D4H (prior to RS-68A) and would be capable of launching the largest unmanned payloads ever launched outside of Saturn V.  I don't think there's bee a payload yet on D4H that needed it's additional capacity.  (someone please correct me if I'm wrong).
And you get all 3 cores back to the launch site, with basically just an upper stage expended.  Horribly inefficient from a performance standpoint, but very efficient from an economics standpoint.  And you have an LV that can launch any payload out there for the cost of an upper stage.

I don't really see the problem with that, if that's the case.  IF that FH cannot put 11mt to GTO, but say 7mt to GTO (1500 m/s to GSO), that's still as much or more capacity as an Atlas V-551 and every other LV out there except D4H and Ariane V with the ECA upper stage.  And the D4H is the only LV to fly a single payload that needed more than 7mt to GTO.  Ariane V has only flown dual payloads to GTO.  Unless flying dual payloads, a FH with all 3 reusable boosters at 7mt to GTO could fly anything that's ever flown outside of Saturn V's payload, and 5 NRO payloads on D4H.

If they want more performance, they expend the central core, or recover it farther down range on a barge or something.  If they want even more performance, they crossfeed the boosters, but still recover them, and expend the core.
If they want even more performance, they crossfeed the boosters (so they are empty at separation) and expend them as well as the central core.  But payload would have to grow far larger than any are now to need that.

[deruch]

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?

Yes and no.  Yes, the first stage is going to fly a considerable distance down range, then turn around and come back, but it won't be hundreds of miles (closer to just 100 miles).  This is made potentially possible by a few factors.  The first is sort of counter intuitive and where I think many people get confused.  It doesn't have to fly as fast to get back.  It can fly back much slower.  To illustrate, on the Orbcomm OG2 launch, the booster touched down 8 minutes after the stages separated.  Even allowing for time for the stages to move apart and for the booster to turn around, it should have about twice the amount of time to fly back that it took going downrange.  Consequently, it can cover the same distance at a slower speed.  Slower means less acceleration needed--->take less force and therefore less fuel to accomplish this.  The second factor that makes RTLS potentially possible is mass changes.  On it's outbound trip, the first stage is pushing the mass of a full second stage as well as the payload.  It also starts with full tanks of its own.  By the time it turns around to come back to land, it has already separated from the second stage and its own tanks are much closer to being empty.  Hence, for the return trip, the amount of mass needing to be propelled back is way, way less.  Consequently, it will take way, way less force to accelerate the stage back towards the launch site (F=m*a or a=F/m. i.e. For a given amount of acceleration needed, lowering the mass means it will be accomplished with less force).  Less force needed means less fuel required for that burn. 

So, because it doesn't need to fly as fast, it needs much less acceleration.  And because it has so much less mass at boost-back, it will take much less force to accelerate it to the required velocity.  Both of those factors together mean that there is sufficient propellant remaining to accomplish boost-back to land.  Whether there is sufficient margin to boost-back, control reentry, and land safely at a specific point remains to be seen.  At this point I would say that it's very likely possible for at least some payloads flown on the F9v1.1.

If you want to see what it may look like, there a decent demonstration from a modded Kerbal Space Program:

[MP99]

If a FH sending all of it's cores back to the launch site could put 23mt into LEO and 11mt to GTO (1500m/s to GSO), then it would match D4H (prior to RS-68A) and would be capable of launching the largest unmanned payloads ever launched outside of Saturn V.

SpaceX quotes to a GTO at 27 degrees, which is 1800m/s short.

FH is 7t with all-RTLS and 14t with centre core expended.



Note that F9E's payload drops to 3.5t when delivering to a minus-1500 orbit, which they call "SuperSynchronous", or SSO.

There is some question whether FH with core expended could loft 11t to SSO.

Cheers, Martin

[Lobo]

If a FH sending all of it's cores back to the launch site could put 23mt into LEO and 11mt to GTO (1500m/s to GSO), then it would match D4H (prior to RS-68A) and would be capable of launching the largest unmanned payloads ever launched outside of Saturn V.

SpaceX quotes to a GTO at 27 degrees, which is 1800m/s short.

FH is 7t with all-RTLS and 14t with centre core expended.



Note that F9E's payload drops to 3.5t when delivering to a minus-1500 orbit, which they call "SuperSynchronous", or SSO.

There is some question whether FH with core expended could loft 11t to SSO.

Cheers, Martin

Understood.  I was just using speculative numbers as I didn't know for sure what the predicted performance for an FH with all 3 cores RTLS and with a central core expended were.
Just saying that even with the obviously large performance hit of all 3 cores RTLS, it's fairly immaterial if the majority of the payloads FH would be launching don't need more than that, and you get to reuse all 3 cores.
So if FH with all 3 cores RTLS actually can do 7mt to GTO (1800m/s to GSO), then that's right on par with Atlas V-531 and Delta IV (5,4) (about 1/2mt short of each).  So the only payloads that have every been flown that require -more- than that to GTO have been the 5 D4H missions (the first was only a test), and 5 Atlas-541/551 total launches. 
So while very inefficient to bring all 3 cores back to the launch site, it would still cover about 86% of all the EELV launches every done.  By expending just the core, it looks like it would cover at least most of the rest of that. 
Ariane V can do more, but it doesn't look like it's ever launched more than 9.4mt payload to GTO, but those were always dual payloads, so no single payload to GTO of more than probably 5-6mt.  So FH with all 3 cores RTLS would be able to launch any of those payloads as single payloads....or perhaps dual payloads with an expended central core, if that would be more cost effective.  Not sure if SpaceX is looking to do dual payloads to GTO the way Ariane V does though?



 

 

[FinalFrontier]

So someone please explain to me why the Falcon Heavy cant just fly straight up, directly up, in a straight line to orbit? Seems this would solve their boost back problem?

So the rocket flys diagonally across the sky, it doesn't fly straight up? There must be a good reason for this, please explain?

Oh dear...

This must be trolling.

Please let it be just that.

Personally I'd place my bet on "general level of education in [insert country of the original poster here]".

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

I doubt most high schools all around the world teach even basic orbital mechanics to be honest.

Ha. They don't. Math and science in america in k-12 doesn't exist, its a myth. What they teach now is whatever is "politically correct" to teach. Spaceflight is considered by many not to be politically correct (don't ask its beyond the scope of the thread).
This being said I was not at all surprised to see this thread.

[ChrisWilson68]

Ha. They don't. Math and science in america in k-12 doesn't exist, its a myth. What they teach now is whatever is "politically correct" to teach. Spaceflight is considered by many not to be politically correct (don't ask its beyond the scope of the thread).
This being said I was not at all surprised to see this thread.

That's nonsense.  The level of math and science education is uneven, varying from place to place in the U.S. but to just make blanket attacks on it is wrong.

[Owlon]

Ha. They don't. Math and science in america in k-12 doesn't exist, its a myth. What they teach now is whatever is "politically correct" to teach. Spaceflight is considered by many not to be politically correct (don't ask its beyond the scope of the thread).
This being said I was not at all surprised to see this thread.

That's nonsense.  The level of math and science education is uneven, varying from place to place in the U.S. but to just make blanket attacks on it is wrong.

This.

I got a solid foundation of physics, chemistry, and calculus in my Texas high school (hint: Texas is not renowned for it's excellent k-12 education). I did seek out AP classes and whatnot, but the minimum graduation requirements would have you taking very similar chemistry and physics classes. As I mentioned previously in the thread, my regular physics class that everyone had to take covered the basics of orbit and gravitation using algebra. You would have failed that unit if you didn't understand the difference between space and orbit.

[cleonard]

You could launch in a straight line and get into orbit.  The catch is what you are orbiting.  Launching from the surface of the earth you could go in a "straight line" directly into solar orbit by getting just past escape velocity. 

[Dave G]

Can we kill this thread already?

[M_Puckett]

Isn't straight line orbit an oxymoron?