Delta-v to orbit

Factual Questions
1

I see in this morning’s Reno, NV paper that Nevada is on the list as a possible test/launch site for Lockheed Martin’s VentureStar spaceplane (the old NV Test Site down by Las Vegas–Area 51, here we come!). Our governor, Kenny Guinn, is hot on the idea, but is quoted as saying “Our launch pad would be a mile up (in elevation) already, and the first mile is the most expensive” for launch. It’s too soon for me to judge Kenny as a governor–he came into office in January–but I thinks his physics is (are?) a little suspect. As I understand it, the cost of the first mile or so is due to the fact that the vehicle has to generate enough thrust to get its weight and the weight of the neccessary fuel to orbital velocity. As speed increases, mass decreases as the fuel burns. I can’t see that one vertical mile, and, I suppose, the lower air pressure, would make that much difference. Any comments? It’s been fun seeing some practical discussion of orbital mechanics around here lately, especially considering some of the off-the-wall scenarios they’ve started from!

–Alan Q

2

The spaceplane engines are supposed to be desinged so airpressure doesnt make a difference. I not so sure about saving the cost of the first mile though. You dont weigh much less at a mile high. The distance to orbital altitude would be shorter, but you still have to get the lateral speed to keep from hitting the earth when you start falling.

I’ve always wondered if anyone seriously thought about launching the space shuttle by catapulting it up the side of a mountain though. Seems like a lot of fuel is spent just clearing the tower.

3

Are you sure Gov. Guinn wasn’t just joking? Perhaps we should drill for oil in Death Valley, so we can save that extra 300 feet of drilling. :slight_smile:

4

I can’t say about “seriously”, but the idea of launching from a track instead of a pad has been used in a number of older science fiction stories.

Abort strategies would worry me a little, depending on what the initial launch mechanism is. You wouldn’t want to get to the business end of the rail and discover that your first stage ignition is hanging fire.

5

I think his statement is right because that first mile would be where you would overcome the most inertia. But by moving the launch site up a mile you are not really getting rid of the first mile you are actually getting rid of the last mile. You still need to start from a stop and it is the starting from a stop that takes the energy.

6

They been doing on aircraft carriers for years.(With substantialy lighter and cheaper aircraft) You would have to find a “mountain” near the ocean in case you had to go in the drink.

7

Alan Q:

*Yeah, he needs a little physics lesson. By having a launch site one mile higher, it’s the last mile that’s taken out of the equation, not the first.

This is like being offered “buy one get the second free” and responding “OK, I’ll take the second one.” Nope, sorry you have to buy the first one, just like you have to buy the first mile of a launch.*

8

falcon2; Yes there is a company that is currently designing a space craft whos launch mission reads very much like that. It starts with the unfueled spacecraft being launched via a ‘catapult’ (Currently the design calls for a railgun type launcher.) It would then rendevous with a normal plane that would offload the fuel needed for the second orbital insertion portion of the mission.
A similar approach (except it uses a turbofan engine for liftoff.) is being explored by Pioneer Rocketplane. www.rocketplane.com Another good page to look at is www.lunacity.com , It has a lot of good links to different Co’s.

>>Being Chaotic Evil means never having to say your sorry…unless the other guy is bigger than you.<<

—The dragon observes

9

IMHO, politicians occasionally get sound bites dealing with physics right, but usually because somebody else told them what to say. Very few politicians have much science background, as far as I can tell.

Anyway, there are a few factors that come into play here. Getting something into orbit requires both lifting its mass vertically to the required altitude, and giving it enough horizontal velocity to achieve orbit (otherwise it falls straight back down again). It’s the horizontal-velocity which requires most of the total energy expenditure, especially because a lot of that happens down in thick atmosphere where there’s a lot of aero drag to overcome.

For this reason, just from an energy perspective, launch sites closer to the equator are better than ones further north, because you get more horizontal velocity “for free” that way due to the earth’s rotation. That’s why NASA launches from Florida (it has oceans to launch over, and it’s pretty far south), instead of someplace further north, and it’s also why retrograde orbits are much harder to achieve.

Also, higher elevation is better, obviously, but this is almost a second order effect. Some of it comes from starting in thinner atmosphere; some obviously also comes from just having to lift something over less distance. But since that’s not the biggest factor in the first place, it only matters a little bit. The numbers I’ve seen tossed around in the past said about 1-2% total savings if you could somehow start at 10,000m above sea level (which clearly isn’t practical). I didn’t see the derivation for that so I can’t vouch for its accuracy, but my gut feel says its in the right ballpark.

Its interesting to note also that the atmosphere is thicker (as in taller, not as in denser) near the equator, so some of the gains you get from being further south are canceled out by having to plough through more air on your way up.

Anyway, it’s not very hard for logistics concerns to outweigh factors like elevation, but OTOH, if you can get any gains more or less for free, then you might as well!

k0myers

10

I should say, by “clearly not practical” I meant for conventional systems like the shuttle or a Titan - these require a lot of ground based infrastructure. There are other launch systems that launch from aircraft flying higher than that - the Orbital Sciences Corp Pegasus(?) is one that I remember off hand.

There have also been proposals floated to use balloons to lift something in a similar manner.

k0myers

11

k0myers,
Yes, there is a team of amateurs who are attempting to use ballons as the first stage lifting device to place a payload into orbit. They are currently felt to be the forerunners in the running for the Foundation Prize (? I can’t remember which prize it is, is $250,000 for first payload sent to 50 miles, by Nov 2000.) They attempted to launch earlier this year, but had some problems with the ballon phase, high winds I believe. The Lunacity link I posted earlier has some information on this.
Also, by amateur, I mean without corporate backing. not amateur in ability…some of these guys are excedingly good engineers and scientists.

>>Being Chaotic Evil means never having to say your sorry…unless the other guy is bigger than you.<<

—The dragon observes