Can We Reach the Moon with a Slow, Steady Launch?

[nution]

This is all theoretical of course. I would like to pt together a small presentation on what it would take to get an object to the moon. For the sake of argument, we will say the object weighs 10 pounds. The shape can be determined by the most efficient shape given the method of propulsion etc.

Escape velocity being something like 17,500 mph does that mean that no matter what an object MUST reach that speed to get to the moon?Or since the moon is still technically interacting within the Earth's gravitation field, this wouldn't be technically needed since you would not in reality be escaping? Are there options for a more slow and steady approach?

Lets say for example a balloon can get to the outer stratosphere before it gets so large it just explodes. Let's say we attach a rocket to a platform on the balloon and once it reaches it maximum altitude before exploding, we launch the rock and begin to push further out. Using some form of "slow launch" because a rapid blast off would be pretty unstable on the surface of a balloon in the outer reaches of the atmosphere. Eventually, with a scale of this size the rocket would loose fuel and fall back to earth. Since the rocket is not equipped to enter an orbit due to low speed, could you instead of trying to completely escape Earth's orbit, make a "stop" at the moon and allow the gravity of the moon to hold you there?

Lets also assume this "slow launch" uses some method to slowly begin to propel the rocket off the platform of the balloon. Over some period of a few minutes the force of propulsion would equal the speed of ascent, then slowly pass that speed at around the time the balloon would become ineffective, thus slowly lifting the small rocket from the platform and continuing its own journey upward and the balloon eventually falls back to earth.

Also, once into space, what different fuels or methods of propulsion could be employed to aid it on its continued path to the moon?

There are a lot of things to consider mathmatically, but what are we really looking at here in possibilities other than sheer blast off and trying to go thousands upon thousands of mph?

 

[Janus]

This is all theoretical of course. I would like to pt together a small presentation on what it would take to get an object to the moon. For the sake of argument, we will say the object weighs 10 pounds. The shape can be determined by the most efficient shape given the method of propulsion etc.

Escape velocity being something like 17,500 mph does that mean that no matter what an object MUST reach that speed to get to the moon?Or since the moon is still technically interacting within the Earth's gravitation field, this wouldn't be technically needed since you would not in reality be escaping? Are there options for a more slow and steady approach?

You don't need to reach escape velocity to get to the Moon, just pretty close. Its like throwing a ball into the air. The harder you throw it, the higher it will go before falling back down. Escape velocity happens when you throw it so hard that it never falls back. With the Moon, you want to throw it just hard enough that the top of its arc brings it close enough to the Moon for the Moon's gravity to take over. This is more or less how we do it with rockets. We get the rocket up to speed as fast as we can and then let it follow a ballistic path to the Moon

A "slow and steady" approach, while theoretically possible, is just not as efficient. With the same rocket, you would end up burning a lot more fuel to do it this way. The idea is that you use up your fuel while you are still close to Earth. That way you don't have to lift the mass of that fuel higher. The slower you burn the fuel, the more fuel you use up just lifting the fuel that you will use later on.

 

[nution]

A "slow and steady" approach, while theoretically possible, is just not as efficient. With the same rocket, you would end up burning a lot more fuel to do it this way. The idea is that you use up your fuel while you are still close to Earth. That way you don't have to lift the mass of that fuel higher. The slower you burn the fuel, the more fuel you use up just lifting the fuel that you will use later on.

But in this scenario, let's say we take the Balloon and this platform holding the rocket to around 25 miles up in the air. By balloon this would take about 45 minutes to an hour give or take some depending on the balloon and load it carries. Either way, at this point, the rocket itself has pulled away 25 miles into the air without expending fuel in the rocket. Am I correct in assuming that gravity is less at this distance also? Allowing the rocket to have eliminated interacting with much of Earth's atmosphere and wasting energy converted to heat having made this "skip" and also allowing a launch with maybe minimal but less gravitation force acting on it negatively?

 

[D H]

The gravitational force at 25 miles altitude is about 98.75% that at the surface. So not all that much of a reduction.

Two things must be done to get some object into low Earth orbit. These are raising the object up to orbital altitude and speeding the object up to orbital velocity. The latter represents the lion's share of the energy budget; for an orbit of 200 km about 96% of the energy goes into attaining orbital velocity while only 4% goes into attaining orbital altitude. A balloon launch addresses that small 4%. It just isn't buying all that much.

There are advantages and disadvantages to a balloon-launched rocket. The advantages include a slight reduction in required energy due to the increased altitude, another reduction in required energy due to the fact that the rocket isn't plowing through the thickest of the atmosphere, and reduced problems with weather.

The disadvantages are many. Here are just a couple. First and foremost is the huge reduction in the size of the rocket. You can't loft an Atlas via a balloon. Another problem is range safety. A bad launch, whether done from the ground or at altitude, can place people's lives at risk. Controlling where the launch pad is is easy: It is fixed. Controlling where the balloon is is anything but easy. Safely launching from a balloon is a challenging task.

You are still going to need a powerful rocket to launch something into space from a balloon. You still have to attain that orbital velocity rather quickly. Suppose you launch with a low thrust but high specific impulse rocket from a balloon. The rocket would fall to Earth. Those low thrust/high ISP rockets are fine once a vehicle is on orbit. They are pretty much worthless as launch devices.

 

[nution]

You are still going to need a powerful rocket to launch something into space from a balloon. You still have to attain that orbital velocity rather quickly. Suppose you launch with a low thrust but high specific impulse rocket from a balloon. The rocket would fall to Earth. Those low thrust/high ISP rockets are fine once a vehicle is on orbit. They are pretty much worthless as launch devices.

Lets say though we are not looking for orbital velocity but just a point a to point b path to the moon. I know that the traditional approach is to go into orbit, then basically "take off" from there as you can use your already stored up speed etc etc and in the end is probably a better idea, but I am looking at the possibility mainly of not being able to reach orbital or escape velocity, but still making it to the moon. I am talking about much lower speeds of maybe 1000mph range.

Just forgetting going into orbit, I was thinking, you would really need to conserve as much as possible at every step. Most of the fuel being spent just to get into space, then could possibly deploy some solar sail to help a little in the "push". Just various ways to take a straight shot with no tricks of sling shotting or anything like that. Also keeping it at reasonable speed because it cannot go to fast. Once it gets there it has to stop and fuel being on a choke hold, landing would have to be a well thought out process :).

 

[D H]

That is the very last thing you want to do. The energy requirements of your proposal are immense.

 

[nution]

So you mean to say it is better to get it overwith instead of the slow approach or you mean it is easier to get there by first getting into orbit?

 

[D H]

Exactly.

The only reason not to do so is because the thrusters don't have the oomph to do it all at once. A good example was the SMART-1 spacecraft . It used a high ISP/low thrust engine to get from insertion orbit to the Moon. The thrust was incredibly low: 1/50,000 g. That low an acceleration obviously won't work for launch. A minimum of 1g acceleration is needed just to counteract gravitation.

 

[nution]

So we could say again being as far away as possible before launch point, say we are searching for a constant speed of 2 g's to over come and ultimately reverse the the force of gravity basically like were free falling away from Earth to the moon. Over distance we can increasingly reduce the for amount of propulsion to maintain the speed we had at 2nd on Earth since the further we get the less it takes to overcome the pull of Earth and eventually reach a point that we can be taken over by the gravity of the moon then start concentrating on decent. Wouldn't this be pretty feasible considering the initial hard launch to force it out then take the high ISP approach? Also considering traditional rockets carry a large payload this would only be carrying itself so it would be relatively light weight.

 

[D H]

Do the math.