# Ballistic space probes?

1. Oct 25, 2013

### fulltime

Hi. I am writing a scifi story that involves some (i hope) realistic science. The conceit is to show, in the near future, some use of cheap "ballistic" probes of the inner system and eventually TNS and heliopause.

They would be small objects launched in large numbers, designed to gather very specific data and perhaps to work together somehow.

I am thinking they would be launched directly from the surface, perhaps via a magnetic chute.

Am i on the right track? Is something like this plausible? What would be the physical properties of the probes? What kind of construction would a launcher have?

Last edited by a moderator: Oct 25, 2013
2. Oct 25, 2013

### Bobbywhy

fulltime, I imagine when you wrote "The conceit is to show," you meant "The concept is to show,"

What's the TNS?

This Forum deals with real science. Fictional stories and scenarios, I'm pretty sure, are not dealt with here. Have you read the Forum Rules? Maybe a Mentor will assist us here.

3. Oct 25, 2013

### Staff: Mentor

We have a sci-fi subforum, including a writing sub-subforum. Hmm. This is beginning to sound like inception...

4. Oct 25, 2013

### Staff: Mentor

The main thing is that your probes would need to reach escape velocity within the time frame of the launch. Escape velocity for Earth's gravity is approximately 11 km/s, while for the Sun, seeing that we are already 1 AU out, is about 42 km/s. Launching from Mercury would require 67 km/s to escape the Sun's gravity.

So, either your probes would need to be extremely durable and able to withstand very high G-forces, or your launch mechanism would need to accelerate them over a long period of time. Currently, we do the latter. We use rockets which give a low acceleration over a long period of time.

5. Oct 25, 2013

### voko

As long as your probe acquires the escape velocity, it is not impossible.

The problem is in the method.

The currently used method uses a two or three-stage rocket that puts a spaceship into a low Earth orbit. The spaceship, which may also include expendable stages, then accelerates and acquires the escape velocity. The critical part of this method is that the densest part of the atmosphere is traversed at a relatively low speed, so drag is manageable.

Which is not very likely in your method.

6. Oct 25, 2013

### Staff: Mentor

This thread has now been moved to the Science Fiction Writing subforum.

7. Oct 25, 2013

### Decimator

Launching ballistically off Earth isn't really feasible for drag and compression heating reasons, but perhaps you could launch them off Luna with a large mass driver?

8. Oct 25, 2013

### fulltime

I came here looking for good advice from knowledgeable of people. First thing i get is someone going for the jugular with an ignorant jab at my writing style. Not cool.

Trans neptunian space, an area extending from 30 to 600+ au, containing pluto and many large bodies (sedna, eris, makemake, etc), the keiper belt and scatter disk, and possibly the beginnings of the oort cloud. Of course science fiction stories can have real science in them...

Lets say these are small - 1 cubic meter volume - bullet or sphere shaped payloads with no boosters of any kind. I assume that to be commercially viable the launchers would be electrically powered magnetic rails or some similar system which would use low friction in combination with a long ramp, perhaps down the side of a mountain and then up into a valley.

The mechanics of that would be interesting to research too, but would a structure like that produce higher velocity launches, at a higher rate than launching rockets?

Regarding managing drag - does that mean that fast movers will be very uncontrollable?

Regarding durability. Are there extant materials and sensors which would suffice for these velocities? I imagine there are...

9. Oct 25, 2013

### Enigman

That was NOT a jab. Just a bit of well-meant advice about the forum you were posting in.

You cannot use gravitational potential energy to overcome gravitational potential energy. So the mountains and valleys won't matter except to elongate the path. You will have to have a source of energy. As long as you can use the rails to power it up somehow it should be fine. Then comes the trajectory- it would have to be carefully calculated so as to avoid gravitational effect of planets and avoid all the debris in between as there's no steering mechanism. Also about physical property the projectile will have to face a lot of air friction and will get heated up- so it better be durable (I don't know enough physics to say what material).

Last edited: Oct 25, 2013
10. Oct 25, 2013

### voko

"Managing" meant it could be overcome. In this case, you want an escape velocity close to the sea level. The force will be huge (meaning huge stress on the probe), and so will be the heating. Plus you will need to pump lots of energy into this over some very short time. Seems very unreasonable as a concept. I am not qualified to say whether this is impossible, but that seems so to me.

11. Oct 25, 2013

### fulltime

I read what you said, then used the forum tool to fix it on my end. Regarding using the slopes though, i wonder whether coriolis effect and gravity together provide more energy than just gravity... might just have to go and ask a stupid question in some wrong forum ;)

I too am no expert but it seems reasonable enough to me, if there was a power station near by. Does anyone have any other opinions?

12. Oct 25, 2013

### D H

Staff Emeritus
A power station is not going to help "manage drag". The solution to "managing drag" at low altitudes is exactly the same as the solution to "Doctor! It hurts when I do this:" «bonk!».

13. Oct 25, 2013

### Hornbein

This is a good idea, probes are moving in this direction, especially probes for magnetic fields. EM fields in vacuum are not remotely detectable, you have to have a probe there. The best way to get detail would be large numbers of small probes not too far from one another.

The atmosphere is a huge problem, so I don't think anyone would ever do it this way. You want to launch from outside the atmosphere. So you could send a rail gun or something up on a rocket or up a tether and launch a large number of ballistic probes from up there. I'd do it via a large number of cheap solid state probes about the size of a fist that have no or almost no propulsion. No engine, no fuel. Each would be sort of like a rock, so no problems with acceleration. I think (maybe) they could network in some way to simulate a large antenna to transmit the data.

Last edited: Oct 25, 2013
14. Oct 28, 2013

### Ryan_m_b

Staff Emeritus
Reaching escape velocity at sea level means that the object would be travelling at Mach 32. I doubt there are many materials that could survive the heat from the dense lower atmosphere that would arise from travelling at that speed.

Shooting things into space with a cannon has experimented with but there are a range of issues:
http://en.wikipedia.org/wiki/Space_gun

Your best bet for an SF novel is to propose something like a launch loop, space elevator or even some form of beam launch.
http://en.wikipedia.org/wiki/Launch_loop
http://en.wikipedia.org/wiki/Space_elevator
http://en.wikipedia.org/wiki/Beam-powered_propulsion

15. Oct 28, 2013

### voko

Heat per se might be manageable via ablative cooling. What seems more of a problem here is the stress in the material of the probe produced by the drag.

And the stress in the material when it gets accelerated to those speeds over some relatively short distance.

More significantly, we need M32 just outside the atmosphere; which means we need a much greater speed at its bottom, which makes things much nastier.

16. Oct 28, 2013

### Ryan_m_b

Staff Emeritus
Totally forgot to mention that, the escape velocity figure doesn't take into account how the atmosphere will slow you down.

17. Oct 28, 2013

### voko

Some back of envelope calculations. Drag is given by $c \rho(h) v^2$, where $c$ is some constant (I know it is not really constant). I will neglect gravity, too, because it will be much weaker than the drag at the speeds we are considering. $$m\dot v = - c \rho(h) v^2$$ Dividing both sides by $m \dot h = mv$ yields $$\frac {\dot v} {v} = - \frac c m \rho(h) \dot h$$ giving $$\ln \frac {v_f} {v_i} = - \frac c m \int\limits_0^h \rho(x) dx$$ Now, using the US Standard Atmosphere data published here: http://en.wikipedia.org/wiki/U.S._Standard_Atmosphere and Wolfram Alpha via http://www.wolframalpha.com/input/?...,+868},+{47,+111},+{51,+61},+{71,+4}},+x])+dx (somebody please check - much appreciated), for h = 100 km I obtained $$\ln \frac {v_f} {v_i} = -2 \cdot 10^9 \frac c m$$

At high Reynolds numbers, for a sphere, $c$ is approximately one quarter of the area of its cross-section. Assuming the probe to be spherical, with m = 100 kg and radius 0.3, $\frac c m \approx 0.001$, giving $$\ln \frac {v_f} {v_i} = -2 \cdot 10^6$$ which basically means whatever initial velocity we might have at the sea level, it is not possible to reach the LEO altitude with anything even remotely resembling the escape velocity.

18. Oct 28, 2013

### fulltime

I could do that thing and move my chute to the moon. But to do this i need some simple explanation of why sending people to the moon is cheap. Near future solutions arent cheap or likely. So - united alliance permanent moon mission? Pan oceanic factories? Asianese biodomes?

As you can see, i like the earth launcher idea! A, it seems to comply with forum rules 100% and b it doesnt require any serious flights of fancy.

If possible i want to use something commercial, something that could be built right now if only there was a will.

Thank you for that, i im certain those space gun chels have calculated everything! This may be what im looking for. Its not much of a looker but its a proven concept, nearly perfect. It just needs to be sexified.

In answer to ryan as well, this is interesting! I did some basic math using the american shuttle as a model (and discovered the laughable way the boosters are recovered from the ocean, at a huge cost in fuel for helicopters and craft), which seem to confirm that shooting things straight up is actually a good idea?

It reduces the need for ablative armor as well i assume. Not great at maths though me...

But the most intriguing thing about this is that there are sensors available right now that will easily withstand these g forces! There are even sensors that can assemble themselves after launch i believe, as well as purely mechanical sensors like chemical films for example, that can be imaged and transmitted.

So what if these were small projectiles, like the shells fired from 100 mm guns mentioned in the space gun article?

S as above i assume we need to make it smaller?

19. Oct 29, 2013

### voko

The volume of a sphere is $\frac 4 3 \pi r^3$; thus its mass is $m = \frac 4 3 \rho_m \pi r^3$; its cross-section is $\pi r^2$, thus $c \approx \frac 1 4 \pi r^2$, so $\frac c m \approx \frac {3} {16 \rho_m r}$.

So it is exactly the opposite of your intuition: the smaller the sphere is, the stronger the deceleration. The bigger the sphere, the lesser the deceleration.

You need $\frac c m \approx 10^{-9} \frac {\text{m}^2} {\text{kg}}$ so that the initial and final velocities be in the same ballpark. That means $\rho_m r \approx 10^8 \frac {\text{kg}} {\text{m}^2}$ Taking $\rho_m \approx 2500 \frac {\text{kg}} {\text{m}^3}$ (a bit less dense than aluminum), $r \approx 40 \ \text{km}$, which is clearly impossible.

The bottomline is that a purely ballistic launch from the Earth of an interplanetary probe is impossible.

20. Oct 29, 2013

### Staff: Mentor

Plus, when you are shooting through the atmosphere (even assuming you shoot from high enough to overcome drag problems voko explained) you have only a very rough control over the final trajectory.

21. Oct 29, 2013

### D H

Staff Emeritus
That interpolating polynomial yields nonsense above 55 km or so. It has pressure negative between ~56 km and ~70 km and above 71 km, pressure rises as a seventh order polynomial. As a general rule, you should never use an interpolating polynomial to extrapolate. That region from 71 km to 100 km is doing exactly that, and at 100 km, the pressure is greater than 1 atmosphere per the interpolating polynomial. It's better to do the integration from 0 to 55 km and use this as a lower bound. Your factor of 2×109 becomes 7×108.

That is still a very bad result. You can't launch ballistically from the ground.

Making it smaller makes the problem that much worse! Acceleration due to drag is proportional to cross section area and inversely proportional to mass. Cross section area is proportional to length squared while mass is proportional to length cubed. This is one of those places where the cube-square law says the bigger the better (so long as average density remains roughly constant). A boulder fall to the ground at roughly 9.81 m/s2. Shave a tiny grain of dust off that rock and the grain of dust will remain suspended for hours, or even longer.

I'll repeat what I said above: You can't launch ballistically from the ground.

22. Oct 29, 2013

### fulltime

OK guys, thank you. Would a launch from the top of kilamanjaro or elbrus help? And if not, what if i did put it on the moon? I assume having these launchers in orbit would cost a lot, with fuel needed for the shooting tobe straight.

23. Oct 29, 2013

### voko

I made a silly mistake in #17. Instead of taking the values of density of the air when interpolating the air density function, I took the values of pressure. Plus there is the bad extrapolation as D H pointed out. Very embarrassing.

The correct density values taken from the original publication are:

0 1.2250
1 1.1117
3 9.0925 × 10-1
5 7.3643 × 10-1
9 4.6706 × 10-1
15 1.9476 × 10-1
25 4.0084 × 10-2
40 3.9957 × 10-3
50 1.0269 × 10-3
75 3.9921 × 10-5
100 5.604 × 10-7

Where the left column is altitude, in kilometers, and the right column is density in $\frac {\text{kg}} {\text{m}^3}$. This I simply integrate using the trapezoidal rule, which gives 10771 $\frac {\text{kg}} {\text{m}^2}$.

So the result for a 0.3 m 100 kg sphere is $$\ln \frac {v_f} {v_i} = -20$$ which is still "impossible".

However, this modifies the argument in #19 somewhat. It should read:

You need $\frac c m \approx 10^{-4} \frac {\text{m}^2} {\text{kg}}$ so that the initial and final velocities be in the same ballpark. That means $\rho_m r \approx 2000 \frac {\text{kg}} {\text{m}^2}$ Taking $\rho_m \approx 2500 \frac {\text{kg}} {\text{m}^3}$ (a bit less dense than aluminum), $r \approx 0.8 \ \text{m}$, and the mass of 5.5 metric tons, which seems impossible, but is actually still impossible due the heat and stress.

24. Oct 29, 2013

### Staff: Mentor

Moon would be my choice. And it was already mentioned much earlier in the thread.

25. Oct 29, 2013

### voko

This seems much more likely, but.

You would still have to shoot at a velocity way higher than 11.2 km/s.

The Chelyabinsk meteor we all saw past winter disintegrated at 22 km altitude, at 15 km/s, which is milder conditions than your setup.

Regardless, you still have the problem how the probe is accelerated and how it withstands the acceleration.