3 ms·

The difference between such a ballistic trajectory (1/5th of escape velocity) and LEO is huge. >you don't need to scale "go fast rocket" up by that much My na

The difference between such a ballistic trajectory (1/5th of escape velocity) and LEO is huge. >you don't need to scale "go fast rocket" up by that much My napkin math says you need at least 2 orders of magnitude more fuel mass which classifies as "a lot"

dogma1138 11y ago
Not sure what your math is, but mine is about 20% larger rocket to get to 160KM and have enough speed for orbit, at least as far as cube-sats that can skip on the atmosphere for a bit go. There's also https://en.wikipedia.org/wiki/Copenhagen_Suborbitals https://en.wikipedia.org/wiki/Copenhagen_Suborbitals with their plans for a manned suborbital flight, their rocket (although it blew up) could also be used as a base for amateur orbital flights. I'm not entirely sure what is so surprising here, it's a question of money at this point, but amateur rockets are quick close to being able to reach space, this doesn't mean that they will be commercial viable or even viable for any non-anecdotal use.
- avmich 11y ago
  LEO difficulty is mostly speed, not height. SpaceShipOne flew above 100 km - but it's a long way to reach Vostok-1 performance, as delta-V for SS1 is more than four times less than for Vostok-1.
- mikeash 11y ago
  What's surprising is that amateur rockets can reach space on suborbital trajectories but none of them come even remotely close to orbit. Altitude is the easy part, relatively speaking. Getting the speed for LEO is the hard part. The GoFast rocket you linked to has demonstrated up to 4,200MPH. Low earth orbit requires about 17,000MPH. That factor of ~4 requires massively larger rockets, because the rocket equation is cruel. Each incremental increase in final speed requires an exponential (actually using this word correctly, for once) increase in the amount of fuel. Fuel in return requires more hardware like tanks and engines, which means yet more fuel is required, etc. Edit: using RP-1 as the fuel in a single stage, getting to 4,200MPH requires that 45% of your total rocket mass be fuel at launch. Getting to LEO requires 94% of the mass to be fuel.
  - dogma1138 11y ago
    Yes I understand it quite well, ~20% scale up which is 20% longer, and 20% wider is quite a big change in mass. The big problem is either staging or being able to shift it to a shallow enough trajectory and get enough velocity, but it might be solvable if you can do a zoom climb type launch from an aircraft similar to how Pegasus/SpaceShip One do it. https://en.wikipedia.org/wiki/Pegasus_(rocket) https://en.wikipedia.org/wiki/Pegasus_(rocket)
    - mikeash 11y ago
      How is a 20% scale up going to get you anywhere close to the extra dv needed? 20% on each dimension is still only 70% more rocket, where you need something more like 10x more rocket to make that leap.
    - lutorm 11y ago
      That's 1.2^3, or 70% more... not even close. Also remember that making the rocket bigger also increases the dry mass, which further increases the fuel requirements. The difference in ISP between a liquid-fueled rocket (say 450s for LH2/LO2, 350s for RP1/LO2) and solid propellant (250s) makes a huge difference to your prop fraction. To get, say 8km/s delta-v, just plug it into the rocket equation: LH2/LO2: 6.1 initial/final mass. RP1/LO2: 10.3 solids: 26 Staging can help this a bit, but the basic fact remains. It's very difficult to get useful payload to orbit with solids.
- fivesigma 11y ago
  A 160km altitude cubesat would require a minimum 7.8km/s of delta-v. Anything less than that and it will burn up in the atmosphere after a few orbits. GoFast Rocket achieved a top speed of 1.6 km/s (3580 mph). It needs to go 4 times faster, and no, a 20% larger rocket won't cut it. 20000% larger by mass would be more like it. It also needs a 2nd stage burning liquid propellant with high specific impulse which is way, way harder to build than a dumb solid propellant rocket. Noone (not even governments) has been able to send a single stage rocket to orbit.
  - dogma1138 11y ago
    Israel and India use 3 stage HTPB solid rockets for space launches you don't need to have a 2nd stage liquid fuel rocket to get to orbit. Also 20000% larger by mass? want to be a bit more serious? if you scale this rocket by 35-40% you get to about the size of the first stage of the Shavit solid fuel space launcher which is a commercial space launcher. You do understand how scaling works on 3 dimensional objects right?
    - fivesigma 11y ago
      10000% by mass is a more accurate figure and since you like dimensional scaling, that's 364% scaling. The GoFast Rocket weighs 350kg. Scaling it by 35% (1.35 ^ 3) would make it weigh 861kg. The rocket you mentioned weighs between 30 and 70 _tons_. I'm not sure you understand how anything works.
      - simoncion 11y ago
        > I'm not sure you understand how anything works. Based on my previous conversations with this guy, this seems to be a recurring pattern. I do hope that he's using these conversations to recognize and fill in the holes in his knowledge. :)
    - mikeash 11y ago
      What good does it do to scale something up to the size of the first stage of a four stage rocket? Those other three stages are pretty important to that whole putting-stuff-in-orbit business. Edit: I see Shavit can also launch in a three stage configuration. My overall point remains.
- btilly 11y ago
  I'm not sure what your math is because you're not showing us any. But it sure doesn't agree with mine. Math is done with numbers, so let me give you actual numbers. You can double-check these against https://en.wikipedia.org/wiki/Orbital_speed#Tangential_velocities_at_altitude https://en.wikipedia.org/wiki/Orbital_speed#Tangential_veloc.... Standing on the surface of the Earth your potential energy relative to infinity is −62.6 MJ/kg. If you were standing still 160 km up, you've added roughly 1.6 MJ/kg. (You climbed 160,000 meters against a force of 9.8 Newtons per meter.) To then go into orbit you have to add a sideways velocity of about 7800 m/s. Apply the famous kinetic energy = 0.5mv^2 and going into orbit requires 30.4 MJ/kg of energy. That's 19x more energy than is required to get to altitude! But it gets worse! If you're going to go into orbit, you need to think about getting down. Going up we go straight up out of the air and then add sideways velocity. Going down we let the atmosphere do most of the work of slowing us down. But in order to survive that you need a much more rugged device with a heavy heat shield. You're now both doing more work per mass lifted AND you're lifting more mass! But we're not done with the bad news. A fundamental fact about rockets is that you have to lift the fuel you use later in the launch. That means that as you're increasing the final velocity you're getting an ever-decreasing ratio of energy expended to useful kinetic energy for your payload. Put it all together and actual rocket scientists have told me that it is about 100x easier to get to orbital altitude than it is to actually get into orbit. That said, LEO is the half-way point. Getting into LEO takes about as much energy as it does to launch from LEO entirely out of Earth's orbit.
- TeMPOraL 11y ago
  A honest hint: spend some hours in Kerbal Space Program. It does wonders for one's intuitions about rockets and its calibration with reality.

dogma1138 11y ago

Not sure what your math is, but mine is about 20% larger rocket to get to 160KM and have enough speed for orbit, at least as far as cube-sats that can skip on the atmosphere for a bit go. There's also https://en.wikipedia.org/wiki/Copenhagen_Suborbitals https://en.wikipedia.org/wiki/Copenhagen_Suborbitals with their plans for a manned suborbital flight, their rocket (although it blew up) could also be used as a base for amateur orbital flights. I'm not entirely sure what is so surprising here, it's a question of money at this point, but amateur rockets are quick close to being able to reach space, this doesn't mean that they will be commercial viable or even viable for any non-anecdotal use.

avmich 11y ago
LEO difficulty is mostly speed, not height. SpaceShipOne flew above 100 km - but it's a long way to reach Vostok-1 performance, as delta-V for SS1 is more than four times less than for Vostok-1.
mikeash 11y ago
What's surprising is that amateur rockets can reach space on suborbital trajectories but none of them come even remotely close to orbit. Altitude is the easy part, relatively speaking. Getting the speed for LEO is the hard part. The GoFast rocket you linked to has demonstrated up to 4,200MPH. Low earth orbit requires about 17,000MPH. That factor of ~4 requires massively larger rockets, because the rocket equation is cruel. Each incremental increase in final speed requires an exponential (actually using this word correctly, for once) increase in the amount of fuel. Fuel in return requires more hardware like tanks and engines, which means yet more fuel is required, etc. Edit: using RP-1 as the fuel in a single stage, getting to 4,200MPH requires that 45% of your total rocket mass be fuel at launch. Getting to LEO requires 94% of the mass to be fuel.
- dogma1138 11y ago
  Yes I understand it quite well, ~20% scale up which is 20% longer, and 20% wider is quite a big change in mass. The big problem is either staging or being able to shift it to a shallow enough trajectory and get enough velocity, but it might be solvable if you can do a zoom climb type launch from an aircraft similar to how Pegasus/SpaceShip One do it. https://en.wikipedia.org/wiki/Pegasus_(rocket) https://en.wikipedia.org/wiki/Pegasus_(rocket)
  - mikeash 11y ago
    How is a 20% scale up going to get you anywhere close to the extra dv needed? 20% on each dimension is still only 70% more rocket, where you need something more like 10x more rocket to make that leap.
  - lutorm 11y ago
    That's 1.2^3, or 70% more... not even close. Also remember that making the rocket bigger also increases the dry mass, which further increases the fuel requirements. The difference in ISP between a liquid-fueled rocket (say 450s for LH2/LO2, 350s for RP1/LO2) and solid propellant (250s) makes a huge difference to your prop fraction. To get, say 8km/s delta-v, just plug it into the rocket equation: LH2/LO2: 6.1 initial/final mass. RP1/LO2: 10.3 solids: 26 Staging can help this a bit, but the basic fact remains. It's very difficult to get useful payload to orbit with solids.
fivesigma 11y ago
A 160km altitude cubesat would require a minimum 7.8km/s of delta-v. Anything less than that and it will burn up in the atmosphere after a few orbits. GoFast Rocket achieved a top speed of 1.6 km/s (3580 mph). It needs to go 4 times faster, and no, a 20% larger rocket won't cut it. 20000% larger by mass would be more like it. It also needs a 2nd stage burning liquid propellant with high specific impulse which is way, way harder to build than a dumb solid propellant rocket. Noone (not even governments) has been able to send a single stage rocket to orbit.
- dogma1138 11y ago
  Israel and India use 3 stage HTPB solid rockets for space launches you don't need to have a 2nd stage liquid fuel rocket to get to orbit. Also 20000% larger by mass? want to be a bit more serious? if you scale this rocket by 35-40% you get to about the size of the first stage of the Shavit solid fuel space launcher which is a commercial space launcher. You do understand how scaling works on 3 dimensional objects right?
  - fivesigma 11y ago
    10000% by mass is a more accurate figure and since you like dimensional scaling, that's 364% scaling. The GoFast Rocket weighs 350kg. Scaling it by 35% (1.35 ^ 3) would make it weigh 861kg. The rocket you mentioned weighs between 30 and 70 _tons_. I'm not sure you understand how anything works.
    - simoncion 11y ago
      > I'm not sure you understand how anything works. Based on my previous conversations with this guy, this seems to be a recurring pattern. I do hope that he's using these conversations to recognize and fill in the holes in his knowledge. :)
  - mikeash 11y ago
    What good does it do to scale something up to the size of the first stage of a four stage rocket? Those other three stages are pretty important to that whole putting-stuff-in-orbit business. Edit: I see Shavit can also launch in a three stage configuration. My overall point remains.
btilly 11y ago
I'm not sure what your math is because you're not showing us any. But it sure doesn't agree with mine. Math is done with numbers, so let me give you actual numbers. You can double-check these against https://en.wikipedia.org/wiki/Orbital_speed#Tangential_velocities_at_altitude https://en.wikipedia.org/wiki/Orbital_speed#Tangential_veloc.... Standing on the surface of the Earth your potential energy relative to infinity is −62.6 MJ/kg. If you were standing still 160 km up, you've added roughly 1.6 MJ/kg. (You climbed 160,000 meters against a force of 9.8 Newtons per meter.) To then go into orbit you have to add a sideways velocity of about 7800 m/s. Apply the famous kinetic energy = 0.5mv^2 and going into orbit requires 30.4 MJ/kg of energy. That's 19x more energy than is required to get to altitude! But it gets worse! If you're going to go into orbit, you need to think about getting down. Going up we go straight up out of the air and then add sideways velocity. Going down we let the atmosphere do most of the work of slowing us down. But in order to survive that you need a much more rugged device with a heavy heat shield. You're now both doing more work per mass lifted AND you're lifting more mass! But we're not done with the bad news. A fundamental fact about rockets is that you have to lift the fuel you use later in the launch. That means that as you're increasing the final velocity you're getting an ever-decreasing ratio of energy expended to useful kinetic energy for your payload. Put it all together and actual rocket scientists have told me that it is about 100x easier to get to orbital altitude than it is to actually get into orbit. That said, LEO is the half-way point. Getting into LEO takes about as much energy as it does to launch from LEO entirely out of Earth's orbit.
TeMPOraL 11y ago
A honest hint: spend some hours in Kerbal Space Program. It does wonders for one's intuitions about rockets and its calibration with reality.