Newbie help with getting into orbit

35 posts in this topic

All,

1) How do you know when you are in orbit?
2) Once in orbit, how do you get out so you can get back to earth?  Do you need a rocket?
3) What is the difference between orbit and ground speed?

I have been watching various videos by Scott Manley.

Thx
jonpfl

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You are in orbit of Kerbin when your path goes entirely around the planet without dipping below 70kms in altitude. That's because the atmosphere goes up to 70kms, so if you pass into it you will be slowed and eventually fall out of orbit.

Getting up to orbit requires reaching roughly 2200m/s in speed, which can be hard to do. But once there it's really easy to get back out of orbit again. Point your ship opposite the current direction of your motion and burn the engine a little bit until your path line passes below 70kms again at some point. Because that's where the air is (see above), and it will slow your ship to an eventual landing. The lower you set the dip in your path, the hotter your fall will be, so don't go too low.

The planet is rotating east at 174m/s, so your speed relative to the ground is your orbital speed +174m/s east. So an orbital speed (heading east) of 2200m/s would measure 2026m/s in ground speed.

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1 hour ago, Vanamonde said:

You are in orbit of Kerbin when your path goes entirely around the planet without dipping below 70kms in altitude. That's because the atmosphere goes up to 70kms, so if you pass into it you will be slowed and eventually fall out of orbit.

Getting up to orbit requires reaching roughly 2200m/s in speed, which can be hard to do. But once there it's really easy to get back out of orbit again. Point your ship opposite the current direction of your motion and burn the engine a little bit until your path line passes below 70kms again at some point. Because that's where the air is (see above), and it will slow your ship to an eventual landing. The lower you set the dip in your path, the hotter your fall will be, so don't go too low.

The planet is rotating east at 174m/s, so your speed relative to the ground is your orbital speed +174m/s east. So an orbital speed (heading east) of 2200m/s would measure 2026m/s in ground speed.

Can you expand on what you mean by "set the dip in your path"?

Thx
jonpfl

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There is a technical term, "periapsis," which is the lowest point in an orbit. Because I didn't know if you were familiar with that term, I called it a dip. If it is under 70kms at Kerbin, you will be passing into the atmosphere and increasingly slowed the lower you go, which is bad if you're trying to stay in orbit but saves fuel if you are trying to land.

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3 hours ago, jonpfl said:

All,

1) How do you know when you are in orbit?
2) Once in orbit, how do you get out so you can get back to earth?  Do you need a rocket?
3) What is the difference between orbit and ground speed?

I have been watching various videos by Scott Manley.

Thx
jonpfl

1) How do you know when you are in orbit?

The blue line which shows your course must be above the height of any atmosphere (on a world with an atmosphere) and above the height of any high ground (on worlds which don't have an atmosphere.)
In the case of Kerbin it must be higher than 70K.
Both ap and pe on the blue line.
ap is short for apoapsis. That is the highest part of your course.
pe is short for periapsis. That is the lowest part of your course.
If you can not see a pe at all... then it is still below the surface of the ground of the planet and you are not in orbit.

2) Once in orbit, how do you get out so you can get back to earth?  Do you need a rocket?

In KSP The craft will stay in orbit for ever. To get back you have to point your ship the other way and fire your engines until your pe goes below 70K.
How far below 70K determines how fast your ship goes back down.
You need fuel to go up to orbit and you need fuel to come back down again.
If you are in an orbit which is going around in the same direction as the planet is spinning it as called a 'prograde orbit' if you are going around the opposite way to the direction that the world is spinning it as called a 'retrograde orbit'
You came up to orbit by flying east and thrusting 'prograde' and so to return you must thrust 'retrograde.' This will make your pe fall on the opposite side of the world to you It will eventually go into the ground and you will not be able to see it any more. Your pe will be a negative number.
With a rocket you will usually detach the fuel and engine before entering the atmosphere so that your heat shield can protect your capsule as it comes back.
With a space plane your you return the same way except that you need a longer descent. Your ship will feel the heat of re-entry longer but not get as hot as quickly as a rocket.

3) What is the difference between orbit and ground speed?

Orbital velocity is how fast you are flying through space in orbit compared to the speed that the ground is moving below you of whatever world you are orbiting.

If you go into your street and throw a ball straight out in front of you. Going for distance rather than height. It will go so far and then come down to the ground again. But it's flight path was an arc.
Now imagine that the arc was bigger and eventually the ball would fall over the horizon. If you throw it so hard that the horizon falls at the same speed as the arc of your ball (and air wasn't rubbing against it slowing it down) then it would be in orbit.
If you threw it too fast, it would escape the gravity of earth completely and go off into outer space.

Any more problems.. let me know.

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Wow, awesome info guys!!

Quick question, when designing a spaceship to go into orbit, is it trial and error (in regards to how much fuel, which engines,etc)?  Or are there things to look for to help (ie stats for engines, etc)?

Thx
jonpfl

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43 minutes ago, jonpfl said:

Quick question, when designing a spaceship to go into orbit, is it trial and error (in regards to how much fuel, which engines,etc)?  Or are there things to look for to help (ie stats for engines, etc)?

Once you get the hang of it, it's not trial and error anymore.

1. Your thrust-to-weight ratio ("TWR") must be greater than 1 to be able to lift off the ground
2. You need around 3400 m/s of delta V to get through the atmosphere and up to orbital speed
3. An efficient launch starts pointing straight up, ends pointing horizontally, and turns gradually, generally by following the craft's prograde vector

Delta V, to sum up, is about how much your ship can speed up with the engines and fuel it has. You can find many other threads on the forum going into great detail about delta V.

Both TWR and delta V can be calculated by hand using information available in the game, or automatically with user-created mods such as Kerbal Engineer Redux. When dealing with multiple stages (parts of rockets that are detached and dropped), #1 applies to each stage individually (your TWR should be >1 at all times during flight unless you're doing something fancy), and #2 applies across all stages (you need enough total delta V, calculated by summing the delta V of the individual stages).

You can get stats for all the parts by right clicking them in the parts list. The important ones for launch engines are mass, thrust, and Isp (specific impulse).

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For me, everything was trial-and-error until I learned the ropes.

When you start, use your mistakes as a learning experience.

• Just built an under-powered rocket? It's too heavy. Try a different engine, or maybe multiple engines. Some perform better in atmosphere versus a vacuum.
• Ran out of fuel? Check what kind of engines you're using (some are gas-guzzlers). Perhaps you should add more fuel tanks, but then consider what the added weight will do for those engines you picked out.
• Rocket getting wobbly and twisting up? Consider adding struts to strengthen it.
• Rocket tumbles end-over-end? Make sure you have a weighted tip, like a dart. Think about what happens when your fuel burns off and the center of mass shifts. Consider the effects of staging on the balance of your ship as well.
• Can't control your rocket? Make sure you've added a pod with SAS control and make sure your engines can gimbal (aim themselves). Consider adding tail fins as a last resort. Don't bother trying to fix it with RCS, either. Make sure you've enabled symmetry mode when attaching radial parts to balance your weight, otherwise it'll always pull in one direction over another. Check if you've designed something that is aerodynamically unstable.
• Still can't control your rocket? Make sure you didn't use overpowered solid rocket boosters (SRB), which cannot gimbal and can make steering sluggish.
• Rocket overheats and explodes? Try throttling back.

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On 5/12/2017 at 3:03 PM, Vanamonde said:

There is a technical term, "periapsis," which is the lowest point in an orbit. Because I didn't know if you were familiar with that term, I called it a dip. If it is under 70kms at Kerbin, you will be passing into the atmosphere and increasingly slowed the lower you go, which is bad if you're trying to stay in orbit but saves fuel if you are trying to land.

I looked at your ship but I couldn't tell what parts you were using.  I am still at the beginning of the game so I am assuming I don't have access to your parts.

Anyway, my next mission is to orbit and I have tried about 20 times and keep running out of fuel.  I am curious if you could give me a good design with some beginner parts and I can go from there.  Should I be using solid fuel rockets to take off and then switch to liquid fuel.  I keep adding more liquid fuel but I assume at that point I weigh too much.

Getting pretty frustrated (I tried to follow your guide by turning east).

Also, I noticed on your screen it says time until you hit the peak, I don't have that on my screen.  Also, when I try to move around to view the planet in map mode, it doesn't seem to move on the X, Y and Z axis, I am having problems clicking and moving the mouse.  It doesn't seem to be free scroll.

Thx
jonpfl

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I made that guide in sandbox mode where you are not limited in part choices and have full flight information displays. It might be easier for you to figure this out there.

I'm not sure what you're describing about the scrolling. In this game, the camera is always centered on something, whether that's a ship or a world. You can zoom in and out and rotate around it, but not scroll to the sides.

And if your rocket is not going far enough, try making it lighter rather than adding fuel. Too much fuel actually holds the ship back. Maybe share a picture of your ship?

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On 5/12/2017 at 8:58 PM, Daveroski said:

2) Once in orbit, how do you get out so you can get back to earth?  Do you need a rocket?

In KSP The craft will stay in orbit for ever. To get back you have to point your ship the other way and fire your engines until your pe goes below 70K.
How far below 70K determines how fast your ship goes back down.
You need fuel to go up to orbit and you need fuel to come back down again.
If you are in an orbit which is going around in the same direction as the planet is spinning it as called a 'prograde orbit' if you are going around the opposite way to the direction that the world is spinning it as called a 'retrograde orbit'
You came up to orbit by flying east and thrusting 'prograde' and so to return you must thrust 'retrograde.' This will make your pe fall on the opposite side of the world to you It will eventually go into the ground and you will not be able to see it any more. Your pe will be a negative number.
With a rocket you will usually detach the fuel and engine before entering the atmosphere so that your heat shield can protect your capsule as it comes back.
With a space plane your you return the same way except that you need a longer descent. Your ship will feel the heat of re-entry longer but not get as hot as quickly as a rocket

on top of that i recomend 35Km Periapsis as that usualy works for the soyuz i go around 25Km as the impactless separation ruins the descent

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8 hours ago, Vanamonde said:

I made that guide in sandbox mode where you are not limited in part choices and have full flight information displays. It might be easier for you to figure this out there.

I'm not sure what you're describing about the scrolling. In this game, the camera is always centered on something, whether that's a ship or a world. You can zoom in and out and rotate around it, but not scroll to the sides.

And if your rocket is not going far enough, try making it lighter rather than adding fuel. Too much fuel actually holds the ship back. Maybe share a picture of your ship?

It seems like when I got to planet mode, my camera doesn't freely move by rmb and moving the mouse.  It seems to not rotate around the world freely.  It seems like it will scroll around to a point and not go further (like it isn't truly freeform).  Something seems wonky, would I have some weird setting messing it up?

I will post a picture later of my ship, going to try some more.

Thx

jonpfl

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Posted (edited)

9 hours ago, jonpfl said:

Anyway, my next mission is to orbit and I have tried about 20 times and keep running out of fuel.  I am curious if you could give me a good design with some beginner parts and I can go from there.  Should I be using solid fuel rockets to take off and then switch to liquid fuel.  I keep adding more liquid fuel but I assume at that point I weigh too much.

Getting pretty frustrated (I tried to follow your guide by turning east).

Also, I noticed on your screen it says time until you hit the peak, I don't have that on my screen.  Also, when I try to move around to view the planet in map mode, it doesn't seem to move on the X, Y and Z axis, I am having problems clicking and moving the mouse.  It doesn't seem to be free scroll.

Thx
jonpfl

This rocket will almost fly itself to orbit.

The tanks have been set up so that they are used from the bottom upwards helping to keep the rocket top heavy.
Most of the ablator has been removed from the heat shield as it will not use very much on re-entry.
Set the tank fuel priorities and the ablator of your heat shield as I have done here.
I did not remove the monoprop from the pod. To keep it the same neither should you.

Launch this rocket with or without launch clamps. Wait a few seconds for the physics to settle down before turning on SAS (T) and applying full thrust. (Z)

Step 1 : apply SAS (T) and full Thrust (Z)

Step 2: Stage the engines (Spacebar)

Step 3: As soon as the engines fire tap your (D) key 10 times quickly.
Tap it. Don't hold it down for a second.

Step 4: When the rocket reaches 60m/s turn off SAS (T)
That's right -turn it off.  Gravity will slowly turn your rocket toward the east as you fly.

Step 5: When the first stage runs out of fuel Stage the rocket twice (Engines stop then  STAGE count 1-2-3 then STAGE again)
This will help stop the stage you just dropped from being blown up by your engine.

Step 6: At 30000m cut your engines (X)
By this time your rocket will be experiencing some heat. and heat indicators will be showing. Don't worry about it.

Step 7: Go to map view (M) now you have an apoapsis well above 70K. Set a node at ap to circularise the orbit.

Step 8 Go to ship view (M) and turn on SAS (T)

Step 9: aim for the blue node you created in step 7

Step 10 Circularise the orbit.

I have just now built this ship and tested it several times in the manner listed above.
If you carry out the instructions as I have given them to you, you should have no problem getting it to orbit.

Edited by Daveroski
Typo
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Posted (edited)

Here is the ship I just built to try and get into orbit.

14 minutes ago, Daveroski said:

This rocket will almost fly itself to orbit.

The tanks have been set up so that they are used from the bottom upwards helping to keep the rocket top heavy.
Most of the ablator has been removed from the heat shield as it will not use very much on re-entry.
Set the tank fuel priorities and the ablator of your heat shield as I have done here.
I did not remove the monoprop from the pod. To keep it the same neither should you.

Launch this rocket with or without launch clamps. If without wait a few seconds for the physics to settle down before turning on SAS (T) and applying full thrust. (Z)

Step 1 : apply SAS (T) and full Thrust (Z)

Step 2: Stage the engines (Spacebar)

Step 3: As soon as the engines fire tap your (D) key 10 times quickly.
Tap it. Don't hold it down for a second.

Step 4: When the rocket reaches 60m/s turn off SAS (T)
That's right -turn it off.  Gravity will slowly turn your rocket toward the east as you fly.

Step 5: When the first stage runs out of fuel Stage the rocket twice (Engines stop then  STAGE count 1-2-3 then STAGE again)
This will help stop the stage you just dropped from being blown up by your engine.

Step 6: At 30000m cut your engines (X)
By this time your rocket will be experiencing some heat. and heat indicators will be showing. Don't worry about it.

Step 7: Go to map view (M) now you have an apoapsis well above 70K. Set a node at ap to circularise the orbit.

Step 8 Go to ship view (M) and turn on SAS (T)

Step 9: aim for the blue node you created in step 7

Step 10 Circularise the orbit.

I have just now built this ship and tested it several times in the manner listed above.
If you carry out the instructions as I have given them to you, you should have no problem getting it to orbit.

Sorry, I am not in sandbox mode so I don't have access to most of those items (I think).  Some of the stuff you typed I didn't even understand.  What are fuel priorities?

Thx
jonpfl

Edited by jonpfl

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9 minutes ago, jonpfl said:

Here is the ship I just built to try and get into orbit.

Sorry, I am not in sandbox mode so I don't have access to most of those items (I think).  Some of the stuff you typed I didn't even understand.  What are fuel priorities?

Thx
jonpfl

Scroll down and select AVANCED TWEAKABLES

Then you will see what I am talking about.

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Posted (edited)

Hello @jonpfl,

I'm afraid that rocket will not get you into orbit, no matter how hard you try; you'll need at least three of those FLT-400 tanks. If you're math is decent, we can show you how to calculate if you can make it into orbit or not.

In addition, although many will not agree with me, I see Career mode as a challenge mode, not as a learn mode. While you get "eased" into the various parts, you will also have the challenge of not having the right parts to make a simple rocket to get you into orbit. My personal suggestion is to start with sandbox mode (just try to keep it simple & light!) , get a hang of getting into orbit and then advance to career mode (but again, that's personal opinion. Not everyone will agree with me on that).

Good luck!

Edited by Kerbart

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Posted (edited)

7 hours ago, Kerbart said:

Hello @jonpfl,

I'm afraid that rocket will not get you into orbit, no matter how hard you try; you'll need at least three of those FLT-400 tanks. If you're math is decent, we can show you how to calculate if you can make it into orbit or not.

In addition, although many will not agree with me, I see Career mode as a challenge mode, not as a learn mode. While you get "eased" into the various parts, you will also have the challenge of not having the right parts to make a simple rocket to get you into orbit. My personal suggestion is to start with sandbox mode (just try to keep it simple & light!) , get a hang of getting into orbit and then advance to career mode (but again, that's personal opinion. Not everyone will agree with me on that).

Good luck!

So you suggest three FLT-400 tanks and anything else?  Do I need a solid fuel rocket to start?

Here is my last attempt (with the rocket you pointed out was weak), do you see anything else I need to do?

7 hours ago, Kerbart said:

Hello @jonpfl,

I'm afraid that rocket will not get you into orbit, no matter how hard you try; you'll need at least three of those FLT-400 tanks. If you're math is decent, we can show you how to calculate if you can make it into orbit or not.

In addition, although many will not agree with me, I see Career mode as a challenge mode, not as a learn mode. While you get "eased" into the various parts, you will also have the challenge of not having the right parts to make a simple rocket to get you into orbit. My personal suggestion is to start with sandbox mode (just try to keep it simple & light!) , get a hang of getting into orbit and then advance to career mode (but again, that's personal opinion. Not everyone will agree with me on that).

Good luck!

And yes, my math is decent but rusty (got a computer engineering degree in the early 90s).

Also, my first job was doing computer work for NASA for a couple of years.

Thx

Edited by jonpfl

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24 minutes ago, jonpfl said:

So you suggest three FLT-400 tanks and anything else?  Do I need a solid fuel rocket to start?

Let's calculate the TWR and delta V of your rocket:

I'll use the mod Kerbal Engineer Redux to simplify the process:

The TWR is 2.74 at liftoff and jumps to 5 after staging, so that's acceptable. It's a bit on the high side, but that won't keep you out of space in itself, and a high TWR gives you the option to add more fuel without having to increase thrust. The total delta V however is 2397 m/s, which is less than the 3400 m/s you need. So we can regard this problem as a lack of about 1000 m/s of delta V. If we add those extra FL-T400 tanks, it changes to this:

Now the total delta V is 3597 m/s, which should be enough, and the TWR is still comfortably greater than 1. So this fix checks out number-wise. You might encounter other issues, but they'll have different solutions depending on what they are.

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48 minutes ago, jonpfl said:

So you suggest three FLT-400 tanks and anything else?  Do I need a solid fuel rocket to start?

@HebaruSan showed (domo arigato!) that with three FLT-400's you should be able to get into orbit. KSP is notorious for not showing data that is pretty essential if you want to take building beyond wildly guessing, and the Kerbal Engineer mod does a good job in giving you that data.

There are two important factors into "making it into orbit":

• Enough fuel: this is the delta-V bit. Just as we can express the fuel capacity of a car in "miles" because it's more meaningful (one gallon in a Ford Fiesta gets you further than one gallon in a Ford F150), fuel for a rocket is expressed as "delta-v" which is the total change in velocity a rocket can achieve. Without diving into details, you will need about 3500 m/s delta-V to get into low Kerbin orbit.
• Enough thrust: DV is a theoretical number and is based on some assumptions. One of those assumptions is that you're moving; if your rocket isn't powerful enough it's not getting of the launch pad! Thrust-to-weight (TWR) tells you that. And as pointed out by Mr. HebaruSan, you'll need something "comfertable" north of 1.0 -- the 1.8 is about the sweetspot of "enough" without overdoing (and without spending needless cash and weight on too much thrust).

Do you need a solid fuel rocket to start? No. In general, Liquid Fuel rockets will get you better bang for the buck (weight) than solid rocket boosters. Because of their weight, the contribution in DV is very little. But... you need to get off the pad. And that's where SRB's excel: they offer tremendous TWR's. So, getting started with an SRB (or a pair strapped to the side of your rocket) is not a bad idea just to get that initial bit of speed.

As you get higher you get into a thinner atmosphere, and as your flight path is more horizontal you don't have to fight gravity as much, and your TWR can be lower.

Also keep in mind that Kerbal Engineer lists data based on vacuum performance, which for some engines is significantly better than for others. The LV-909 for instance (the small 1.25m engine) has an atrocious atmospheric performance. The two larger 1.25m engines that you have are a lot better in atmospheric flight, in that respect (but the LV-909 beats them once in space when it comes to efficiency)

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1 hour ago, jonpfl said:

And yes, my math is decent but rusty (got a computer engineering degree in the early 90s).

Take a look at this section of the wiki.

If we take your original design into account, then we can say that there's two stages (let's ignore the capsule when it does reentry).

The first (top) stage, is the capsule, FL-T400 and engine. I'm including heat shield, parachute, and four wings for stabilizing, as you did, and a decoupler for the capsule. And I'm assuming an LV-T45 Swivel (as it gimbals and gives you better control)

Total weight without fuel: 3080kg
Total weight with fuel: 5080 kg
Efficiency (Isp) of the engine: 320s -- Real efficiency is measured in exhaust velocity, but then you have that whole unit (m/s or ft/s) thing. So it's often divided by one standard g (9.81m/s) to get a meassurement in seconds, which is the same regardless of the units you're using. However, that standard g then comes back in delta V calculations:

dV = Isp×g×[ln(total weight with fuel) - ln(total weight without fuel)]
Plugging in the numbers gives us:
dV = 320 × 9.81 × [ln 5080 - ln 3080] = 1570 m/s

Now we can calculate the dV for the bottom stage.

Total weight without fuel: 5080 kg + 800  = 5880 kg (Total "wet" weight of the top stage, and dry weight of decoupler and RT-10 booster)
Total weight with fuel: 8692 kg
Isp of RT-10 SRB: 170 (numbers quoted are 170-195, 170 being at sea level which is where we use them. Note how much lower than the Swivel!)

dV = Isp×g×[ln(total weight with fuel) - ln(total weight without fuel)]
Plugging in the numbers gives us:
dV = 170 × 9.81 × [ln 8692 - ln 5880] = 650 m/s

If you add another stage, you can calculate the dV for that one in a similar matter.

Total dV of this rocket: 2220 m/s — not enough to take it into orbit. Which is why you need three FL-T400's.

Kerbal Engineer is great because it does these calculations for you, but doing the math "by hand" a couple of times is useful because you get a better insight in how things work. The rocket equation teaches a couple of things that are not all that intuitive:

• The number of engines doesn't increase or decrease total efficiency. In fact, more engines means more weight which will lower your dV. But very low TWR's can be bad too!
• A high Isp looks great but the dry mass the engine adds to the total weight of the ship is important as well. This is why the LV-909 is such a great performer (it weighs nearly nothing) for small (1-kerbal) ships and why the LV-N nuclear engine isn't (for small ships), because its high weight (despite superior Isp)
• The tyranny of the rocket equation means that, given a certain Isp, you need a certain fuel-to-dry-mass ratio to achieve a goal (say, low Kerbin Orbit). There's no way around that. Also, those ratios tend to propagate through stages. The lighter your space craft (top stage) is, the bigger the weight savings at the bottom. Bringing an extra 1000 m/s of dV with you "just in case" means paying a horrible price in fuel and mass at the bottom stages.

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2 hours ago, HebaruSan said:

Let's calculate the TWR and delta V of your rocket:

I'll use the mod Kerbal Engineer Redux to simplify the process:

The TWR is 2.74 at liftoff and jumps to 5 after staging, so that's acceptable. It's a bit on the high side, but that won't keep you out of space in itself, and a high TWR gives you the option to add more fuel without having to increase thrust. The total delta V however is 2397 m/s, which is less than the 3400 m/s you need. So we can regard this problem as a lack of about 1000 m/s of delta V. If we add those extra FL-T400 tanks, it changes to this:

Now the total delta V is 3597 m/s, which should be enough, and the TWR is still comfortably greater than 1. So this fix checks out number-wise. You might encounter other issues, but they'll have different solutions depending on what they are.

Wow, great stuff!

I still have vanilla KSP but this sounds like a great mod to install huh?  Is this information available without the mod or do you have to calculate on your own?

Thx

jonpfl

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1 hour ago, Kerbart said:

Take a look at this section of the wiki.

If we take your original design into account, then we can say that there's two stages (let's ignore the capsule when it does reentry).

The first (top) stage, is the capsule, FL-T400 and engine. I'm including heat shield, parachute, and four wings for stabilizing, as you did, and a decoupler for the capsule. And I'm assuming an LV-T45 Swivel (as it gimbals and gives you better control)

Total weight without fuel: 3080kg
Total weight with fuel: 5080 kg
Efficiency (Isp) of the engine: 320s -- Real efficiency is measured in exhaust velocity, but then you have that whole unit (m/s or ft/s) thing. So it's often divided by one standard g (9.81m/s) to get a meassurement in seconds, which is the same regardless of the units you're using. However, that standard g then comes back in delta V calculations:

dV = Isp×g×[ln(total weight with fuel) - ln(total weight without fuel)]
Plugging in the numbers gives us:
dV = 320 × 9.81 × [ln 5080 - ln 3080] = 1570 m/s

Now we can calculate the dV for the bottom stage.

Total weight without fuel: 5080 kg + 800  = 5880 kg (Total "wet" weight of the top stage, and dry weight of decoupler and RT-10 booster)
Total weight with fuel: 8692 kg
Isp of RT-10 SRB: 170 (numbers quoted are 170-195, 170 being at sea level which is where we use them. Note how much lower than the Swivel!)

dV = Isp×g×[ln(total weight with fuel) - ln(total weight without fuel)]
Plugging in the numbers gives us:
dV = 170 × 9.81 × [ln 8692 - ln 5880] = 650 m/s

If you add another stage, you can calculate the dV for that one in a similar matter.

Total dV of this rocket: 2220 m/s — not enough to take it into orbit. Which is why you need three FL-T400's.

Kerbal Engineer is great because it does these calculations for you, but doing the math "by hand" a couple of times is useful because you get a better insight in how things work. The rocket equation teaches a couple of things that are not all that intuitive:

• The number of engines doesn't increase or decrease total efficiency. In fact, more engines means more weight which will lower your dV. But very low TWR's can be bad too!
• A high Isp looks great but the dry mass the engine adds to the total weight of the ship is important as well. This is why the LV-909 is such a great performer (it weighs nearly nothing) for small (1-kerbal) ships and why the LV-N nuclear engine isn't (for small ships), because its high weight (despite superior Isp)
• The tyranny of the rocket equation means that, given a certain Isp, you need a certain fuel-to-dry-mass ratio to achieve a goal (say, low Kerbin Orbit). There's no way around that. Also, those ratios tend to propagate through stages. The lighter your space craft (top stage) is, the bigger the weight savings at the bottom. Bringing an extra 1000 m/s of dV with you "just in case" means paying a horrible price in fuel and mass at the bottom stages.

Is all this info about stats necessary from researching on the web?  This is stuff I will learn in time?

Thx

jonpfl

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1 hour ago, jonpfl said:

I still have vanilla KSP but this sounds like a great mod to install huh?

In my opinion, yes.

1 hour ago, jonpfl said:

Is this information available without the mod or do you have to calculate on your own?

You can calculate it by hand using information available in the game: mass of vessel, mass of fuel, thrust of engines, specific impulse of engines. That's effectively what @Kerbart did in his subsequent posts. (Differences between our numbers are due to his inclusion of a heat shield, which I didn't see in your screenshot, and using a slightly different engine; otherwise we would have calculated the same numbers.)

TWR and delta V themselves are not shown directly anywhere in the stock game, which is a common complaint/request of players.

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2 hours ago, HebaruSan said:

In my opinion, yes.

You can calculate it by hand using information available in the game: mass of vessel, mass of fuel, thrust of engines, specific impulse of engines. That's effectively what @Kerbart did in his subsequent posts. (Differences between our numbers are due to his inclusion of a heat shield, which I didn't see in your screenshot, and using a slightly different engine; otherwise we would have calculated the same numbers.)

TWR and delta V themselves are not shown directly anywhere in the stock game, which is a common complaint/request of players.

Thanks for the help!!

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Posted (edited)

8 hours ago, jonpfl said:

So you suggest three FLT-400 tanks and anything else?  Do I need a solid fuel rocket to start?

Here is my last attempt (with the rocket you pointed out was weak), do you see anything else I need to do?

And yes, my math is decent but rusty (got a computer engineering degree in the early 90s).

Also, my first job was doing computer work for NASA for a couple of years.

Thx

judging by the trajectory, you've got plenty of altitude there for space, but you are too steep.  you'll get more downrange velocity off of a shallower climb, which makes achieving orbit easier, so trade some of that altitude for speed.  For the first try just get it above 70km.  Also, you need to do another burn as you go over the top of the trajectory to actually put it in orbit.

Edited by Capt. Hunt