Graveyard challenge.

[bobbblair123]

The situation is thus. Almost circular orbit with mercury. I was looking to enter Mercury SOI just barely moving. Got impatient. This is were it stands. Calculate ∆V remaining. Which the formula you guys showed me doesn't make sense. G 3.7 m/s² (mercury gravity) times 240 Isp (kolibri) times 7.3 tons Me = 2664 meters of ∆V. That ain't right. I need burn time for a kolibri with 3.5 tons of fuel. There is no way in hell I have 2664 m/s of ∆V. But that's the formula y'all tried to beat into my head.

 

[Pink]

G is always earth gravity, it's a constant.


Handy calculator to see if you're on the right track:
Delta-V Calculator

I'm going to double check if the formula I found that uses isp instead of exhaust velocity is the right one.

 

[Pink]

 

[Altaïr]

Mooncrasher is right. Your main task in practice is to calculate the "finish mass" (aka dry mass), and the "starting mass" (aka wet mass), which is finish mass + fuel mass. Be careful, you have to account for everything in the calculation of those masses: each part (landing leg, solar panel, RCS...) has its own mass.
Be careful about fuel tanks too: 10% of their mass are for the tank itself and should be counted as dry mass, and the remaining 90% are fuel: a 10 tons fuel tank actually holds 9 tons of fuel, and the tank itself weighs 1 ton.

 

[bobbblair123]

Mooncrasher is right. Your main task in practice is to calculate the "finish mass" (aka dry mass), and the "starting mass" (aka wet mass), which is finish mass + fuel mass. Be careful, you have to account for everything in the calculation of those masses: each part (landing leg, solar panel, RCS...) has its own mass.
Be careful about fuel tanks too: 10% of their mass are for the tank itself and should be counted as dry mass, and the remaining 90% are fuel: a 10 tons fuel tank actually holds 9 tons of fuel, and the tank itself weighs 1 ton.

Sorry. Would help if I included information. I'm calculating 7.3 tons empty mass with 3.5 tons propellant left.

G is always earth gravity, it's a constant.


Handy calculator to see if you're on the right track:
Delta-V Calculator

I'm going to double check if the formula I found that uses isp instead of exhaust velocity is the right one.

Whoa. Wait a minute. ALWAYS use earth gravity constant?

 

[Horus Lupercal]

But that's the formula y'all tried to beat into my head.

Calm down. Not only did we not beat anything into you, you're using the wrong formula. Take a breath, go back, check what you're doing against what you were shown and you'll see where the mistake is.

Whoa. Wait a minute. ALWAYS use earth gravity constant?

As my boy Hans Snape would say

Otherwise you get all kinds of different numbers depending on where you are. Earths gravity is used as a universal gravitational constant so you get the same ΔV number regardless of where in the universe you happen to be (including those places where you're in deep space and the local gravity would be almost zero).

I'm going to double check if the formula I found that uses isp instead of exhaust velocity is the right one.

Yeah, that's the one I use. It's a different way of going about it, but it gives the same results afterwards.

 

[bobbblair123]

The situation is thus. Almost circular orbit with mercury. I was looking to enter Mercury SOI just barely moving. Got impatient. This is were it stands. Calculate ∆V remaining. Which the formula you guys showed me doesn't make sense. G 3.7 m/s² (mercury gravity) times 240 Isp (kolibri) times 7.3 tons Me = 2664 meters of ∆V. That ain't right. I need burn time for a kolibri with 3.5 tons of fuel. There is no way in hell I have 2664 m/s of ∆V. But that's the formula y'all tried to beat into my head.

In same orbital path as mercury obviously not circular orbit around mercury. I was and am looking to get into Mercury's SOI barely moving. I've come close then very next attempt I'm waving hello to Jupiter. The Veega tutorial did not prove as helpful as I hoped. Still looking for gravity maneuvering guidance. Going to try some different keywords and see if anyone is up and responding.

 

[Pink]

Whoa. Wait a minute. ALWAYS use earth gravity constant?

The reason for that is because we're using isp, not the "proper" exhaust velocity.
isp is a comparative value, it's simply exhaust velocity divided by the earth's gravity.

In American units, isp looks like this:
Velocity (ft/s) divided by gravity (ft/s²) which gives a result in time (seconds)

In SI units, isp looks like this:
Velocity (m/s) divided by gravity (m/s²) which gives a result in time (seconds)

That's why it was selected, because it works no matter what units you're using. That way an American scientist could tell a German scientist that this engine has an isp of 260s and they'd understand.

To use isp in the rocket equation, you need to multiply it by earth's gravity again.
260 seconds (s) times gravity (9.81 m/s²) which gives exhaust velocity of 2550 m/s

260 seconds (s) times gravity (32 ft/s²) which gives exhaust velocity of 8320 ft/s

That's why Stef used isp for his game intended to be played around the world.

 

[bobbblair123]

View attachment 54814

What am I missing? Is it I will have to constantly shed velocity gained from gravity? I can see where that could be a problem cause I'm going to be in Mercury's gravity well for awhile.

 

[bobbblair123]

The reason for that is because we're using isp, not the "proper" exhaust velocity.
isp is a comparative value, it's simply exhaust velocity divided by the earth's gravity.

In American units, isp looks like this:
Velocity (ft/s) divided by gravity (ft/s²) which gives a result in time (seconds)

In SI units, isp looks like this:
Velocity (m/s) divided by gravity (m/s²) which gives a result in time (seconds)

That's why it was selected, because it works no matter what units you're using. That way an American scientist could tell a German scientist that this engine has an isp of 260s and they'd understand.

To use isp in the rocket equation, you need to multiply it by earth's gravity again.
260 seconds (s) times gravity (9.81 m/s²) which gives exhaust velocity of 2550 m/s

260 seconds (s) times gravity (32 ft/s²) which gives exhaust velocity of 8320 ft/s

That's why Stef used isp for his game intended to be played around the world.

Ok don't lose me now. How did you get 260 seconds burn time? I don't think I've got 4 minutes at full throttle.

 

[bobbblair123]

I

 

[Pink]

What am I missing? Is it I will have to constantly shed velocity gained from gravity? I can see where that could be a problem cause I'm going to be in Mercury's gravity well for awhile.

Dude, I was just showing you how to calculate the dV at your disposal. I also gave a calculator so you could double check your work. Nothing deeper than that.


As for your mercury troubles, you're going about it the inefficient way. What you want is to use the slingshots to come in as slow as possible right above the mercury surface when you make your mercury capture burn. If you're almost losing paint because of how low you are, you're doing it right.

if you're on the edge of its SOI when you make the capture burn, you're not benefiting from Oberth effect and your fuel will run dry before you land.

If you end up at Jupiter, that means you went the wrong way around Venus.

Ok don't lose me now. How did you get 260 seconds burn time? I don't think I've got 4 minutes at full throttle.

It's not burn time.

But if you want to figure out burn time, the kolibri uses 0.05625 tonnes of fuel per second, according to my experiment.

 

[Altaïr]

Ok don't lose me now. How did you get 260 seconds burn time? I don't think I've got 4 minutes at full throttle.

That's not a burn time. Mooncrasher just talked about 260s as an example. Anyway, Isp is not related with burn time.

In your case, with 3.5 tons of fuel and 7.3 tons of dry mass, your full mass is 10.8 tons, and your final mass 7.3 tons. Then:
Delta-V = 9.8 × 260 × ln(10.8/7.3) = 998 m/s

 

[Pink]

That's not a burn time. Mooncrasher just talked about 260s as an example. Anyway, Isp is not related with burn time.

In your case, with 3.5 tons of fuel and 7.3 tons of dry mass, your full mass is 10.8 tons, and your final mass 7.3 tons. Then:
Delta-V = 9.8 × 260 × ln(10.8/7.3) = 998 m/s

Uh oh. And he's in very high mercury orbit?
So 250 m/s to get to low mercury orbit, and another.... over 800m/s? to land?

 

[bobbblair123]

Dude, I was just showing you how to calculate the dV at your disposal. I also gave a calculator so you could double check your work. Nothing deeper than that.


As for your mercury troubles, you're going about it the inefficient way. What you want is to use the slingshots to come in as slow as possible right above the mercury surface when you make your mercury capture burn. If you're almost losing paint because of how low you are, you're doing it right.

if you're on the edge of its SOI when you make the capture burn, you're not benefiting from Oberth effect and your fuel will run dry before you land.

If you end up at Jupiter, that means you went the wrong way around Venus.



It's not burn time.
View attachment 54819

But if you want to figure out burn time, the kolibri uses 0.05625 tonnes of fuel per second, according to my experiment.

Getting there. Been trying to just drift down to mercury as I heard someone describe it. Using Venus atmosphere for braking. In your opinion is this mission still achievable? 19% fuel where I sit in orbit around mercury??

That's not a burn time. Mooncrasher just talked about 260s as an example. Anyway, Isp is not related with burn time.

In your case, with 3.5 tons of fuel and 7.3 tons of dry mass, your full mass is 10.8 tons, and your final mass 7.3 tons. Then:
Delta-V = 9.8 × 260 × ln(10.8/7.3) = 998 m/s

There's the problem. Lol you really have to divide wet and empty mass not just subtract? Another formula I won't soon be forgetting. Thank you for your time Altair and Mooncrasher.

 

[bobbblair123]

Uh oh. And he's in very high mercury orbit?
So 250 m/s to get to low mercury orbit, and another.... over 800m/s? to land?

I don't like the look in your eyes

 

[bobbblair123]

I don't like the look in your eyes

cursus offerre majis

 

[Altaïr]

But if you want to figure out burn time, the kolibri uses 0.05625 tonnes of fuel per second, according to my experiment.

There's an even simpler way for that. If FC is fuel consumption, then you have:
Thrust = FC × g × Isp
But that's when thrust is expressed in kN!

When Thrust is expressed in tons, then it's even simpler: Thrust = FC × Isp
So fuel consumption is simply Thrust/Isp.

In the case of the Kolibri, you get 0.05769 tons/s, you were close

Using Venus atmosphere for braking. In your opinion is this mission still achievable? 19% fuel where I sit in orbit around mercury??

If you braked using Venus atmosphere when performing a gravity assist, it was a mistake: you reduce the gravitational slingshot power that way. Also, make sure you perform a retrograde flyby of Venus (which is anticlockwise) if you want to reach Mercury. Going clockwise is used for VEEGA, but that one is used to reach cheaply the external solar system (Jupiter and above). This is not what you need.

For your current situation I would say while you're at it, just try it, you'll see the result! But there's a chance that you'll be short on fuel.

 

[bobbblair123]

There's an even simpler way for that. If FC is fuel consumption, then you have:
Thrust = FC × g × Isp
But that's when thrust is expressed in kN!

When Thrust is expressed in tons, then it's even simpler: Thrust = FC × Isp
So fuel consumption is simply Thrust/Isp.

In the case of the Kolibri, you get 0.05769 tons/s, you were close


If you braked using Venus atmosphere when performing a gravity assist, it was a mistake: you reduce the gravitational slingshot power that way. Also, make sure you perform a retrograde flyby of Venus (which is anticlockwise) if you want to reach Mercury. Going clockwise is used for VEEGA, but that one is used to reach cheaply the external solar system (Jupiter and above). This is not what you need.

For your current situation I would say while you're at it, just try it, you'll see the result! But there's a chance that you'll be short on fuel.

Got you. Retrograde. Watch perigee. Atmosphere braking reduces gravity effect.

 

[bobbblair123]

Ok. Round 2. I'm right at what I've been trying to do. Escape Venus SOI basically neutral. My question. Is it more productive to reverse direction and get another assist from venus or forge onwards to mercury? And how can you tell?

 

[TtTOtW]

bobbblair123. Have you seen my VEEGA video?

 

[Altaïr]

Here is how to do it bob:

First, with your ship in LEO (Low Earth Orbit), aim for Venus:


Then, once you reach the transfer point, don't perform the transfer right now, but instead, time-warp again until your projected transfer looks like this:

Personally, I aim for an angle between the Earth and the encounter point around 120°:

This doesn't need to be exact, I didn't use a protractor myself, but the result should be interesting enough to require only a few corrections. Such a transfer is more expensive than a normal one, but it's required to get a sufficient deflexion angle.

Make sure your fly-by is retrograde:


Then time-warp until you enter Venus SOI, and look at the result:


We are very close to the goal! First step is to lower the periapsis, without entering Venus atmosphere, by burning sideways:


We are a bit closer, but that's still not enough. So now we are going to perform a powered slingshot: time-warp until you reach Venus' periapsis, and then burn prograde (yes, you have to accelerate!) until your trajectory encounters Mercury orbit:


Now you just have to get an encounter with Mercury, and it's done. I tried myself the landing maneuver, I had 11% of fuel left, so this should be fine

 

[bobbblair123]

bobbblair123. Have you seen my VEEGA video?

Saw more of a lecture. I did not gather anything useful yet I'm afraid. Altair here is helping things jell together. So there's an actual video? What I saw was two guys discussing some aspects of it

 

[bobbblair123]

Here is how to do it bob:

First, with your ship in LEO (Low Earth Orbit), aim for Venus:
View attachment 54868

Then, once you reach the transfer point, don't perform the transfer right now, but instead, time-warp again until your projected transfer looks like this:
View attachment 54869
Personally, I aim for an angle between the Earth and the encounter point around 120°:
View attachment 54870
This doesn't need to be exact, I didn't use a protractor myself, but the result should be interesting enough to require only a few corrections. Such a transfer is more expensive than a normal one, but it's required to get a sufficient deflexion angle.

Make sure your fly-by is retrograde:
View attachment 54872

Then time-warp until you enter Venus SOI, and look at the result:
View attachment 54873 View attachment 54874

We are very close to the goal! First step is to lower the periapsis, without entering Venus atmosphere, by burning sideways:
View attachment 54875 View attachment 54876

We are a bit closer, but that's still not enough. So now we are going to perform a powered slingshot: time-warp until you reach Venus' periapsis, and then burn prograde (yes, you have to accelerate!) until your trajectory encounters Mercury orbit:
View attachment 54877 View attachment 54878 View attachment 54879

Now you just have to get an encounter with Mercury, and it's done. I tried myself the landing maneuver, I had 11% of fuel left, so this should be fine

✓ nicely done

 

[TtTOtW]

Saw more of a lecture. I did not gather anything useful yet I'm afraid. Altair here is helping things jell together. So there's an actual video? What I saw was two guys discussing some aspects of it

VEEGA 1.5 Edition
In this video, we go to Jupiter. But, to go to Mercury all you have to do is go the opposite way around Venus, at 40.1km. You'll see it works every single time.