J. Peraire, S. Widnall 16.07 Dynamics Fall 2008 Version 2.

Lecture L14 - Variable Mass Systems: The Rocket Equation

In this lecture, we consider the problem in which the mass of the body changes during the motion, that is, m is a function of t, i.e. m(t). Although there are many cases for which this particular model is applicable, one of obvious importance to us are rockets. We shall see that a signicant fraction of the mass of a rocket is the fuel, which is expelled during ight at a high velocity and thus, provides the propulsive force for the rocket. As a simple model for this process, consider the cases sketched in a) through f) of the gure. In a) we consider 2 children standing on a stationary at car. At t = 0, they both jump o with velocity relative to the at car of u. (While this may not be a good model for children jumping o a atcar, it is a good model for the expulsion of mass from a rocket where the mass ow from the choked nozzle occurs at a relative velocity u from the moving rocket.) In b) we consider that they jump o in sequence, each jumping with a velocity u relative to the then velocity of the atcar, which will be dierent for the 2nd jumper since the car has begun to move as a result of jumper 1. For a), we have a nal velocity of the at car as V2R = for b) we have V22 = mu mu + M + 2m M + m (2) 2mu M + 2m (1)

where M is the mass of the atcar, and m is the mass of the jumper. We can see the case b) gives a higher nal velocity. We now consider cases with more blocks; we introduce the notation VN N to be the nal velocity with N blocks coming o one at a time. We use the notation VN R to denote the reference velocity if the N blocks come o together. If we consider 3 blocks, we can see by inspection that V 3R = and V33 = While for 4 blocks V4R = and V44 = mu mu mu mu + + + M + 4m M + 3m M + 2m M + m (6) 4mu M + 4m (5) mu mu mu + + M + 3m M + 2m M + m (4) 3mu M + 3m (3)

Generalizing this to arbitrary N, we can write VN R = and VN N = N mu M + Nm mu M + im (7)

N i=1

(8)

The gure shows the results for V100,100 /V100R , the increase in velocity as each block comes o. We choose 100 blocks with a mass m equal to .01 M; therefore the 100 blocks with m = .01 equal the mass M, the propellant ratio of the rocket is 50%. The nal velocity of the cart is 40% higher than would be obtained if the masses come o together. Momentum is conserved. The sum of the momentum of the cart plus all the individual mass points is equal to zero. However, the momentum of all the particles left behind is of very little interest to us. We care only about the nal velocity of the cart. 2

To analyze this question we must consider a system of variable mass, and the process by which it gains velocity as a result of ejecting mass. While our previous discussion provided a discrete model of this process, we now consider a continuous process, obtaining a relation for v (t) as a function of the momentum interaction between the system and the external world. We shall start by considering a body with velocity v and external forces F , gaining mass at a rate m = dm/dt. Let us look at the process of gaining a small amount of mass dm. Let v be the velocity of dm before it is captured by m, and let f represent the average value of the impulsive forces that dm exerts on m during the short interval dt, in which the capturing takes place. By Newtons third law, dm will experience a force f , exerted by m, over the same dt.

We can now examine the capture process from the point of view of dm and equate the impulse, f dt, to the change in linear momentum of dm, f dt = dm(v + dv v ).

(9)

Here, v + dv is the velocity of m (and dm) after impact. Analogously, from the point of view of m, we write

F dt + f dt = m(v + dv ) mv = mdv .

(10)

The term dm dv in equation (9) is a higher order term and will disappear when we take limits. The impulse due to the contact force can be eliminated by combining equations (9) and (10), F dt dm(v v ) = mdv , or, dividing through by dt, dv dm dm = F (v v ) = F + (v v ) . dt dt dt

(11)

Here, v v is the velocity of dm relative to m. This expression is valid when dm/dt > 0 (mass gain) and when dm/dt < 0 (mass loss). If we compare this expression to the more familiar form of Newtons law for
a particle of xed mass m dv dt = F , we see that the term (v v )dm/dt is an additional force on m which is

due to the gain (or loss) of mass. Equation (11) can also be written as d(mv ) dm = F + v , dt dt 3

where v is the velocity of the captured (or expelled) mass relative to the velocity of the mass m. This shows that, for systems involving variable mass, the usual expression stating conservation of linear momentum, d(mv )/dt = F , is only applicable when the initial (nal) velocity of the captured (expelled) mass, v , is zero. The behavior of m(t) is not an unknown, but is specied according to the characteristics of the rocket. In most cases,
dm dt

is a constant and negative. In some cases, the behavior of m(t) may be determined by a

control system. In any case, it is a given quantity.

The Rocket Equation

We consider a rocket of mass m, moving at velocity v and subject to external forces F (typically gravity and drag). The rocket mass changes at a rate m = dm/dt, with a velocity vector c relative to the rocket. We shall assume that the magnitude of c is constant.

The velocity of the gas observed from a stationary frame will be v = v + c. In this frame, c is a vector aligned along the ight path in a negative direction. c = ct, where t is the unit direction along the ight path. Thus, dv dm =F +c . dt dt

(12)

The term T = c dm/dt is called the thrust of the rocket and can be interpreted as an additional force on the
rocket due to the gas expulsion.
Equation (12) is a vector equation which can be projected along the direction of v (tangent to the path).
Thus,
dv dm = Ft c = Ft + T, dt dt

(13)

where Ft is the tangential component of F , v and c are the magnitudes of v and c respectively, and we have . Note assumed that c is parallel and has opposite direction to v . The magnitude of the thrust is T = cm that for a rocket, m will be negative (mass is lost). If the force Ft is known, this equation can be integrated in time to yield an expression for the velocity as a function of time. Let us consider some simple cases:

No External Forces: Ft = 0 If gravity and drag eects are neglected, we have, dv dm = c dt dt

or, integrating between an initial time t0 , and a nal time t, m m0 = c ln . mf m0

v = v v0 = c(ln mf ln m0 ) = c ln

(14)

Alternatively, this expression can be cast as the well known rocket equation, m = m0 ev/c ,

(15)

which gives the mass of the rocket at a time t, as a function of the initial mass m0 , v , and c. The mass of the propellant, mpropellant , is given by, mpropellant = m0 m = m0 (1 ev/c ). From the above equations, we see that for a given v and m0 , increasing c increases m (payload plus structure) and decreases mpropellant . Unfortunately, we can only choose c as high as the current technology will allow. For current chemical rockets, c ranges from 2500-4500 m/sec. Ion engines can have cs of roughly 105 m/sec. However, since relatively few particles are involved, the thrust is quite low, whereas chemical rockets have high mass ows. Equation (13) has some quite remarkable properties, as sketched below. First, the V for a given m0 /mf is independent of time. More strikingly, if we compare the nal V to the at car situation where all the mass was ejected instantaneously rather than continuously, we see a substantial gain in V from the continuous process. However, the nal V for the continuous process is independent of the actual time span of the process, as shown in the sketch. Thus in the limit as the continuous process occurs quickly, we will obtain the result for the continuous process. We will refer to the rapid continuous process as a V impulse. When we consider rocket thrusting during orbital maneuvers, we will consider only impulsive thrust. A second property, is that the nal V from a series of rings is independent of the details and a function only
0 of the total mass change over the processes. Consider a rst thrust producing a V . Then V1 = c ln m m1 . 1 Now the mass is m1 and a second thrust produces a V2 = c ln m m2 . By a property of logarithms, adding

m1 m0 0 these two V s together gives Vf = V1 + V2 = c ln m m1 + c ln m2 = c ln m2 where m2 is the nal mass

of the vehicle mf .

Another property is that the relationship between thrust and V is a positive scaler, always additive. Whether we thrust in the positive or negative direction,or to the side it always consumes propellant, we never get anything back , and substitute into equation 14 to obtain an Note that if m is constant, we can write m(t) = m0 + mt expression for v as a function of t, v = v0 c ln(1 + m t) m0 .

Recall that according to the convention used, m is negative as the mass decreases with time. Gravity: Ft = mg A constant gravitational eld acting in the opposite direction to the velocity vector can be easily incorporated. In this case, equation 13 becomes, m which can be integrated to give v = v0 c ln m m m m0 gt = v0 c ln g . m0 m0 m (16) dv dm = mg c , dt dt

This solution assumes that cm > m0 g at t=0. If this is not true, the rocket will sit on the pad, burning fuel until the remaining mass satises this requirement. Gravity Plus Drag: Ft = mg D The eect of the drag force, D, is harder to quantify. It turns out that for many important applications drag eects are very small. The drag force is characterized in terms of a drag coecient, CD . Thus, D= 1 2 v ACD , 2 6

where A is the cross-sectional area of the rocket. The air density changes with altitude z , and may be approximated by = 0 ez/H , where H 8000 m is the so-called scale height of the atmosphere, and 0 is the air density at sea level. It turns out that the dierential equation that results for the velocity cannot be integrated explicitly and, in practice, needs to be integrated numerically. It is interesting to note, however, that the eect of drag losses is usually quite small, and it is often reasonable to ignore it in a rst calculation. In order to see the importance of D versus the eect of gravity, we can estimate the value of the ratio D/mg . At conditions typical for maximum drag, 0.25 kg/m3 and v = 700 m/s. Considering a rocket of 12, 000 kg with a cross section of A = 1 m2 and CD = 0.2, we have, ACD v 2 0.25 1 0.2 7002 = = 0.021 , 2mg 2 12, 000 9.8 which indicates that the drag force is only about 2% of the gravity force. Note (Optional) Gravity Losses [1]

Let us consider a rocket providing a constant thrust T = cm which is launched vertically upwards from rest. Neglecting drag but considering gravity forces, the velocity is given by expression (16) which can be re-written as 1 1 v = c ln , n (17)

where we have introduced = m/m0 and the thrust induced acceleration measured in gees, n = cm/ (gm0 ). Now, v = dz/dt, and the above equation can be integrated to give c2 1 (1 )2 z= 1 ln . gn 2n

(18)

If we consider equations (17) and (18) at burnout time (when all the propellant is consumed), the mass fraction will be = (m0 mpropellant )/m0 . After this time, the rocket will coast in free-ight. The total energy per unit mass at this (and all subsequent times) is 2 v2 1 1 1 ln(1/) 2 E= + gz = c ln + . 2 2 n Since, 0 < < 1, 1 ln(1/) is negative and consequently, higher n means larger nal energy per unit mass E, tending to the ideal limit Eideal = 1 2 2 1 c ln ,

which is that obtained for an impulsive start. Reducing n means applying smaller thrust for a longer time, and, as the above equation shows there is a price to be paid in the eventual energy of the payload. One way to quantify this loss is to dene an equivalent impulsive velocity (v0 )eq such as to achieve the energy per

unit mass E in a very short time, hence, at ground level, 2 1 1 ln(1/) (v0 )eq = c ln +2 , n and to compare this to the ideal impulsive velocity (v0 )ideal = c ln 1 .

Some representative fractional losses ((v0 )ideal (v0 )eq )/(v0 )ideal , are shown below, = 0.7 n = 1.5 n=3 n = 10 0.3628 0.1615 0.0456 = 0.5 0.3188 0.1444 0.0410 = 0.3 0.2676 0.1235 0.0354

The table shows losses of 15% for n = 3, 4% for n = 10. (Recall that n must be greater than 1 for immediate lift o from the launch pad.) The losses are severe for n as low as 1.5. The gure shows a graph of these losses as a function on n for = .8, .7, .5, and .3.

Example Single stage

Single vs. Two Stage Rockets [1]

To achieve an orbital speed of v = 7600 m/s, we require an ideal v of about 9000 m/s where the extra velocity is needed to overcome gravity and drag. Chemical rockets produce exhaust jets at velocities of c 2500 4500 m/s. Using the higher c, if we wish to place a payload in orbit with a single stage rocket, we have a mass ratio (mass at burn out, mf , divided by initial mass, m0 ) of mf = e9000/4500 = e2 = 0.135 . m0 8

The mass mf must include all components of the rocket infrastructure, including the engine, empty tank, guidance equipment, etc., as well as the payload. The mass of the propellant will be m0 mf = (1 0.135)m0 = 0.865m0 . If we assume that the tank plus engine are the main contributors to the weight of the rocket and weigh 10% of the propellant, 0.087m0 , we have,
stage m1 payload = (0.135 0.087)m0 = 0.048m0 .

Clearly, there is not much margin here, and, in fact, single-stage-to-orbit vehicles have yet to be engineered successfully. The alternative solution is to subdivide the rocket, so that empty tanks are dropped when they are no longer needed. Two stages Consider now a two stage vehicle. We shall again assume that each empty tank plus its engine weighs 10%
of the propellant it carries. The required v is now subdivided into two v s of 4500 m/s each.
First stage
If m0 is the initial mass and m1 is the mass after burn out, we have,
m1 = e4500/4500 m0 = e1 m0 = 0.368m0 The fuel burnt will be m0 m1 = (1 0.368)m0 = 0.632m0 , and the weight of the tank and engine to be dropped will be 0.0632m0 , leaving an initial mass for the second stage, m2 , of m2 = 0.368m0 0.063m0 = 0.295m0 . Second stage After burn out, mf = e1 m2 = e1 0.295m0 = 0.109m0 . The fuel burnt in this stage will be m2 mf = (0.295 0.109)m0 = 0.186m0 and the weight of the tank plus engine will be 0.019m0 , leaving for the payload
stage m2 payload = (0.109 0.019)m0 = 0.090m0 ,

which is still very small but about twice the size of the payload obtained for the single stage rocket.

Note

Launch Strategy

The earth rotates towards the East with a period of about 23.9 hours. This results in a tangential velocity of vt = 466m/sec. at the equator. The corresponding value for higher latitudes is vt = 466m/sec Cos0 , where 0 is the latitude of position on the earth. In most situations, we desire to use this tangential velocity to add to the energy available to place a satellite in orbit. Therefore, most launch sites are located as close

to the equator as possible and the launch occurs with a velocity component directed towards the east. The table shows the latitude of the current major international launch sites. Of interest is Sea Launch, which leaves the US west coast with a sea-going launch platform and control ship and launches very close to the equator. Other criterion for launch site is safety; a failed launch should not put populations in danger. Kennedy Vandenburg Baikonur Sea Launch 28.5o 34.4
o

Tanegashima Xichang Kourou

30.4 28.25 5.2

45.6o 0 19 min
0

Note

Gravity Turn

The model we have introduced for rocket launch is incomplete if our goal is to place a spacecraft into orbit. Orbital launch requires that the ight end with a roughly horizontal velocity at orbital speed. One useful maneuver to accomplish this transition is called the gravity turn. In this maneuver, gravity acts to turn the trajectory of the rocket towards the horizontal.

This maneuver oers two main advantages over a conventional thrust-controlled trajectory where the rockets own thrust is used to steer the vehicle. First, any thrust used in changing the rockets direction is not being used to accelerate the vehicle into orbit, constituting a loss which can be reduced by using gravity to steer the vehicle onto its desired trajectory. Second, and more importantly, because the force of gravity is doing the steering during the initial ascent phase of the launch the ship can maintain low or even zero angle of attack. This minimizes transverse stress on the launch vehicle; allowing for a weaker, and thus lighter, launch vehicle. For a constant thrust, and a variable mass, the details of the trajectory and motion require a numerical solution. Using local intrinsic coordinate, the governing equations for the gravity turn are v = T /m(t) g cos = g sin v 10 (19) (20)

Note

Specic Impulse

The specic impulse Isp , with units of seconds, is often used in practice to characterize the performance of a rocket engine. The denition of the specic impulse is the magnitude of the thrust divided by the propellant weight ow rate, Isp = T c = . mg g

Typical values of Isp are around 300 s for solid propellants and up to 500 s for higher energy fuels.

ADDITIONAL READING J.L. Meriam and L.G. Kraige, Engineering Mechanics, DYNAMICS, 5th Edition 4/6, 4/7

References
[1] M. Martinez-Sanchez, Unied Engineering Notes, Course 95-96.

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16.07 Dynamics
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